ACTIVATION AND DEACTIVATION OF RIBULOSE-1,5-BISPHOSPHATE CARBOXYLASE OXYGENASE (RUBISCO) IN 3 MARINE MICROALGAE/

The activity of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubis co) was examined in three marine microalgae: the chlorophyte Dunaliell a tertiolecta and the chromophytes Pavlova lutheri and Thalassiosira p seudonana. The three species differed in the sensitivity of Rubisco ac tivity in crude extracts to magnesium ion concentration, the presence of protease inhibitors, the duration of the incubation on activity, an d the potential for full activation of Rubisco with 20 mM magnesium ch loride and 20 mM bicarbonate in vitro. D. tertiolecta had responses th at were similar to those described in vascular plants: regulation of i nitial activity on a gradient of irradiances; maximum initial activiti es that were 80-90% of light-saturated photosynthesis; total activitie s that exceeded light-saturated photosynthesis by 30-100%; and deactiv ation of Rubisco in darkness. Both initial and total activity declined in darkness and increased on a return to growth irradiance. First-ord er time constants were about 9 min for deactivation and 3 min for reac tivation of initial activity. The decline in total activity after a tr ansition into darkness could not be reversed in vitro but could be rev ersed by exposing D. tertiolecta to light, a characteristic of regulat ion by CA1P. The responses of T pseudonana were qualitatively similar, except that recovery of initial activity was low and could only accou nt for 30-40% of light-saturated photosynthesis. Rubisco from T pseudo nana exposed to low irradiance could be activated in vitro but at grow th irradiance and higher, total activity was lower than initial activi ty. The time constants for deactivation and reactivation of initial ac tivity after reciprocal switches between growth irradiance and darknes s were 12-18 min and 3 min in T pseudonana. T! lutheri showed no regul ation of Rubisco activity in response to changes in irradiance or ligh t-dark transitions. This may have been an artifact of the conditions c hosen to measure activity.