Volume 5, Number 3, 1992

FR:

Cette étude est destinée à tester expérimentalement les capacités d'épuration des eaux usées par lagunage à macrophytes (jacinthe d'eau : Eichhornia crassipes), sous les conditions climatiques de Marrakech.

L'installation fonctionne en continu avec un débit constant à l'entrée de 10 l/min. La charge admise est de 40 g DCO/M²/j.

Sous l'aspect de la production de biomasse végétale, les effluents domestiques constituent un bon substrat nutritionnel. Les taux de croissance et les productions obtenues montrent dans l'ensemble une excellent adaptation d'Eichhornia crassipesà ce milieu. Le maximum de biomasse et de productivité ont été obtenu en période estivale et sont respectivement de: 40 kg MF/m² et 38,6 MS/m^2/j. Il s'est avéré également que la jacinthe d'eau est persistante toute l'année sous le climat méditerranéen aride de Marrakech.

L'épuration des eaux usées domestiques par lagunage à macrophyles aboutit à des rendements satisfaisants surtout en période estivale où on obtient un abattement de 87 % de la DCO et une réduction de 95 % des MEST.

Sur te plan sanitaire, l'abattement de la charge bactérienne exprimée par les bactéries témoins de contamination fécale peut atteindre jusqu'à 2ULog pour un temps de séjour théorique très court (7 jours).

Ce système e par ailleurs fourni des abattement de 100 % des oeufs d'helminthes parasites au niveau de l'eau épurée.

EN:

The aim of the present study is to experimentaly test the capacities of the mater hyacinth (Eichhornia crassipes) in order to purify wastewater under Manakesh climatic conditions.

The experiment was carried al wastewater spreading zone of Marrakesh pretraitement.

The experimant's installation is made of two lined water yacinth ponds that receive domestic wastewater.

The proposed process is a continuous system with a constant flow rate of 10 l/mn. The theoritical retention time was estimated to 7 days. The allowed load is 40 g COD/m²/day. Macrophytic biomass was observed for both ponds during the experimental period (Match, 1986 - February, 1987). Parameters of organic, bacterial and parasitological loads are studied in order to determine the system efficiency under arid climate.

Obtained results show the water hyacinth ability to adapt to Marrakesh climatic conditions. The number of plants doubled at 12 days, this is coherent with results obtained by PENFOUND (1956), BOCK (1969), WESTLAKE (1963, 1975) and SCULTHORPE (1967). Maximum biomass level and productivity were achieved during the summer period : 40 kg WW/m²/day and 38,6 g DW/m²/day respectively. Biomass and productivity obtained under arid climate are similar to results obtained by WOOTEN and DODD (1976), and by DINGES (1976) under subtropical conditions, and higher than chose obtained by JOHN (1985) under temperate climate. The growth period of water hyacinth is estimated to 9 months at Marrakesh, 10 months at subtropical climate (WOLVERTON and MC DONALD, 1976) and limited to 6 months under cold climate (COPELLI et al., 1982; DUBOIS, 1983; SAUZE, 1983; DE CASABIANCA, 1985). Temperature is considered as a limited growth factor of water yacinth. According to FRANCOIS et al. (1977), the water hyacinth growth was stopped when the temperature is lower than 10 °C. Linder Marrakesh arid climate, the temperature is always higher than 10 °C. It was also found that the water hyacinth survive all a year around in the arid climate of Marrakesh.

Domestic sewage purification by water hyacinth leads to satisfactory efficiency during the summer concerning reduction of COD: 87 % and TSS : 95 %. This phenomenon may be jointed to the retention time wich was lengthed (9,4 days) in the summer, and the higher biomass density of water hyacinths in this one. The purifying action of floating macrophytes (Eichharnia crassipes) is physical and biological. The root system stabilizes the medium thus favoring sedimentation of TSS and particulate COD both on the bottom of the tank and by trapping in the root hairs. Elimination of COD is realized by means of the action of bacteria which are present, by sedimentation of particulate COD and root filtration.

The biological action of the plants is not an important mechanism for COD elimination. The system efficiency is low at the winter and the reduction of COD and TSS have not exceed 60 % and 82 % respectively because the degenering of the water hyacinths.

From sanitary point of view, bacterial load reduction expressed by control faecal contamination bacteria achieved 2 log Units for a short theoritical retention time (7 days). This is higher than the result obtained by DUBOIS (1985). Two hypothesis are given to explain reduction of bacterial load by water hyacinths :1) the bacteria are sedimented or trapped in the root hairs of the water hyacinths whith TSS. 2) Water hyacinths may have a capacity to secrete a chemical substance wich could have bactericid or bacteriostatic effect. The improvement of retention time and the addition of one or two supplementary ponds will probably lead to better results. Moreover, this process had also reduced parasitical helminth eggs to undetectable levels (100 %). The parasitical helminth eggs distinguisched at domestic sewage received by the experimental installation, are Taenia, Hymenolepis, Trichuris and Ascaris geints. Their total number vary tram 0 to 120 eggs/l with a mean of 32. Other types of eggs could be encountred generally in waste water as : Toxocara, Oxyure, Capillaria and Taxoascaris (FOX and FITZGERALD,1976) but was not detected by our technique. No helminth eggs were found in purified wastewater descended through water hyacinth ponds. This phenomenon is explained by supposing that the helminth eggs are present in the effluent but it was the detection limit of the employed technique (Bailenger method), or there is no eggs really at the effluent because of their higher specific weight. Ascaris, Taenia and Trichuris eggs have a sedimentation rate of 0,65 m/h, 0,26 m/h and 1,5 m/h respectively (FEACHEM et al.,1983). The eggs sedimented rapidly in the water hyacinths ponds involving a transfer of the effluent pollution to the sediment. Several authors affirmed that the stabilization ponds are an effective means to reduce parasitical helminth eggs of the wastewaters (GLOYNA, 1972; KOWAL, 1985). Hence, if the parasitical risk could be controled in the purified water (effluent), particular attention should be given to sediments. It is also important to point out, that no parasitical nematode is found at the influent. Nematofauna associated to wastewater and roots of water hyacinth, was represented by bacteria consumer nematode. The abundance of nematode is definite by the existence of bacterial food in the wastewaters (CALAWAY, 1963; SHIEMER, 1976). The principal genus determined are Rhabditis sp, Plectus sp. and Mononchoïdes sp. It appears that Rhabditis genus, is dominant in the first pond (94,7 %) of the nematode population. However, the two genus Rhabditis sp. and Plectus sp. are dominant in the second one and represent 50 % and 49 % respectively. The presence of Plectidae in the second basin indicates that is less loaded (ZULINI, 1976). However, under the arid climate conditions of Marrakesh, the process based on water hyacinth for wastewater purification, is faced with two major problems : first, the water loss by evapotranspiration reachs 60 % during the summer time under arid climate of Marrakesh. The development of Mousquito particularly in the summer, constitutes the second problem of our proposed process. Moreover, front economical point of view, the water hyacinths show a good productivity in the summer under arid climate and could be exploited in several field.

