EFFECT OF INHIBITORS ON AMMONIUM ASSIMILATION IN CHLORELLA SOROKINIANA IN LIGHT AND DARKNESS

In N-sufficient cells of Chlorella sorokiniana Shihira and Krauss stra in 211/8K (CCAP of Cambridge University), assimilation of ammonium was strictly dependent on light and CO2, and was severely inhibited by 10 0 mu M atrazine or 10 mu M 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DC MU). In N-limited cells, assimilation of NH4+ took place at similar ra tes in both light and darkness, which were 1.6-fold higher than the ra te of light-dependent assimilation by N-sufficient cells. Assimilation by N-limited cells was inhibited by L-methionine-DL-sulfoximine (MSX) , but not by atrazine or DCMU. The rate of photosynthetic O-2 evolutio n was 2.9+/-0.9 mmol ml-(1) packed cell volume (PCV) h-(1) in N-suffic ient cells, and 0.64+/-0.12 mmol ml-(1) PCV h-(1) in N-limited cells. In the latter resupply of ammonium resulted in a rapid activation by 2 2%, followed by a time-dependent increase of the photosynthetic O-2 ev olution, which after 12 h reached the same rate as in N-sufficient cel ls. Respiratory consumption of oxygen in darkness in N-sufficient and N-limited cells was 0.10+/-0.03 and 0.11+/-0.02 mmol ml-(1) PCV h-(1), respectively. Addition of ammonium was without effect on respiration of N-sufficient cells, but resulted in a 4-fold stimulation of respira tion of N-limited cells. Such stimulation took place also in cells tre ated with DCMU, atrazine, or MSX, and it was also promoted by methylam monium. The stimulation of respiration lasted for several hours.