Abstract
Measurements of ribulose‐1,5‐bisphosphate carboxylase (RUBPCase) and phosphoenolpyruvate carboxylase (PEPCase) in several phytoplankton cultures and natural phytoplankton populations showed a positive correlation of photosynthesis with RUBPCase activity but not with PEPCase activity. Observed ratios of RUBPCase:PEPCase were high in log phase cultures when photosynthesis was maximal and low in stationary phase. There was no significant correlation between photosynthesis and the ratio of RUBPCase:PEPCase activity in natural populations. The rate of carbon assimilation by the intact algae was significantly greater than that by the isolated carboxylases. In both natural phytoplankton populations and laboratory cultures, the ratio of photosynthesis:carboxylase activity depended on the physiological state of the cells, being greatest when the photosynthetic rate was highest.
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