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-2 exchange. NADH : NR was assayed in crude extracts. The two cryophilic algae grew below 15degreesC, and C. sorokiniana above 20degreesC. Photosynthetic and respiration rates of K. antarctica and 'C.' saccharophila were elevated at 5degreesC, and peaked at 30degreesC. Arrhenius plots from 5 to 25degreesC were linear in K. antarctica, whereas in 'C.' saccharophila and C. sorokiniana they exhibited breaks at 15 and 20degreesC, respectively. Values for activation energy (E-a) and the factor by which the rate increases with raising the temperature 10degreesC (Q(10)) differed. Nitrate reductase had its optimum at 25degreesC in cryophilic algae and at 35degreesC 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. (C) New Phytologist (2004).