Temperature responses of growth, photosynthesis, respiration and NADH: nitrate reductase in cryophilic and mesophilic algae.

•  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 O 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 (E ) and the factor by which the rate increases with raising the temperature 10°C (Q ) 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.