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N-limited cells of Chlorella sorokiniana Shihira and Krauss 211/8k exhibited rates of respiratory oxygen consumption and photosynthetic oxygen evolution which were 70 and 17%, respectively, of those of N-sufficient cells. N-sufficient cells absorbed NH 4 + in light at a linear rate, and absorption was 100% inhibited by darkness. N-limited cells, by contrast, absorbed NH 4 + at almost similar rates in light and darkness. The rate of uptake in darkness decreased after 20 min, and decreased further after 60 min. In the light, by contrast, it decreased to a constant higher value, which appeared to approximate the sum of the rate of the dark absorption plus the rate of the light-dependent NH 4 + absorption by N-sufficient cells. NH 4 + in darkness did not affect respiration of N-sufficient cells. However, NH 4 + supply to N-limited cells in darkness resulted in a sudden increase of respiratory oxygen consumption, suggesting that short-term control mechanisms were triggered; it also resulted in time-dependent increase in photosynthetic activity and chlorophyll content, suggesting long term control of the photosynthetic apparatus. NH 4 + supply also resulted in a resumption of growth, in light but not in darkness. It is proposed that the use of NH 4 + in darkness by N-limited cells serves to recover from N-limitation, whereas in light it serves to sustain both recovery and growth. This could also explain why cells which are in an N-sufficient status and need nitrogen for growth only, exhibited exclusively a light-dependent NH 4 + uptake.

Nitrate and ammonium utilization in cell suspension of the acidophilic thermophilic non-vacuolate alga Cyanidium caldarium and the related pH variations in the external medium, were measured using ion specific electrodes. Nitrate absorption and pH variations were also monitored. The acidophilic C. caldarium allowed experimentation under acidic conditions where CO 2 exchange did not affect pH measurements. Both ammonium and nitrate assimilation occured in dark and light conditions; however, the rate of assimilation was higher in the light than in the dark. The presence of oxygen was an absolute requirement. It is suggested that the uptake of both nitrate and ammonium are mediated by active mechanisms dependent on the energy charge of the cell. The assimilation of nitrate and ammonium was accompanied by an increase and a decrease, respectively, of the external medium pH. Under steady-state conditions, for each equivalent of ammonium incorporated there were 0.8–1 equiv. of protons released into the external medium and for each equivalent of nitrate absorbed there were 1–1.2 equiv. of protons absorbed from the external medium.

Assimilatory nitrate reductase and glutamate dehydrogenase of the red alga Porphyridium aerugineum have been studied. Nitrate reductase exhibits a pH optimum at approx. 7.75; it is functional with reduced benzyl viologen, FMN; FAD, NADH and is not functional with NADPH. In crude extracts of cells grown on nitrate as the sole nitrogen source, a partially inactive form of nitrate reductase occurs. This inactive enzyme can be activated by heating at 50°C. Glutamate dehydrogenase of P. aerugineum is strictly dependent on NADPH.