The physiological significance of light and dark NH4+ metabolism in Chlorella sorokiniana

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 NH4/+ in light at a linear rate, and absorption was 100% inhibited by darkness. N-limited cells, by contrast, absorbed NH4/+ 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 NH4+ absorption by N-sufficient cells. NH4/+ in darkness did not affect respiration of N-sufficient cells. However, NH4/+ 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. NH4/+ supply also resulted in a resumption of growth, in light but not in darkness. It is proposed that the use of NH4/+ 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 NH4/+ uptake.