Tolérance et accumulation du cuivre et du chrome chez deux espèces de lentilles d’eau : Lemna minor L. et Lemna gibba L.

Résumé

Certains procédés d’épuration extensive des eaux usées utilisent les lentilles d’eau. Les rejets des eaux usées sont de plus en plus sujets à des pollutions de natures diverses, notamment les métaux lourds. Dans ce travail, on procède à une évaluation comparée de la tolérance et de l’accumulation de deux métaux lourds, le cuivre et le chrome, par deux espèces de lentilles d’eau Lemna minor et Lemna gibba. Bien que sous climat de type méditerranéen, L. gibba soit plus utilisée que L. minor, les données disponibles dans la littérature concernent plutôt L. minor. L’évaluation des paramètres toxicologiques montre chez L. gibba une tolérance nettement supérieure aux effets toxiques des deux métaux expérimentés. Le chrome est moins toxique que le cuivre et s’accumule à des taux supérieurs à ceux du cuivre dans les tissus des plantes. Pour les deux métaux, l’accumulation est plus importante chez L. minor. Cependant, on pense que L. gibba serait plus indiquée dans des applications en phytoremédiation que L. minor, vu sa tolérance et sa productivité en biomasse. Les potentialités de ces deux espèces pour des applications de ce type se justifient par des taux d’accumulation fort élevés, spécialement pour le chrome où ils dépassent largement les 1000 µg g^‑1 de poids sec. En effet, les concentrations des plantes en Cr obtenues dans ce travail atteignent 2140 µg g^‑1 chez L. minor et 1710 µg g^‑1 chez L. gibba. Ces performances montrent un potentiel fort intéressant en comparaison à d’autres macrophytes comme la jacinthe d’eau par exemple.

Mots clés : Lemna minor, Lemna gibba, chrome, cuivre, tolérance, accumulation, phytoremediation

Summary

Tolerance and accumulation of copper and chromium in two duckweed species: Lemna minor L. and Lemna gibba L.

Natural wastewater treatment technologies are common practice in many regions of the world. Although these technologies are normally meant for domestic wastewater treatment, they can have broader applications such as the treatment of water contaminated by heavy metals.

Among various existing technologies, systems that use duckweed are exploited for wastewater treatment in various regions of the world. In Morocco, this type of system has been tested on an experimental scale and a pilot scale. The most common duckweeds species are Lemna minor L. and Lemna gibba L. L. gibba is used in Mediterranean climates more often than L. minor, but literature data on the tolerance and accumulation of heavy metals are mostly available for L. minor.

In the current study, we compared the tolerance and accumulation of two heavy metals, copper (Cu) and chromium (Cr), for these two duckweed species. Thus, this study compares the potential of these two duckweed species to be used as a method of decontaminanting Cu‑ and Cr‑contaminated water. The experimental design used hydroponic crops of the two species. Forty fronds of each species were sown in plastic pots containing 100 mL of White nutritive solution with a pH adjusted to 6.8. As experimental treatments, we used exposures corresponding to the following concentrations: 0.5; 1; 2; 3; 4; 5 mg Cu/L and 3; 5; 10; 20; 30; 35 mg Cr/L. Each treatment was repeated five times. In order to compensate for water loss by evapotranspiration, we added 15 mL of the corresponding solution to each treatment daily.

Generally, the results obtained confirm that Cu is more toxic than Cr and that the two species of duckweed have different tolerance levels; L. minor is more sensitive than L. gibba for both metals. For Cu, we observed a highly significant inhibition of growth in response to the gradient of Cu concentrations used, the growth of L. minor being more affected than that of L. gibba. For example, the time required for L. minor colonies to double was more than four days for 1 mg Cu/L, whereas for L. gibba this was observed only at 3 mg Cu/L. The toxicological parameters reflect this observation as L. minor had both a lower NOEC (No Observed Effect Concentration) and a lower IC50 (50% Inhibition Concentration) than those reported for L. gibba. Comparatively, these results show a higher tolerance of Cu contamination for L. gibba.

For Cr, L. minor was more sensitive than L. gibba, but in much less marked way than for Cu. Indeed, the time required to double the colony size and the NOEC value were similar for the two species, whereas the IC50 of L. minor was lower than that of L. gibba. The comparison of the rate of inhibition confirms a slightly greater sensitivity of L. minor to Cr exposure.

The Cu and Cr concentrations in the biomass increased with the concentration of metal; the highest contents were observed in the treatments with the highest concentrations used: 5 mg Cu/L and 10 mg Cr/L. However, the BCF (Bioconcentration Factor) decreased with the concentration of the treatment; the highest values were observed for the treatments with the lowest concentrations (1 mg Cu/l and 3 mg Cr/L). With respect to possible applications in phytoremediation, the potential use of these two species would be more efficient for low levels of contamination.

The two species accumulate the two metals at different rates; the accumulation of Cr is greater than that of Cu and L. minor shows higher rates of accumulation. Copper concentrations of approximately 800 µg/g were obtained in L. minor exposed to 5 mg Cu/L. For Cr, concentrations were approximately 2140 µg/g obtained in L. minor exposed to 10 mg Cr/L. For L. gibba, the maximum concentrations obtained were 745 µg/g and 1710 µg/g respectively for the treatments of 5 mg Cu/L and 10 mg Cr/L. Similarly, the BCF estimated at L. minor was higher than that of L. gibba.

Compared with other macrophyte species, the duckweeds show a very interesting potential for metal accumulation. Indeed, for Cu, accumulation was definitely higher than that for other species such as Eichhornia crassipes and Polygonum hydropiperoides. The BCF obtained was higher than those observed for species of Typha and Spartina. These results confirm that L. minor and L. gibba could be good Cu accumulating species compared to other macrophytes. For Cr, other species of macrophytes accumulate this metal to a greater or lesser extent: E. crassipes (slower rate than that of the duckweeds), Nymphea alba (similar rate to that of the duckweeds) or Azolla pinnata (higher rate than the duckweeds). The Cr contents accumulated by the two duckweed species would justify their classification as hyperaccumulator species.

With respect to tolerance, accumulation potential and biomass productivity, L. gibba shows potential as a species that could be used in phytoremediation and in particular the rhizofiltration of wastewater contaminated by Cu and Cr.

Keywords: Lemna minor, Lemna gibba, chromium, copper, tolerance, accumulation, phytoremediation