FR:

Cette étude a eu pour but de déterminer l'effet d'un traitement par le sulfite de sodium sur la concentration en composés organohalogénés totaux (TOX) et sur l'activité mutagène de solutions chlorées de substances humiques d'origine aquatique (SHA), après avoir cherché à préciser l'influence du pH et du temps sur la concentration en TOX.

Les résultats obtenus à partir d'échantillons chlorés de SHA en absence de chlore résiduel ont permis de mettre en évidence une diminution de la concentration en composés organohalogénés totaux, soit par stockage en milieu neutre ou basique, soit par addition de sulfite de sodium. L'intensité de cette réduction de la concentration en TOX augmente avec le pH, le temps de réaction et la dose de sulfite de sodium introduite.

Les résultats obtenus à partir d'échantillons contenant du chlore libre indiquent que seule une déchloration totale avec un excès de sulfite de sodium peut conduire, en milieu neutre, à une diminution de l'activité mutagène et de la concentration en TOX des solutions diluées de SHA. La comparaison des pourcentages d'abattement obtenus sur le paramètre TOX et sur l'activité mutagène indique que la diminution de la génotoxicité par déchloration totale est due à l'action du sulfite sur des composés mutagènes non chlorés ou sur des composés chlorés fortement mutagènes et ne représentant qu'une très faible fraction du TOX.

EN:

If is a well known tact that mimerous organohalogenated compounds are formed during the chlorination (preoxidation or final disinfection) of drinking water. Some of these compounds have been shown to be mutagenic. Recent studies have suggested that a treatment with oxygenated derivatives of SIV (SO₂, NaHSO₃ and Na₂SO₃) could reduce the genotoxicity of chlorinated drinking water.

The general aim of Ibis study was to determine the effect of dechlorination treatments on the mutagenic activity of chlorinated drinking water. The following experiments were carried out in order to point out the effect of a treatment with sodium sulfite on the concentration of total organohalogenated compounds (TOX) and on the mutagenic activity of chlorinated dilute solutions of Aquatic Humic Substances (AHS).

At first, the affects of pH, sodium sulfite dose and contact time on TOX concentration were investigated. Then, the importance of the dechlorination rate (partial or complete) on TOX concentration and also on the mutagenic activity could be studied.

Experimental

Aquatic Humic Substances (natural mixture of fulvic and humic acids) were dissolved in phosphate-buffered ultra-pure water at 5 and 15 mg l^-1 concentrations (pH 6.1 and 6.9 respectively). Stock solutions of chlorine were prepared in the laboratory and titrated by iodometry. Chlorination and dechlorination treatments were carried out in headspace-free baffles, at 20± 1 °C in the dark. Residual chlorine was determined by spectrophotometric measurements at 510 nm, following the calorimetric method using N,N-diethylphenylene-1,4-diamine (DPD). To avoid the slow oxidation of Slv into Svl by dissolved oxygen, the sodium sulfite solutions were prepared freshly before use. TOX concentrations were measured using a DOHRMAN DX-20 TOX analyser equipped with a MC-1 microcoulometric cell and with an AD-2 adsorption module. Before analysis, the residual chlorine was neutralized with sodium thiosulfate and samples were acidified to pH 1.4.

The mutagenic activity was determined using acetone-dichloromethane extracts (AMBERLITE XAD-8 and XAD-2 resins) of the aqueous samples of chlorinated and dechlorinated solutions of AHS, acidified to pH 2.0 before extraction. The mutagenicity tests were carried out on TA 98 and TA 100 tester strains, following the method described by MARON and AMES (1983).

Results

-Effect of pH, addition of sodium sulfite and storage time on the TOX concentration

The experiments carried out with dilute solutions of AHS ([AHS] = 5 mg 1^-1; DOC = 2.5 mg Cl^-1; pH = 6.1) showed a linear relationship between TOX production and chlorine consumption in the range 0-2.0 mg Cl2 l^-1 (fig. 2).15 % of the chlorine demand was incorporated as organic chlorine in molecules.

Experiments performed on solutions containing no residual free chlorine showed that organohatogenated compounds could be partially destroyed upon storage at neutral or basic pH (table 1). Reductions in TOX concentrations of 10 % at pH 6.1-8.5 in 24 hours and of 20 % at pH 11.5 in 2 hours were observed. This was enhanced by increasing the storage time.

The addition of sodium sulfite (100 µmol l^-1) in solutions containing no residual free chlorine significantly reduced the TOX concentration (10 % in 2 hours at pH 6.1-8.5; table 1). This reduction was enhanced by increasing sulfite dose and storage time and by increasing pH (30 % in 2 hours at pH 11.5). Furthermore, at a given pH value and for a reaction time of 2 hours, the decrease in TOX concentration was larger in presence of sulfite.

- Effect of a dechlorination treatment on the TOX concentration

As shown in figure 3, a dechlorination treatment (reduction of the residual free chlorine concentration) with sodium sulfite could significantly reduce the TOX concentration of the dilute solutions of AHS at pH 6.1 only if an excess of the dechlorinating agent was added. This effect was enhanced by increasing the excess of sulfite but nevertheless seemed to be limited (less than 15 % of reduction for the highest doses used; table 2).

The free chlorine residuals measured after a 2 hours partial dechlorination confirmed the stoichiometric factor of 1 mole/mole for the reaction between chlorine and sodium sulfite.

- Effect of a dechlorination treatment on the mutagenic activity and on the TOX concentration

The dechlorination treatments were carried out on chlorinated dilute solutions of AHS ([AHS] = 15 mg l^-1; DOC 7.5 mg C l^-1; pH = 6.9). The TOX concentrations were measured on aqueous solutions and mutagenicity tests were performed on the corresponding acetone-dichloromethane extracts following a solvent exchange (dimethylsulfoxide). The results obtained showed again that only a total dechlorination treatment could reduce the TOX concentration of the aqueous chlorinated solutions and was able to destroy a significant part of the mutagenic activity of the extracts (table 3 and fig. 4).

Although the effect of sulfite on TOX concentration seemed limited (less than 7 % reduction for the highest sulfite dose tested), the reduction in the genotoxicity was more important when the excess of sulfite was increased. No correlation between the TOX concentration and the mutagenic activity could be established. The mutagenic compounds destroyed by sodium sulfite do not appear to be organohalogenated ones. If they are, they are present at trace levels and thus are extremely patent and account for a very little part of the TOX concentration.

