Abstract
• 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.
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