Distribution of reducing power between photosynthetic carbon and nitrogen assimilation in Scenedesmus

Fixation of CO2 and N assimilation were studied in synchronous cultures of Scenedesmus obtusiusculus Chod. under saturating and limiting light. Within the photon-flux range studied, the cells maintained C to N assimilation ratios of 7-10 with either NO 3 (-) , NO 2 (+) or NH 4 (+) as the N source. Competitive interactions between C and N assimilation were pronounced under light limitation and were proportional to the oxidation status of the N source. Fixation of CO2 at saturating light was also slightly reduced by NO 2 (-) and NH 4 (+) . In the absence of CO2, NO 3 (-) uptake and reduction was light-saturated at a comparatively low photon flux, whereas NO 2 (-) uptake and reduction was considerably faster in the absence of CO2 than in its presence. The pools of reduced pyridine nucleotides (NADPH and NADH) were largely unaffected by the presence or absence of the different N sources. The regulatory influences of CO2 fixation on N assimilation are discussed in terms of coupling between the rates of CO2 fixation and NH 4 (+) assimilation, as well as the existance of control mechanisms for NO 3 (-) uptake and reduction.