FR:

Le travail présente un modèle mathématique conceptuel de transfert et de diffusion de masse destiné à l'étude des migrations d'effluents en rivière. Ce modèle prend en compte l'existence d'écoulements cisaillés ainsi que la présence de gradients de diffusion turbulente. Il permet de calculer les champs de concentrations et les valeurs moyennes de concentration à travers toute section transversale de l'écoulement. La localisation et la taille relative du rejet sont respectées. L'influence des rives sur les processus de dispersion est prise en considération.Pour quantifier l'influence des berges, une relation est établie entre les concentrations calculées en écoulement de largeur infinie et les concentrations en écoulement d'extension limitée. La méthode utilisée est fondée sur l'emploi d'un champ de vitesse et d'un champ de coefficient de diffusion, symétriques par rapport à des lignes riveraines séparant le courant nul d'un courant fictif situé de part et d'autre de ces limites.Les résultats des tests du modèle mathématique, réalisés à l'aide du programme moniteur « Gradient 2 », sont présentés. Dans le cas d'écoulements cisaillés, on a constaté que la valeur moyenne de concentration d'effluent calculée au travers de sections transversales à l'écoulement n'était pas une quantité invariante tout au long de l'écoulement. Un gradient de vitesse négatif induit une augmentation de cette moyenne à mesure que l'on s'éloigne du rejet alors qu'un gradient positif produit l'effet inverse. Un gradient du coefficient de diffusion turbulente détermine un changement du profil de concentration à l'intérieur d'une section transversale donnée, sans en changer cependant la valeur moyenne. Un gradient négatif augmente la valeur maximale de la distribution des concentrations. Un gradient positif fait diminuer la valeur maximale en aplatissant l'allure du profil.Le modèle mathématique a ensuite été vérifié à l'aide d'un modèle physique. Un modèle réduit respectant les similitudes d'écoulement a été bâti. Les gradients de vitesse du fluide et les gradients du coefficient de diffusion étaient provoqués par l'introduction de tirants d'eau non uniformes dans chaque section transversale. Les mesures réalisées ont permis d'estimer les coefficients de diffusion turbulente.

EN:

The report shows mathematical model of diffusion and pollutants mass transport in rivers with transversal to main movement direction velocity and turbulent diffusion coefficient gradients. The occurence of the mentioned gradients in plane cross sections is a result of variable depth channel and stream interaction. The results of sample calculations using microcomputer program and the results of mathematical model verification on laboratory model were shown. The presented model serves to qualification of pollutants concentration distribution in rivers, below the outfall.

A mathematical model is based on differential equations of advection-diffusion. Fundamental equation includes real dimensions of outfall and riversides reaction. To take into account the influence of flow velocity and turbulent diffusion gradients, an intensity of transversal to the main flow direction diffusion flux was determined. An intensity of diffusion flux which flows in any stream filament « I » depends on its coordinate, flow velocity and turbulent diffusion coefficient in this stream filament. Basing on diffusion flux intensity equations with variable parameters vi and Di, a pollutant concentrations field in a plane cross section was determined. To determine the influence of stream boundaries, the method of ratio of concentration field in unlimited and limited by riversides stream was used. Using this method it is necessary to take into account the symmetry of turbulent diffusion coefficient and velocity distributions related to boundaries which separate the real stream from the imaginary streams situated beyond the boundaries. A condition of mirror reflection of turbulent diffusion coefficient and velocity distributions is accounted for transversal diffusion flux reflection off impermeable riverside and the change movement direction to reversal one.

Sample calculations were based on the example of river channel 30 m wide and the boundary outflow with real dimensions. To express explicitly the results of sample calculations and their analysis, a linear positive and negative diffusion coefficient (range 1.5 10^-3 to 80 10^-3 [ms^-2]) and velocity (range 0.2 to 0.8 [ms^-1]) gradients were assumed. Negative gradients mean decreasing flow velocity and diffusion coefficient with increasing y distance from the outflow. Positive gradient means increasing magnitude of D (y) and v (y) with increasing y coordinate.

Quantities of turbulent diffusion coefficient were estimated on the basis of mathematical model verification at hydraulic model described below. Sample calculations were dons using computer program « Gradient 2 ».

Basing on the analysis of testing results it was found out that the condition of concentration field continuity in the plane cross-section, which was valid with nongradient flow, was not satisfied with the occurence of flow velocity gradient. Negative velocity gradient causes a considerable increase of concentration field. The quantity of concentration field increases with distance from outfall to cross-section. Comparison results of sample calculations with carried out earlier for slot outfall shows lower concentrations field increments for mal dimension outfall. It is accounted for the presence of wedge with concentrations equal to the initial concentration in outfall. Positive flow velocity gradient causes concentration field decrease with the increase of distance between the stream plane cross-section and outfall.

The increase of concentrations field with negative flow velocity gradient is accounted for pollutants mass accumulation in stream filaments, which flow with lower velocity. With positive gradient an increased pollutant mass advection process occurs in stream filaments with higher velocity. Turbulent diffusion coefficient gradients do not change the quantity of concentration field, but change distribution of pollutant concentrations. Negative gradient increases the slope of concentrations distribution curve, increases maximum concentrations and decreases range of diffusion flux action. Positive diffusion coefficient gradients decrease the slope of concentrations distribution curve by decreasing maximum concentrations and increasing range of diffusion flux action.

Verification of mathematical model was clone using laboratory model of open channel. The conditions of dynamic similarity of pollutant mass propagation processes on hydraulic model and river channel were determined. Conditions of similarity were determined on the basis of identity of differential equations of advection-diffusion written for model end the real object. Provided there is a similarity condition of processes of advection and diffusion which is the same Peclet number (Pe = idem), the condition of flow similarity must be satisfied, which is the same Froude number (Fr = idem). Using the mentioned similarity conditions between the model and the object a similarity scale of concentrations is determined. Concentrations of pollutant mass in any stream point on hydraulic model are ʆ₁ times larger than concentrations in corresponding river point, where ʆ₁ is a scale of geometric similarity of model to object.

Verification measurements were carried out in laboratory channel 0.3 m in width and 5 m long. Flow velocity gradients were generated by various depth of trapesium channel. Reynolds number was ranging from 2.6 10^-3 to 2 10^-3. Pollutants were simulated by rhodamine B dye solution. Concentrations of rhodamine solution were measured using the spectral colorimeter, equipped with instrument for measuring fluorescence with accuracy of 10^-9 [N m^-3]. The measurements of flow velocity were done with using current flow meter, and the measurements of depth with the use of point limnimeter. Turbulent diffusion coefficient was estimated on the basic of measured parameters by empirical relation Di = ʆ_p vi hi, where ʆ_p is a proportionality coefficient, and its value is ʆ_p = (0.7 -1) 10^-2. The results of measurements of tracer concentrations, and results of mathematical model calculations show sufficient convergence for practical tanks. Maximum relative deviation of concentrations reached 18 %. For the detailed investigation of mathematical model suitability, the next stage of verification is predicted on the base of measurements in the natural channel, with gradient of flow velocity.

