Carboxylating enzyme activity and photosynthetic end products of phytoplankton in the shallow and deep chlorophyll layers of Castle Lake1

Abstract

The patterns of carbon assimilation of the Castle Lake phytoplankton were measured under various environmental conditions. The flow of carbon into polysaccharides and lipids plus low molecular weight metabolites was generally highest at low temperatures and low light (i.e, under‐ice and deep‐water populations). Enrichment with nitrogen to nitrogen‐deficient algae did not significantly alter the patterns of carbon assimilation during time‐series experiments. However, the relative rates of polysaccharide and protein synthesis were increased when the entire epilimnion of the lake was fertilized with nitrogen.On the average about 15% more carbon could potentially be fixed by the anaplerotic enzyme phosphoenolpyruvate carboxylase (PEPCase) than the Calvin Cycle enzyme ribulose‐1,5‐biphosphate carboxylase (RuBPCase). The parameter, RuBPCase : PEPCase/photon saturated in vivo photosynthesis (Pmax), was consistently >1.0 liter ·h‒1·µg‒1 C and showed significant differences between the 3‐m and 20‐m algal populations, indicating that they may be physiologically distinct with respect to carbon metabolism.The proportion of 14C incorporated into protein increased as the ratio of RuBPCase to PEPCase increased. The converse was true for the relative synthesis of lipid + low molecular weight (LMW) compounds while the proportion of 14C incorporated into polysaccharide varied little with RuBPCase : PEPCase. These findings and others from the literature do not corroborate existing simple biochemical models for this relationship, implying that other carboxylating enzymes may be important in the carbon metabolism of certain algae.