Diel pattern of photosynthate partitioning in phytoplankton population in an oligomesotrophic lake

The aim of this study was to measure the short-term changes in inorganic ca rbon allocation into various macromolecular compounds (proteins, polysaccha rides and lipids) throughout a diurnal cycle in the oligomesotrophic Lake P avin (Massif Central of France) at the depths of 5, 15 and 30 m. Biochemica l fractionation was done by consecutive differential extractions in order t o separate proteins, poly saccharides, lipids and low molecular weight comp ounds (LMW) by virtue of their relative solubilities in different extractio n solvents. Over the entire diurnal cycle inorganic carbon was preferential ly incorporated into proteins (M = 30%), then into polysaccharides (M = 28% ), LMWs (M = 27%) and lipids (M = 15%). However, at 5 m, diurnal variations were reflected by the high percentage of the inorganic carbon incorporated into polysaccharides during periods of high light intensity and decreased at dawn and dusk. The reverse pattern was observed for the allocation of in organic carbon to proteins.