FR:

Une étude concernant le transfert du césium radioactif dans une chaîne trophique simplifiée d'eau douce, a été réalisée dans le cadre des Contrats (CCE) n° BI6-B-198-P et n° BI6-0245-P. Les objectifs étaient la connaissance de la radioécologie du fleuve Tejo (Tage) et le comportement du césium radioactif dans un écosystème naturel. Dans le présent document, sont présentées l'accumulation par voies différentes et la rétention du ¹³⁴Cs par Daphnia magna Straus (Crustacea, Cladocera).

L'accumulation directe à partir de l'eau du fleuve contaminée avec du ¹³⁴Cs, a été effectuée avec une concentration initiale, approximative de 10 Bq ml^-1. Le facteur de concentration (FC), rapporté au poids frais des daphnies, a évolué selon la fonction : FC(t) = 57,4 (1 - e^-0,275t), avec le temps (t) en jours.

L'élimination du radionucléide est exprimée par la fonction de rétention : R(t) = 8,5e^{-12,211 t} + 91,5e^-0.441t, qui indique donc deux périodes biologiques, Tb₁ = 0,06 jour et Tb₂ =1,6 jours.

Quand la contamination de D. magna se réalise à partir de la nourriture (Selenastrum capricornutum) contaminée au ¹³⁴Cs,le facteur de transfert trophique suit la fonction : F T (t) = 5, 1 (1 - e^-0,245t).

Dans ce cas, le processus de rétention est représenté par une fonction exponentielle à un seul terme : R(t) . 100 e^-0,410t et correspond donc à une seule période biologique, Tb = 1,7 jour.

La contamination simultanée par les deux vecteurs semble montrer que la voie de contamination la plus importante est le transfert à partir de l'eau. Etant donné que la concentration cellulaire et la concentration en ¹³⁴Cs de la microalgue S. capricorrrutum étaient approximativement stabilisées, la nourriture n'a été responsable que de 28,5 ± 8,3 % de la concentration du radionucléide dans D. magna.

En ce qui concerne l'étude de la rétention, on observe que, pour la contamination par l'eau, la période biologique la plus courte doit correspondre à une désorption au niveau des surfaces, tandis que la période la plus longue doit correspondre à l'élimination du ¹³⁴Cs assimilé. La rétention consécutive à la fixation par l'ingestion de nourriture contaminé, conduit à une seule période biologique, qui ressemble beaucoup à la période longue du premier cas.

EN:

Considering the potential radiological risk, due to the normal operation of the spanish nuclear power plants (Jose CABRERA, TRILLO and ALMARAZ) sited in the Tejo (Tagus) River watershed, the radioecological study of the river is of the utmost interest.

Aiming for a better understanding of radionuclides behaviour in the Fratel dam, a particular freshwater ecosystem in the Tejo River located a few kilometers from the border, several studies have been carried out.

Besides the field follow up, an experimental study concerning the radioactive cesium transfer in a simplified freshwater trophic chain was developed, under the Contracts (CEC) n° BI6-B-1 98-P and n° BI6-0245-P, whose objectives were respectively the knowledge of Tejo River radioecology and the radiocesium behaviour in a natural ecosystem.

In a radiological assessment, the use of site specific data is always recommendable, instead of using default values, what is another perspective of the present work.

The importance of cesium behaviour in the environment, is related to its chemical similarity with potassium, which is a wide-spread element in the living tissues. Following a hypotetical ingestion of radiocesium contaminated foodstuff or water by man, the contamination may then be extended to the whale body, mainly the muscles. Recently, the accident in the ucranian Chernobyl power plant, has alerted the international scientific community to the problem of the environmental dispersion of this radionuclide.

¹³⁴Cs, a gamma radiation emitter, was the isotope selected for the experiments. Its usual concentration in the radioactive liquid effluents released by PWR plants, is similar to the one of ¹³⁷Cs, an isotope with a half-life of 30.15 years. ¹³⁴Cs has a half-life of only 2.07 years, which is an advantage regarding the production of radioactive wastes resulting from the experiments.

Our objective is to understand the mechanisms of the ¹³⁴Cs transfer along the trophic chain, and to determine the parameters that might integrate e radioecological model, namely the concentration factors between organisms and water, and the trophic transfer factors between consumers and nourishment.

The simplified trophic chain concerned : a primary producer, the green microalgae Selenastrum capricernutum Printz (CORISCO and CARREIRO, 1990 a); a primary consumer, the planktonic crustacean Daphnia magna Straus; and a secondary consumer, the omnivorous fish Tinca tinta L. (CORISCO and CARREIRO, 1990 b).

In this paper the ¹³⁴Cs uptake and retention by Daphnia magna Straus, (Crustacea, Cladocera) via different pathways, are presented.

D. magna is a filter-feeding organism very common in freshwater ecosystems and frequently used as a test organism in toxicology. Several authors have been using it, as well, in the field of experimental radioecology. These small crustaceans, hardly exceeding a 3 mm mesh size, reproduce predominantly by parthenogenesis and, in laboratorial conditions, this process may be permanent if optimum temperature, abundant feeding and water quality are maintained.

The main chemical characteristics of Fratel water, which was used in this study, are presented. The average stable cesium concentration, determined by instrumental neutron activation analysis, is 6 x 10^-5 ppm.

During the experiments, the temperature was kept at 20 ± 2 °C and artificial light was maintained for 15 h a day, using daylight fluorescent tubes (36 watt).

The radiocesium used was a ¹³⁴Cs chloride in a 0.1 M solution, with a total cesium content of 1 µg ml^-1.

The radioactive measurements were performed in e well type 1 1/4" diameter and 2 1/2" deep sodium iodine (thalium activated) detector of 4" x 4", connected to monochannel analyser.

The direct uptake from ¹³⁴Cs Iabelled river water was carried out using an initial radionuclide concentration in the water near to 10 Bq ml^-1. The concentration factor (CF), referred to wet weight of daphnids, evolved according to the function : CF (t)=57.4 (1 - e^-0.275t) , with time (t) referred to days.

When the contamination of D. magna was carried out through the nourishment, ¹³⁴Cs Iabelled Selenastrum capricornutum, the trophic transfer factor (TF) followed the exponential function : TF(t)=5.1 (1 - e^-0.245t).

Combining both pathways simultaneously, water appears to be the main contamination source. The number of cells and ¹³⁴Cs concentration in microalgae S. capricornutum being approximately stabilized, nourishment was responsable for 28.5 ± 8.3 % of the radionuclide concentration in D. magna.

However, using the functions issued from the separated pathways experiments, the assessed contamination of D. magna suggests that the trophic pathway would be predominant.

The evaluation of the relative importance of the contamination pathways, would very likely be dependent on the experimental conditions. Therefore, experimental results must be carefully taken in consideration, and an approach as best as possible to the environmental conditions should be attempted.

