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Summary • Temperature effects on growth, photosynthesis, respiration and nitrate reductase (NR) were studied in the cryophilic algae Koliella antarctica and ‘Chlorella’saccharophila, and in the mesophilic Chlorella sorokiniana. • Growth rate was measured as increase in optical density. Photosynthesis at saturating light and respiration in darkness were measured as O2 exchange. NADH : NR was assayed in crude extracts. • The two cryophilic algae grew below 15°C, and C. sorokiniana above 20°C. Photosynthetic and respiration rates of K. antarctica and ‘C.’ saccharophila were elevated at 5°C, and peaked at 30°C. Arrhenius plots from 5 to 25°C were linear in K. antarctica, whereas in ‘C.’ saccharophila and C. sorokiniana they exhibited breaks at 15 and 20°C, respectively. Values for activation energy (Ea) and the factor by which the rate increases with raising the temperature 10°C (Q10) differed. Nitrate reductase had its optimum at 25°C in cryophilic algae and at 35°C in C. sorokiniana. • We conclude that growth of cryophilic algae at low temperature is favoured by elevated photosynthesis and respiration rates, but that it could be limited by a high respiration : photosynthesis ratio.

In N-sufficient cells of Chlorella sorokiniana Shihira and Krauss strain 211/8K (CCAP of Cambridge University), assimilation of ammonium was strictly dependent on light and CO 2 , and was severely inhibited by 100 μM atrazine or 10 μM 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU). In N-limited cells, assimilation of NH 4 + took place at similar rates in both light and darkness, which were 1.6-fold higher than the rate 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 possible origin of carbon skeletons for ammonium assimilation in Cyanidium caldarium (Tilden) Geitler was investigated. N-sufficient cells assimilated ammonium at a rate of 182 ± 18 μmol·mL packed cell volume (pcv)-1· h-1. Removal of CO2 or darkening almost immediately prevented ammonium assimilation. N-limited cells in light assimilated ammonium at a rate of 493 ± 45 μmol · mL pcv-1· h-1 in the presence of CO2 and at a lower rate of 168 ± 17 μmol · mL pcv-1· h-1 in the absence of CO2. In darkness they assimilated ammonium at a rate of 293 ± 29 μmol · mL pcv-1 h-1 in the presence of CO2, only 60% of the assimilation rate in light. In the absence of CO2, ammonium was assimilated at a similar rate of 325 ± 14 μmol · mL pcv-1· h-1. Under the latter conditions, however, assimilation was inhibited after 40 min and ceased after 70 min; it resumed upon resupply of CO2. We suggest that N-sufficient cells of C. caldarium obtain carbon skeletons for ammonium assimilation exclusively by photosynthetic reactions. Upon N-limitation they develop the ability, apparently through derepression or activation of regulatory enzyme system(s), to obtain a consistent quantity of additional carbon skeletons and ATP from mobilization of carbon reserves. This enables the N-limited cell to assimilate ammonium not only in light but also in darkness, and at a higher rate than N-sufficient cells. The fact that ammonium assimilation in light occurs at a higher rate than in darkness suggests that ammonium assimilation in light is the sum of both light and dark ammonium assimilation, which implies separate metabolic reactions for the two processes. These results suggest the existence of two distinct and differently controlled pathways in N-limited cells, but not in N-sufficient cells, through which carbon skeletons for ammonium assimilation originate. An important role for dark CO2 fixation in dark or light ammonium assimilation is also indicated.

Cells of Chlorella sorokiniana Shihira & Krauss 211/8k, grown in a chemostat under conditions of nitrate limitation, assimilated ammonium in light (307 ± 12 μmol · mL packed cell volume (pcv)−1· h−1) and darkness (305 ± 30 μmol · mL pcv−1· h−1) at similar rates when supplied with air containing 5% CO2. Ammonium assimilation in cell suspensions flushed with pure nitrogen to establish anaerobic conditions was almost totally prevented both in darkness and in light. Cell suspensions flushed with CO2−-free air and grown in the dark assimilated ammonium at a rate of 314 ± 23 μmol · mL pcv−1· h−1. Light assimilation was 128 ± 10 μmol · mL pcv−1· h−1, a rate which was only 41% of that in darkness. Resupply of ammonium to N-limited chemostat cells resulted in a 3.5-fold stimulation of dark respiration and a suppression of photosynthetic O2 evolution. We hypothesize that the partial decline of ammonium assimilation in Chlorella 211/8k in the light under conditions of limited CO2 supply is due to the fact that the photosynthetic carboxylation reaction competes with the dark carboxylation reaction for the available CO2, and that the production of C4 and C5 precursors foramina acid biosynthesis will be slower than in the dark. Alternatively we suggest that an increase in the concentration of photogenerated compound(s) not used for CO2 assimilation attains an intracellular level suitable to trigger a control mechanism operating at the level of starch break-down or glycolysis.

Cyanidium caldarium (Tilden) Geitler, a non-vacuolate unicellular alga, resuspended in medium flushed with air enriched with 5% CO2, assimilated NH4+ at high rates both in the light and in the dark. The assimilation of NO3−, by contrast, was inhibited by 63% in the dark. In cell suspensions flushed with CO2-free air, NH4+ assimilation decreased with time both in the light and in the dark and ceased almost completely after 90 min. The addition of CO2 completely restored the capacity of the alga to assimilate NH4+. NO3− assimilation, by contrast, was 33% higher in the absence of CO2 and was linear with time. It is suggested that NO3− and NH4+ metabolism in C. caldarium are differently controlled in response to the light and carbon conditions of the cell.

In the unicellular alga Cyanidium caldarium nitrate utilization is strongly inhibited by ammonium and it is resumed when ammonium has been depleted. In the presence of L‐methionine‐DL‐sulphoximine (MSX), which prevents ammonium assimilation through a specific irreversible inhibition of glutamine synthetase, nitrate reduction is no longer inhibited by ammonium, and most of the ammonium derived from nitrate reduction is excreted into the external medium. However, in the presence of MSX, nitrate reduction to ammonium proceeds at a reduced rate (45 to 70% of the control); this is particularly marked at low nitrate concentration. It is hypothesized that either MSX or accumulating ammonium bring about decrease in the rate of nitrate entry into the cell. Copyright © 1982, Wiley Blackwell. All rights reserved