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 NO₃^-, NO₂⁺or NH₄⁺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 NO₂^-and NH₄⁺. In the absence of CO₂, NO₃^-uptake and reduction was light-saturated at a comparatively low photon flux, whereas NO₂^-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 NH₄⁺assimilation, as well as the existance of control mechanisms for NO₃^-uptake and reduction.Abbreviations Chl chlorophyll - PF photon flux