The elimination of the radionuclide , after the uptake from water, is expressed by the retention function : R (t)=8.5e^-12.211t + 91.5e^-0.441t, meaning two biological retention compartments with half-lives Tb₁ = 0.06 day and Tb₂ = 1.6 days. Following the uptake from the food pathway, retention is expressed by an exponential with only one term : R(t)=100 e^-0.494t and, therefore, only one hall-life, Tb = 1.7 days.

From the retention study after ¹³⁴Cs uptake from water, it appears that the shorter hall-life might correspond to the surface desorption, whereas the longer one might correspond to the loss of assimilated ¹³⁴Cs. Following uptake from ingestion of labelled food, retention study lead to a single hall-life, very similar to the longer one in the previous case.

These results would suggest that water and food pathways act differently when tested together or separately, and assimilation must be the main concentration mechanism of ¹³⁴Cs for the crustacean Daphnia magna.

Considering the importance of these planktonic crustaceans as a link in the freshwater trophic chains, the concentration factor at equilibrium (57 ± 3) and mainly the trophic transfer factor et equilibrium (5.0 ± 0.6), a significative contamination concerning the ¹³⁴Cs, could be reached.

FR:

La contamination bactérienne de la phase eau d'un réseau de distribution résulte d'une multiplication des bactéries sur les parois des canalisations d'eau (biofilms) suivie de leur arrachage et de leur transport dans le flux circulant. Ce travail met en évidence l'effet du chlore, d'une part, sur la formation des biofilms et, d'autre part, sur des biofilms déjà constitués. Des éprouvettes de matériaux neufs introduites dans des eaux présentant des concentrations en chlore total variant de 2,4 à 0,02 mg/l et véhiculant entre 0,5 x 10⁶ et 5 x 10⁵ cellules bactériennes/mi (dont 1 à 10 % de bactéries cultivables) sont rapidement colonisées (10⁶ à 10⁸ cellules/cm²). L'effet du chlore est sensible sur les cellules totales pour des concentrations de l'ordre de 1 à 2,4 mg/l. Sur les bactéries cultivables, un ralentissement de la croissance du biofilm est observé dès 0,3 mg/1 de chlore total. Par contre, des résiduels de 0,02 ou 0,05 mg/l sont sans effet sur la cinétique de formation des biofilms. Des résiduels moyens de chlore total compris entre 2,3 et 3,4 mg/l appliqués en continu pendant 14 jours sur un biofilm constitué d'environ 8,7 x 10⁶ cellules par cm² (1,7 % de bactéries cultivables), entraînent l'élimination d'environ 90 % des bactéries fixées (abattement d'1 logarithme) durant les premiers jours d'exposition. L'altération du biofilm exposé à un résiduel de chlore total de l'ordre de 1,3 mg/l est identique, mais toutefois plus étalée dans le temps. Ces essais réalisés sur des éprouvettes de PVC, PE et mortier de ciment n'ont pas permis la mise en évidence de comportements différents de ces 3 supports..

EN:

Bacterial accumulation in drinking water systems results both of cell deposition on the pipe walls and attached bacteria growth. The presence of a complex biofilm (cells embedded in a matrix of exopolymers) leads to a continuous contamination of the water phase resulting from the erosion of the attached growing biomass. Then, many tentatives to lmit the formation of such a biofilm have been suggested as the removal of biodegradable organic matter fram water or as the application of disinfectant. However, the efficiency of chlorination of the distribution system is debatable. Indeed, adhesion is often described as a factor of protection of attached bacteria which counterbalances the expected effect of disinfectant. Then, the aim of this experimental work is using a model distribution system to evaluate (i) the kinetics of biofilm accumulation on coupons of new materials (Polyvinyl chlorure : PVC, polyethylene : PE, cement) disposed in a constantly chlorinated system (residual total chlorine from 0.021o 2.4 mg. l^-1), (ii) the effect of chlorination on previously accumulated biofilms.

The industrial pilot plant used in this study is comprised of five loops serially disposed (fig. 1). From previous study of simulation, one may assume that each loop works like a perfectly mixed reactor when the whole pilot plant is equivalent to an infinite tubular reactor with high axial dispersion coefficient. During the experiment, the pilot was continuously fed with finished drinking water front the surface water treatment plant of city of Nancy (i.e. natural finished water with its own chlorine demand, organic nutrients and heterotrophic bacteria).

Total number of cells (epifluorescence counts) and heterotrophic plate count bacteria (15 days of incubation at 20 °C) were enumerated both in the water and, after sonication, on the surface of the coupons of tested materials.

The first experimentations show that chlorine slows clown the kinetic of deposition of bacteria onto the pipe wall but never prohibits biofilm formation. When the drinking waters carried from 2.4 to 0.02 mg.1^-1 of chlorine and from 0.5 to 5 x 10⁵ ml^-1 bacterial cells, biofilm is observed after 24 hours of immersion of the coupons with at least 10¹ to 10⁶ bacteria/cm². Respectively, the deposition or/and growth rates of total cells are drastically affected only for chlorine residual as high as 1 to 2.4 mg. 1^-1. The number of heterotrophic plate count of the biofilm is affected with lower chlorine residual (around 0.3 mg.1^-1) but residual concentration as low as 0.05 mg.1^-1 are ineffective.

The tentatives carried out in the second experience on preformed biofilms (2 months old biofilms, 8.7 x 106 cells/cm²) show that the continuous application of 2.3 to 3.4 mg. 1^-1 of residual chlorine for 14 days, leads to the removal of only 90 % of attached total cells without modifications of the proportion of attached alive bacteria (around 1.7 %) into the biofitm. In other wards, a highly chlorinated networks shows at minima 10⁶ attached cells/cm². Its generally takes several days to reply to the chlorine demand of the system and to have a quasi steady state reactor in terms of residual chlorine.

These assays carried out with three types of coupons (PVC, PE, cement lined cast iron) did not show any difference between the tested materials.

The limited efficiency of chlorine against the biofilm can be explained by transfert limitations within the visquous layer, high consumption of chlorine by the biopolymers of the attached matrix (proteins...) or low sensitivity to the disinfectant of the slow growing attached bacteria. Then chlorination is really not a panacea in biofilm war but has to be applied in combination with other methods as biodegradable organic matter removal, hydraulic regime improvement...

FR:

La publication fait le point des connaissances sur le genre Lamprothamnium dans les domaines de la morphologie et du cycle de développement, de l'écologie et des végétations associées, de la répartition des espèces.

