Abstract
The overall biochemical composition, the adenylate energy charge and the metabolic activity of the diatom Melosira italica subsp. subarctica were studied during its spring development in Lake Pavin. The phytoplankton community of this lake was largely dominated by this diatom, which formed 80–95% of the phytoplankton biomass in the meta- and hypolimnion during the study period. Sedimentation of living Melosira was observed; 40–60% of the cells reaching the hypolimnion were viable. The spatial and temporal changes of the biochemical composition of this community suggest that Melosira utilized reserve products during sedimentation, essentially carbohydrates and secondarily also lipids. The increase of the protein/carbohydrate ratio with depth and a stable adenylate energy charge level clearly showed the prevalence of catabolic over anabolic reactions for the cells leaving the euphotic zone. Assays of inorganic carbon incorporation into macromolecules suggest that the settling Melosira adapt to the changing environmental conditions by increasing the synthesis of rapidly catabolisable polymers (polysaccharides and lipids) and by decreasing syntheses of proteins and incorporation into low mol. wt compounds.
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