Growth, photosynthesis, and respiration of Chlorella sorokiniana after N-starvation. Interactions between light, CO2 and NH4+ supply

N-sufficient cells of Chlorella sorokiniana Shihira and Krauss, strain 211/ 8k, absorbed NH4+ under light plus CO2 conditions, when growth occurred, bu t not in darkness or in the absence of CO2, when growth was inhibited. N-su fficient cells subjected to conditions of N-starvation for a 24-h period sh owed a marked loss of photosynthetic activity. Upon supply of NH4+, N-starv ed cells sufflated with CO2 air exhibited a time-dependent recovery of phot osynthetic activity, both when suspended in light and in darkness. By contr ast, growth only occurred in cells suspended in light. N-starved cells abso rbed NH4+ in darkness, but at a lower rate than in light. All of these data suggest that dark NH4+ uptake is driven by N assimulation to recover from N-starvation and that the light-dependent NH4+ uptake is driven by growth, being then influenced by conditions that affect recovery or growth. Unlike CO2 conditions, in a CO2-free atmosphere, absorption of NH4+ by N-starved c ells occurred at a higher rate in darkness than in light. Accordingly, resu mption of photosynthetic potential after NH4+ supply occurred in darkened c ells, but not in illuminated cells. Respiratory activity of N-starved cells was enhanced up to 3-fold by NH4+ and 2-fold by methylammonium with differ ent patterns, suggesting that respiratory enzymes were affected by N-metabo lism, especially through short-term control mechanism triggered by the expe nditure of metabolic energy involved in N-metabolism.