Toutes les espèces monoïques de Lamprothamnium, parfois difficiles à distinguer les unes des autres, forment un groupe naturel dans la tribu des Chareae. Elles sont caractérisées par un thalle sans cortication et une seule rangée de stipulodes plus ou moins développés et disposés un sous chaque phylloïde. WOOD et IMAHORI (1965) distinguent trais espèces (avec six formes) auxquelles DONTERBERG (1984) ajoute un nouveau taxon endémique d'Argentine. Une clé systématique remaniée est proposée.

C'est le seul genre de Characées dont toutes les formes, peuplant les eaux saumàtres et salées, peuvent supporter de larges et soudaines fluctuations du taux de salinité (eau hypersaline à eau douce et vice versa).

La germination s'opère en eau de faible salinité, à température moyenne et sur un substrat meuble reposant sur de la vase compacte et salée dans des milieux souvent peu profonds, ensoleillés, donc à échauffement diurne notable.

Les biotopes aquatiques, temporaires ou permanents, sont généralement peuplés de végétations mono- ou polyspécifiques de Characées, souvent denses, où peuvent s'introduire des Algues et des Phanérogames halophiles.

Toutes les espèces sont en voie de raréfaction notable en raison de la profonde modification des milieux très spéciaux qui leur sont favorables.

L. papulosumse trouve être, à l'échelle mondiale, la première espèce de Cryptogames à recevoir le statut de plante protégée en Grande-Bretagne (MOORE, 1991).

EN:

All monoecious species of Lamprothamnium J. Gr., which often are difficult to distinguish from each another, from a natural group within the Chareae Tribe. The plants, from 5 to 80 cm high, are characterized by a thallus without codex round the axes and the branchlets, and one tier of stipulodes these ones are uniform or of irregular size, as numerous as the branchlets, and inserted one below each branchlet. The bract-cells are well developed and verticillate (fig. 1). The common form tends to develop compact foxtail-like upper portions ; but a plant with the diffuse habit and extremely elongated branchlets can grow tell in some deeper brackishwater biotopes.

The male and female gametangia are conjoined or sejoined on the branchlet modes (one of each) and sometimes aggregated at the base of the branchlets. The oogonium below the antheridium is the predominating position. The table 1 is bringing out the measurements of the male and female gametangia published in a few taxa of Lamprothamnium. It is obvious that the differences measured between them are so small that they are not a bit help in the species determination.

The dried plants kept in the herbarium have become often breakable and cannot be investigated. The fresh plants must be preserved in ethanol at 70 % or in formalin at 5 %. The measurements of all parts of the fresh plants have been taken in water under the stereo- or the lightmicroscope with the help of a micrometer eyepiece. The mature plants with sex organs were fixed in acetic ethanol in the ratio of 1/3 for karyological studies, then the standard staining and squashing methods were followed. The chromosomes have been counted in the cells of the antheridial filaments (GUERLESQUIN,1967).

A few chromosome numbers of four taxa have been published in Lamprothamnium papulosum (n = ca 25, 50, 56 in Europe; n = 70, 72 in Ouzbekistan; n =14, 28, 30 in Australia on some doubtful samples); Lamprothamnium macropogon (n = 28, 55 in Australia); Lamprothamnium succinctum (n =14, 42 in India; n =42 in New Caledonia); Lamprothamnium succinctum var. australiensis (n = 42 in Australia).

WOOD and IMAHORI (1965) have distinguished three similar species and six forms to which Donterberg (1984) added a new taxon endemic in Argentina. A revised key to the species and varieties of Lamprothamnium is propounded (table 2):

1) Stipulodes well developed, of uniform size, male and female gametangia growing together:a) gametangtaa atbranchiet nodes and at base of branchlet : L. papulosum;b) gametangia restricted to branchlet nodes : L. hansenii;

2) Stipulodes irregular in size or absent :a) male and female gametangia growing on different branchlet nodes and at base of branchlets : L. succinctum ;b) male and female gametangia growing on the same branchlet nodes only: L. haesseliae.

A few taxa of an uncertain value have been described such as Lamprothamnium mediterraneum (Lovric, 1979, 1980).

Lamprothamnium is the only genus of Characeae of which all brackish and saline water forms can tolerate wide and sudden fluctuations of the salinity rate (fresh- to hypersaline water and vice versa, NaCl, MgCl², calcium sulphate). The physicochemical composition of a few brackishwater biotopes of Western Europe is mentioned in the table 3. The ratio of Cl^- can vary from 9 g. 1^-1 to 59 g. 1^-1 (more than six times); this one of the total salts from 32 g. l^-1 to 170 g. l^-1 (about five times). This tact is making clear the adaptability of these plants to the fluctuations. All the taxa of Lamprothamnium are sun-loving plants and thereby they can grow in the biotopes with an appreciable warming action of the sun during the day. In the temporary ponds, the plants are enough quick to develop their life cycle completely before the whole drying up.

The germination occurs in a slightly brackishwater with a mild temperature, after a dormancy time more or less long (from a few months to several years : six in Morocco, GUERLESQUIN et al., 1987). From the germination of the oospore to the fertile plant with mature gametangia, the developmental process needs two or three months.

The aquatic biotopes can be covered with dense mono- or polyspecific vegetations of Characeae into which halophilous Algae (Cyanophyceae, Chlorophyceae) and Angiosperme (Ruppia sp., Zannichellia sp., Althenia sp., Zostera sp. pl., Potamogeton pecfinatus, Groenlendia densa, etc.), are growing together. The brackish biotopes must be permanent (salt- or brackish lakes, stagnant or lightly running water, etc.), temporary (coastal lagoons, salt-marshes, etc.), continental or by the sea.

The taxa of Lamprothamnium are widespread in scattered sites between 20° N and 59° N, from Sahara to Southern Norway, also in South Africa, Mauritius, Australia and Tasmania, New Zeatand, New Caledonia, Japan, Eastern Asia (Pakistan, India, China), South America (Bolivia, Argentina). Now they are unknown in North and Central America, and the Caribbean Islands. L. papulosum is the mort widely distributed species (maps 1 and 2).

All the forms are becoming rarefied considerably owing to the great modifications of the very special favourable environments such as the draining or the filling in of the salt-water lakes and the coastal salt-pans. The localities where L. papulosum, the only species present in France, was seen again recently, are very few (map 1).

In its quinquennial review (1987), the Nature Conservancy Council has decided to protect L. papulosum, a rare Charophyte in Great Britain, and its favourable habitats. These are often regarded as unproductive « wasteland » and frequently threatened by « the development at recreational amenities, holiday accomodations, shipping and salt industries » which induce pollution, disturbance and complete drying. Thus L. papulosum will be the first species of Cryptogams to receive the status of protected plant in the whole world (Moore, 1991).

