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, but not in darkness or in the absence of CO₂, when growth was inhibited. N-sufficient cells subjected to conditions of N-starvation for a 24-h period showed a marked loss of photosynthetic activity. Upon supply of NH₄⁺, N-starved cells sufflated with CO₂ air exhibited a time-dependent recovery of photosynthetic activity, both when suspended in light and in darkness. By contrast, growth only occurred in cells suspended in light. N-starved cells absorbed NH₄⁺ in darkness, but at a lower rate than in light. All of these data suggest that dark NH₄⁺ uptake is driven by N assimilation 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 cells occurred at a higher rate in darkness than in light. Accordingly, resumption of photosynthetic potential after NH₄⁺ supply occurred in darkened cells, 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 different patterns, suggesting that respiratory enzymes were affected by N-metabolism, especially through short-term control mechanisms triggered by the expenditure of metabolic energy involved in N-metabolism.