FATTY-ACIDS AS SPECIFIC ALGAL MARKERS IN A NATURAL LACUSTRIAN PHYTOPLANKTON

The fatty acid (FA) composition of planktonic particulate matter (size class 1-160 mu m) was established during a seasonal study carried out on an oligotrophic lake, Lac Pavin (France). The succession of phytop lanktonic populations with a monospecific trend permits the large diff erences in FA composition according to phytoplanktonic classes, or eve n according to genera for dominant algae to be pointed out. Thus, we h ave established that Chlorophyceae had a high content of 18:3(n-3), 20 :4(n-3) and 16:3(n-3) FA, whereas Dinophyceae could be identified by h igher 16:0, 18:4(n-3), 20:5(n-3) and 22:6(n-3) FA levels, and lower 18 :3(n-3) and C16 polyunsaturated FA levels. Diatoms could be characteri zed by high values of the FA ratio [14:0 + 16:1(n-7) + 16:2(n-4) + 16: 3(n-4) + 16: 4(n-1)/16:0]. In addition, a close relationship was noted , on one hand, between Aulacoseira italica biomass and a high 16:2(n-4 ), 16:3(n-4) and 16:4(n-1) FA level and, on the other hand, between As terionella formosa biomass and high 14:0 and 16:1(n-7) levels. So, the main phytoplanktonic classes or genera were characterized by some FA or ratios of FA, which could be used as specific natural biomarkers fo r studying the transfer of organic matter through the aquatic food web .