Activation and deactivation of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) in three marine microalgae

The activity of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) was examined in three marine microalgae: the chlorophyte Dunaliella tertiolecta and the chromophytes Pavlova lutheri and Thalassiosira pseudonana. The three species differed in the sensitivity of Rubisco activity in crude extracts to magnesium ion concentration, the presence of protease inhibitors, the duration of the incubation on activity, and the potential for full activation of Rubisco with 20 mM magnesium chloride and 20 mM bicarbonate in vitro. D. tertiolecta had responses that were similar to those described in vascular plants: regulation of initial activity on a gradient of irradiances; maximum initial activities that were 80-90% of light-saturated photosynthesis; total activities that exceeded light-saturated photosynthesis by 30-100%; and deactivation of Rubisco in darkness. Both initial and total activity declined in darkness and increased on a return to growth irradiance. First-order time constants were about 9 min for deactivation and 3 min for reactivation of initial activity. The decline in total activity after a transition into darkness could not be reversed in vitro but could be reversed by exposing D. tertiolecta to light, a characteristic of regulation by CA1P. The responses of T pseudonana were qualitatively similar, except that recovery of initial activity was low and could only account for 30-40% of light-saturated photosynthesis. Rubisco from T pseudonana exposed to low irradiance could be activated in vitro but at growth irradiance and higher, total activity was lower than initial activity. The time constants for deactivation and reactivation of initial activity after reciprocal switches between growth irradiance and darkness were 12-18 min and 3 min in T pseudonana. T! lutheri showed no regulation of Rubisco activity in response to changes in irradiance or light-dark transitions. This may have been an artifact of the conditions chosen to measure activity.