FR:

La croissance pondérale estimée par le taux instantané de croissance (b) d'Asellus aquaticus L. a été évaluée dans des conditions de laboratoire et en milieu naturel dans des mésocosmes. L'influence d'une contamination par du lindane (insecticide organochloré) a été étudiée. Au laboratoire, la durée d'exposition au lindane a été de 20 jours, sa concentration de 4 µg.l^-1 au départ était de l'ordre de 2 µg.l^-1 à la fin de la période d'exposition : la température a été constante (15 °C) et la photopériode 12/12 heures. Dans les mésocosmes, l'expérimentation a duré du mois de juin au mois de février. Au départ, la concentration du lindane était de 4,5 µg.l^-1, elle était voisine de zéro en février. On constate que le taux instantané de croissance (b) est plus élevé dans les mésocosmes qu'au laboratoire, en milieu contaminé qu'en milieu témoin. Dans les conditions de laboratoire il est environ 2,6 lois plus élevé pour des aselles contaminées (de poids compris entre 7 et 12 mg) que pour les aselles témoins. Cette augmentation est significative de l'action du lindane. Dans les mésocosmes, l'augmentation du taux instantané de croissance des aselles maintenues en milieu contaminé par rapport à celles provenant de milieu non contaminé n'est statistiquement significative qu'au septième mois après le début de la contamination par le lindane, elle ne l'est plus au huitième. Il semble que d'autres facteurs puissent expliquer cette augmentation, en particulier la qualité de l'alimentation est discutée. Aussi ne peut-on affirmer que le lindane dans les conditions naturelles est responsable d'une élévation du taux instantané de croissance de l'aselle.

EN:

We estimated the ponderal growth (instantaneous growth rate) of a fresh-water invertebrate Asellus aquaticus L. (Crustacea, isopoda), bred under laboratory conditions and in experimental mesocosms. Contamination by the insecticide lindane (Pepro 99 % purity) was studied.

To estimate the instantaneous growth rate we used the formula Wt = Wo exp Mt) in what Wo is the median weight class of Asellus at the beginning of experiment. Wt is the median weight after 20 days, dj is the days number multiplied by median temperature above 3 °C of considered period (3 °C is considered as minimal temperature below that no development is possible). Seven classe were constituted : class 1 (2 mg to 6.99 mg), 2 (7 to 11.99), 3 (12 to 16.99), 4 (17 to 21.99), 5 (22 to 26.99), 6 (27 to 31.99) and 7 (32 to 36.99). The instantaneous growth rate is calculated for this seven class weight and for each replicate, next median and standard error were calculated for each class. The number of replicate vary among class weight. It was : 22, 33, 29, 22, 16, 15 and 13 respectively for class 1, 2, 3, 4, 5, 6 and 7.

Asellus were preleved in natural ponds, acclimated in laboratory conditions during a minimal period of 15 days before that they are used for experiment. (glass containers of 30 litres, filled with hall tap-water hall pond-water, feed with maple leaves).

In laboratory conditions Asellus were kept in glass aquarium (15 x 20 x 18 cm) filled with 2 litres of water.

Physico-chemical parameters of water were : pH = 8.2; total hardness =155 mg 1^-1 measured as Ca C03; nitrites = 0.055 mg 1^-1; nitrates = 3 mg 1^-1; chloride = 73 mg l^-1 as Na C1. Laboratory breeding conditions were a tempera-tare of 15 °C and a 12/12 hours photoperiod. Contamination tasted 20 days, lindane concentration was 4,5 µg. l^-1 (near the median lethal concentration, 48 hours : 5.14 µg.1^-1) at the beginning of the experiment and approximatively 2 µg.1^-1 at the end. Twenty Asellus of the same weight class were deposed in each glass. The diet was constitued with maple leaves. For contamination study only Asellus of class 7 to 11.99 was study. Ten replicates were realised.

The mesocosms were constituted by two rectangular basins 10 meters in length, 2.5 m in width and 50 to 60 cm in deep. To secure a good water-thightness, bottom and sides were covered with black polyane 150 µm in thick, sediment and sand were deposed on the bottom on 5 to 10 cm in thick. Next basins were gradually (July 1987 to January 1988) filled up with tapwater. Natural colonization by phytoplankton and insects were observed, whereas vegetable (Ranunculus aquatilis, Typha angustifolia, Scirpus palustris and Ceratophyllum submersum) and invertebrates (Asellus, Planaria and Leech) were introduced by us. One mesocosm was contaminated by surface spraying with lindane acetonic solution.

Thirty of a same weight class were placed in 25 x 12 x 7 cm plastic box, with a total of 20 lateral openings on either side (1.2 cm in diameter) disposed in two rows and covered with fine mesh net (150 µm in opening). The center part of the lid was cut out and also covered with the same fine mesh net. A bed of maple leaves was placed on the bottom of the box. This containers were then distributed throughout bath the control and lindane contaminated basins. Between 15 to 20 days later, this containers were collected and brought back to the laboratory. All the specimens were weighted and once divided into size classes and returned to their experimental basins. The experiment started in June 1988 and lasted till February 1989. The lindane concentration was 4.5 µg.l^-1 at the beginning and near zero at the end.

For the control, in laboratory condition or in mesocosm, we observed that the instantaneous growth rate decrease when the median weight of the class increase. It vary to 0.1131 mg.mg- 1.dj^-1 to 0.0183 mg.mg^-1.dj^-1 and to 0.2704 mg.mg^-1.dj^-1 to 0.0879 mg.mg^-1.dj^-1 respectively in laboratory condition and mesocosm. Significant correlation (level 0.001) was observed between the logarithm of instantaneous growth rate and logarithm of the weight. Slope of regression lines does not vary, only position differ. Growth rate was higher in the mesocosm than in laboratory. In laboratory conditions lindane contamination induce a variation of instantaneous growth rate. An significant increase of 2.6 was observed between contaminated and control for Asellus of weight class 7 to 11.99 mg. In the contaminated mesocosm, a correlation is noted between instantaneous growth rate and weight, it is significant only for 3 collections dates (December 1988, January 1989 and February 1989). No variation in the slope of linear regression is noted, position differ significatively (level 0.01) only in February. For this collection date instantaneous growth rate is higher in contaminated mesocosm than in control. It appear that other factors that lindane contamination may also explain this increase. Among these, food quality has been envisaged by different authors. In conclusion is noted that in laboratory condition lindane induce an increase of instantaneous growth rate, in mesocosm we can't affirm that lindane was the responsible for the increase of instantaneous growth rate. Other experiments are necessary to confirm this observation.

FR:

L'amélioration de la qualité de l'environnement passe par la réalisation de contrôles de toxicité in situ et en continu des sources de pollution ou des milieux contaminés, à l'aide de systèmes automatisés à réponse rapide. Les systèmes donnant une réponse en temps réel permettent d'intervenir immédiatement à la source, d'interrompre le rejet d'un flux toxique et de prévenir ainsi les accidents de pollution. Ce type de stratégie ne peut être développé qu'au moyen de biocapteurs : les méthodes d'essais conventionnelles n'autorisent que des contrôles de toxicité épisodiques, en laboratoire, effectués dans des conditions statiques quelque peu éloignées des conditions dynamiques.

