GROWTH AT LOW-TEMPERATURE MIMICS HIGH-LIGHT ACCLIMATION IN CHLORELLA-VULGARIS

Structural and functional alterations to the photosynthetic apparatus after growth at low temperature (5 degrees C) were investigated in the green alga Chlorella vulgaris Beijer. Cells grown at 5 degrees C had a 2-fold higher ratio of chlorophyll a/b, 5-fold lower chlorophyll con tent, and an increased xanthophyll content compared to cells grown at 27 degrees C even though growth irradiance was kept constant at 150 mu mol m(-2) s(-1). Concomitant with the increase in the chlorophyll alb ratio was a lower abundance of light-harvesting polypeptides in 5 deg rees C-grown cells as observed by sodium dodecyl sulfate-polyacrylamid e gel electrophoresis and confirmed by western blotting. The differenc es in pigment composition were found to be alleviated within 12 h of t ransferring 5 degrees C-grown cells to 27 degrees C. Furthermore, expo sure of 5 degrees C-grown cells to a 30-fold lower growth irradiance ( 5 mu mol m(-2) s(-1)) resulted in pigment content and composition simi lar to that in cells grown at 27 degrees C and 150 mu mol m(-2) s(-1). Although both cell types exhibited similar measuring-temperature effe cts on CO2-saturated O-2 evolution, 5 degrees C-grown cells exhibited light-saturated rates of O-2 evolution that were 2.8- and 3.9-fold hig her than 27 degrees C-grown cells measured at 27 degrees C and 5 degre es C, respectively. Steady-state chlorophyll a fluorescence indicated that the yield of photosystem II electron transport of 5 degrees C-gro wn cells was less temperature sensitive than that of 27 degrees C-grow n cells. This appears to be due to an increased capacity to keep the p rimary, stable quinone electron acceptor of photosystem II (QA) oxidiz ed at low temperature in 5 degrees C- compared with 27 degrees C-grown cells regardless of irradiance. We conclude that Chlorella acclimated to low temperature adjusts its photosynthetic apparatus in response t o the excitation pressure on photosystem II and not to the absolute ex ternal irradiance. We suggest that the redox state of Q(A) may act as a signal for this photosynthetic acclimation to low temperature in Chl orella.