Seasonal variations of phytoplankton photosynthate partitioning in two lakes of different trophic level

The aim of this study was to compare vertical and seasonal variations of inorganic carbon allocation into macromolecules by the phytoplankton population in a eutrophic lake (Lake Aydat) and an oligo-mesotrophic lake (Lake Pavin). Biochemical fractionation was conducted by consecutive differential extractions in order to separate proteins, polysaccharides, lipids, and low molecular weight compounds (LMW). The ratio of light absorption at 480 and 665 nm by acetone extracts of phytoplankton pigments was used as an indicator of the nutritional status of natural phytoplankton populations. Our results show that in Lake Aydat, the main photosynthetic end products were polysaccharides, whereas in Lake Pavin, radioactivity was predominantly incorporated into the protein fraction. Moreover, the seasonal cycles of mixing and stratification in these two lakes affected the pattern of C-14 incorporation into LMW and macromolecules. An increase in the relative synthesis of proteins occurred during stratification periods. It was linked to an increase in temperature and nutrient limitation further complicated by the shift in species composition of the populations. Differences recorded both between the two lakes of different trophic status and between seasons confirm that the proportion of carbon incorporated into proteins might be a useful indicator of the physiological status of phytoplankton communities.