ASSIMILATION OF INORGANIC NITROGEN BY PHOTOSYNTHESIZING CHLORELLA CELLS

The relationships between nitrate reduction and photosynthesis in the unicellular green alga Chlorella sp. K were studied with respect to th e role of respiration at the energy and substrate level. The light dep endency of nitrate reduction was confirmed quantitatively by the stoic hiometry of this process, which corresponds to the equation describing the direct photochemical reduction of nitrate in the absence of CO2, as well as by extra O2 evolution in the presence of CO2. Up to 20% of the energy stored in photosynthesis was shown to be contributed to nit rate reduction. Unlike nitrate reduction, ammonium assimilation was le ss affected by light and utilized both stored endogenous products and newly synthesized products of photosynthesis. The rate of ammonium ass imilation correlated with the size of the carbohydrate pools in the ce ll. By contrast to normal cells, nitrogen-deficient cells used the red ucing equivalents for nitrate and nitrite reduction in light from dark respiration to a greater extent than they used the reductants generat ed by photosynthesis.