Composition en acides aminés libres et combinés et en vitamines de la microalgue Micractinium Pusillum Fres. issue d'un lagunage naturel Method for anticipating flood warnings for the Thoré river (Southern France)

Résumé

Dans ce travail est présentée la composition biochimique en acides aminés (AA) libres et combinés et en vitamines de Micractinium pusillum isolée à partir du lagunage naturel expérimental de Ouarzazate où cette microalgue prolifère pendant la période chaude. M. pusillum est produite en culture en vrac, sur milieu synthétique, dans des conditions de lumière et de température fixes. Cette étude révèle la richesse de M. pusillum en acides aminés combinés qui constituent environ 59% de sa matière sèche. Les acides aminés libres forment 3,4% de cette matière. Dix-sept acides aminés et quatre vitamines (D3, B2, B3, et B12) ont été déterminés chez cette microalgue.

Mots-clés:

• Acides aminés libres,
• acides aminés combinés,
• vitamines,
• microalgue,
• Microctinium pusillum Fres

Abstract

The biotechnological exploitation of microalgae requires more investigations in order to improve our knowledge of their biochemistry and to look for new potentially interesting species. Microalgae are used in various areas. Among them, wastewater treatment in wastewater stabilisation ponds and high-rate ponds is widely prevalent. Beside their purifying ability, their abundance in these ecosystems makes them interesting as a potential source of valuable products.

Micractinium pusillum Fres, a green alga (Chlorophyceae) that grows in these ecosystems, was isolated from the experimental wastewater stabilisation pond of Ouarzazate city (south of Morocco), where it becomes important during the hot period. In the present work, we investigated the dissolved free and combined amino acids and the vitamins produced by this microalgal strain cultured on a mineral medium. The biomass production of M. pusillum was conducted in 2 l cotton-stoppered flasks under continuous light (60 µE m^-2 s^-1) provided by neon tubes and at a temperature of 24 ± 2°C. The culture medium was sterilised by autoclaving before inoculation. Free and combined amino acids were respectively analysed by HPLC separation of fluorescent o -phthaldialdehyde (OPA) derivatives. Prior to derivatization, the cells were broken and the cell content was digested with HCl. The analysis of vitamins was also realised by HPLC, with UV - visible detection methods, after extraction by appropriate solvents.

The results reveal a high content of combined amino acids of approximately 59% (dry matter). Free amino acids constitute 3.4% (dry matter). Seventeen amino acids have been identified in this microalga (Table 1): aspartic acid, glutamic acid, arginine, methionine, phenylalanine, alanine and lysine were the principal components. The analysis of spectra indicated the presence of six categories of amino acids (AA) (Table 2): aliphatic AA, hydroxy AA, sulphur-containing AA, basic AA, aromatic AA and the diacid AA. Aliphatic AA (glycine, valine, leucine, and isoleucine) largely dominated the total AA pool, with more than 27% of total AA. Diacids AA (aspartic and glutamic acids), at approximately 25%, came in the second rank. Dibasics AA (15%) were relatively more abundant than aromatics AA (11%) and sulfur-containing AA (8%). It should be noted, however, that among the sulphur-containing AA, only methionine was detected by the method used in this work. Hydroxy AA are much less represented with only 4% of the total AA.

Four vitamins were detected in M. pusillum (Table 3): vitamins D3, B2, B3, and B12. The content of vitamin D (6970 µg g^-1) was highest, followed by vitamin B3 (2820 µg g^-1) and vitamin B12 (1860 µg g^-1). Vitamin B2 constituted 406 µg g^-1. According to the reported values of vitamin contents in several other microalgal species, as well as some conventional foods (Table 5), it appears that M. pusillum species contains relatively important amounts.

Keywords:

• Free amino acids,
• combined amino acids,
• vitamin,
• microalga,
• Microctinium pusillum Fres