Next to CO2 fixation, the photosynthetic reduction of NO3~ to NH4+ is quantitatively the most important reduction process in the algal cell. With both CO2 and NO3~ assimilation requiring photosyn-thetic activity, it might be expected that competition for reducing capacity would occur between these processes. This does not normally occur, however, since CO2 assimilation and nitrate assimilation have to be matched to each other. Nitrate assimilation can only progress to completion when CO2 assimilation provides the carbon skeletons for amino-acid formation, lack of which may lead to accumulation of toxic nitrites in the algal cell. As noted earlier, control of nitrate assimilation is closely coupled to processes that promote carbohydrate formation in algal cells - including the external presence of glucose (and other carbohydrates) and light.
Laboratory studies on cultures of the green alga Scenedesmus have confirmed that competition between these processes does not occur at moderate to high light intensities, but that it is important at low light intensities (Larsson et al., 1985). Competitive interactions between carbon and nitrogen assimilation were pronounced under these low light conditions, where the rate of non-cyclic electron transport becomes limiting for reductive purposes. These results may have important environmental implications where variations in light intensity occur and suggest that increasing depth in the water column, for example, will have major effects on the balance between these assimilation processes.
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