Growth at Low Temperature Mimics High-Light Acclimation in Chlorella vulgaris

Structural and functional alterations to the photosynthetic apparatus after growth at low temperature (5deg]C) were investigated in the green alga Chlorella vulgaris Beijer. Cells grown at 5deg]C had a 2-fold higher ratio of chlorophyll a/b, 5-fold lower chlorophyll content, and an increased xanthophyll content compared to cells grown at 27deg]C even though growth irradiance was kept constant at 150 mu]mol m-2 s-1. Concomitant with the increase in the chlorophyll a/b ratio was a lower abundance of light-harvesting polypeptides in 5deg]C-grown cells as observed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and confirmed by western blotting.The differences in pigment composition were found to be alleviated within 12 h of transferring 5deg]C-grown cells to 27deg]C. Furthermore, exposure of 5deg]C-grown cells to a 30-fold lower growth irradiance (5 mu]mol m-2 s-1) resulted in pigment content and composition similar to that in cells grown at 27deg]C and 150 mu]mol m-2 s-1. Although both cell types exhibited similar measuring-temperature effects on CO2-saturated O2 evolution, 5deg]C-grown cells exhibited light-saturated rates of O2 evolution that were 2.8-and 3.9-fold higher than 27deg]C-grown cells measured at 27deg]C and 5deg]C, respectively. Steady-state chlorophyll a fluorescence indicated that the yield of photosystem II electron transport of 5deg]C-grown cells was less temperature sensitive than that of 27deg]C-grown cells. This appears to be due to an increased capacity to keep the primary, stable quinone electron acceptor of photosystem II (QA) oxidized at low temperature in 5deg]C- compared with 27deg]C-grown cells regardless of irradiance. We conclude that Chlorella acclimated to low temperature adjusts its photosynthetic apparatus in response to the excitation pressure on photosystem II and not to the absolute external irradiance. We suggest that the redox state of QA may act as a signal for this photosynthetic acclimation to low temperature in Chlorella.