Effect of Temperature on Inorganic Carbon Acquisition ofChlamydomonas reinhardtii

Abstract

Carbon dioxide availability for microalgae in aquatic environments increases with decreasing water temperature, while photosynthetic activity generally decreases. Therefore, inorganic carbon acquisition by algal cells is greatly affected by temperature. We investigated half-saturation constants [Krn(DIC), Km(C02)] of inorganic carbon in photosynthesis under various temperatures for a strain of Chlamydomonas reinhadtii. C. reinhardtii showed an active carbon concentrating mechanism (CCM) at all temperature conditions investigated (5 -25'C), implying that CCM activity is not diminished at low temperatures. The maximum photosynthetic rate was recorded at 15'C, while maximum CCM activity was detected at 2022. A higher optimum temperature for CCM activity than for photosynthesis may compensate for lower photosynthetic rates above the optimum temperature. CCM may play a more significant role at higher temperatures in algal photosynthesis in aquatic environments. INTRODUCTIONMany microalgae, including cyanobacteria, possess a carbon-concentrating mechanism (CCM) to elevate C02 concentrations around the COrfixing enzyme of ribulose-1, 5-bisphosphate carboxylase/oxygenase (RUBISCO), which has a relatively low affinity for C02 in ambient air (Kaplan and Reinhold 1999, Giordano et al. 2005). The CCM is generally activated in cells cultured under the ambient C02 level (0.035%) as compared to those grown under high C02 conditions (2-5%). Since the CCM expression requires extra energy (Beardall 1991), CCM activity possibly decreases under severe conditions, such as under low light or low temperature. Indeed, Beardall (1991) demonstrated that CCM in the cyanobacterium Anabaena variabilis is suppressed when cells were cultured under low amounts of light. In contrast, information on CCM activity in microalgae under low water temperature is relatively limited.Under low temperature, C02 availability for microalgae tends to increase due to higher solubility. On the other hand, photosynthetic activity is generally low at lower temperatures. Therefore, the need for the active transport of DIC to maintain the supply of C02 to the RUBISCO active site may decrease under low temperatures. Indeed, the carbon isotope ratio, which indicates carbon fractionation in photosynthesis, is lower in polar microalgae, suggesting lower CCM activity in the polar region (Rau et al. 1989). In contrast, an increase in water temperature leads a decrease in C02 solubility and increase in photosynthetic activity until reaching an optimum temperature. Near the optimum temperature for photosynthesis, CCM activity may strongly affect the photosynthetic rate.