Nous nous sommes intéressés à la mesure de l'activité photosynthétique d'algues unicellulaires immobilisées. La photosynthèse induite par des stimuli lumineux est en effet un processus dont la réponse est immédiate et aisément mesurable à l'aide de transducteurs électrochimiques. Il apparaît donc intéressant d'utiliser ces réactions photosynthétiques pour la détection des polluants.

Deux dispositifs mesurant l'activité photosynthétique d'algues unicellulaires ont été testés. Le premier dispositif mesure le transfert d'électrons le long de la chaîne photosynthétique lors d'une illumination des micro-organismes. Le second système permet de quantifier la production d'oxygène résultant de cette excitation lumineuse.

La mesure du transfert d'électrons photosynthétiques nécessite l'addition d'une substance oxydo-réductible (médiateur) dans le milieu pour capter ces électrons. De la série de médiateurs testés, seuls les dérivés à caractère lipophile (2,6-diméthylbenzoquinone et p-benzoquinone) ont permis de mesurer un transfert d'électrons. Toutefois la durée de vie de ce biocapteur s'est révélée limitée à moins de 24 heures, ce qui exclut toute utilisation en continu.

Le second dispositif développé présente en revanche une longévité d'une semaine, ce qui le rend intéressant en vue d'une utilisation in situ. Les performances de ce capteur à oxygène ont été testées sur des produits de type herbicides, cyanures, métaux et comparées aux valeurs obtenues à l'aide de tests algues classiques ou de méthodes de détection rapide de la toxicité.

EN:

Environmental monitoring of pollutants with automatic systems, applied on-line and allowing rapid response constitutes one of the most successful ways to improve the quality of the environment. Real time analysis offers the advantage of detecting rapidly sources of pollution and preventing any accidental release of pollutants. Such a strategy is possible only by means of biosensors : current methods, commonly used far toxicity testing are usually carried out in Laboratory in static conditions, making real lime analysis impractical.

Two types of amperometric environmental sensor incorporating eukaryotic algae were investigated for use in monitoring industrial pollution of aquatic systems. Both sensors allowed the monitoring of photosynthetic events.

The first sensor follows photosynthetic electron chain events within the cell resulting in the reduction of mediator acting as terminal electron accepter. Reoxidation of the mediator at the biosensor electrode surface rues in a flow of current, the magnitude of which is proportional to the level of photosynthetic activity of the microalgae.

In the second approach photosynthetic oxygen evolution by the illuminated biocatalyst is measured by reduction at a cathodic electrode. Enzymic systems associated with the water splitting and oxygen evolution are amongst the most fragile components of the photosynthetic apparatus, and the monitoring of algal oxygen production is therefore a useful approach to early detection of toxic environmental pollutants.

Several species of unicellular algae were used for these experiments : Chlorella vulgaris, Scenedesmus subspicatsus and Selenastrum capricomutum. Algal cultures were harvested in the exponential growth phase and diluted to 0.5 O.D. (655 nm); then 1 ml aliquots were centrifuged at 900xg for 3 min. After centrifugation, cells were resuspended in growth medium, LEFEBVRE and CZARDA (LC), and immobilized by aspiration onto a filter disc. This filter disc was placed onto the carbon working electrode surface. Filters were held in place by a fine nylon mesh.

This biosensor is a two electrode system comprising a carbon working electrode and Ag/AgCl reference/counter electrode. Solution was continuously flowed through the electrochemical cell at a flow rate of 2 ml min^-1. Illumination of the algal biocatalyst was supplied by light emitting diodes with a peak wavelength of 635 nm and a light intensity of 125 millicandellas. Periodicity of illumination was chosen in order to obtain a stable photosynthetic response.

Biosensors exploiting direct electron transfer from a biocatalyst to an electrical system are not feasible. Indeed, the tell wall of the biocatalyst act as a barrier to the exchange of electrons between the electrode and the redox intermediates oft the cell. Electroactive compounds (mediators) must be used to shuttle electrons from the photosynthetic electron transfer chain to the electrode. Mediators were added to the flowing solution of LC medium, and a potential of 550 mV applied at the working electrode to reoxidize mediator reduced by the biocatalyst. The mediator must be lipophilic to access the chloroplast electron transport chain of eukaryotic algae. We tested a wide range of mediators but only p-benzoquinone (p-BQ) and 2,6-dimethylbenzoquinone gave measurable responses.

A concentration of 0.2 mM p-BQ (21.5 mg/l) was employed to measure photosynthetic activity. Experiments showed that 15 minutes light period followed by a 15 minutes dark period gave a steady photosynthetic response. However, this high concentration of mediator was toxic for the cells. Static algal tests using Chlorella vulgaris have shown that growth is totally inhibited after 72 hours at a concentration of 5.4 mg/l. The working life of this sensor was therefore very short, less than 24 hours : after 16 hours of continuous monitoring, the recorded photosynthetic current was less than 20 % of initial response. Sensor life was not increased when the probe was used alternately with recovery periods in nutrient medium (4 hours of working period/4 hours of recovery period).

The same apparatus was used for the oxygen electrode based biosensor. The working electrode was coated with a Teflon gas permeable membrane to protect the sensor against poisoning by electrochemically active compounds. Separation of the working and reference/counter electrode requires addition of electrolyte in the flowing solution. With such a semi-protected oxygen electrode, mass transport controlled oxygen reduction currents were obtained when the Teflon covered cathode was poised at -700 mV.

The oxygen biosensor responded more rapidly than the mediated biosensor to changes in the light regime, and alternating light and dark periods of 1 min of light followed by 4 min of dark were used. The sensor also showed good long term stability, with a working life of up to seven days using Chlorella vulgaris or Scenedesmus subspicalus as biocatalysts.

The sensitivity of this oxygen electrode based biosensor was tested on herbicides (isoproturon, propanil and atrazine), cyanide and heavy metals (copper and mercury). Results were compared with chose obtained with three toxicity tests : a standard algal growth inhibition test, the inhibition of photosynthetic activity in spinach leaves and the alga Chlamydomonas reinhardii, and the Microtox test using the luminescent bacterium Photobacterium phosphoreum.

IC₅₀ obtained for isoproturon and atrazine were very similar for the growth inhibition and the oxygen sensor tests. The inhibition of oxygen production by spinach leaves was less sensitive to atrazine; no toxic affect could be detected with the Microtox test. The oxygen sensor was also very sensitive to cyanide but the response of the probe was quite different if Selenastrum capricornutum or Chlorella vulgaris was used.

The sensor allowed metals detection but this detection of toxicity was slow compared to that of herbicides or cyanide. Inhibition growth tests and Microtax test were more sensitive than the algal sensor for copper and mercury.