COMPARATIVE-STUDY OF DISSOLVED INORGANIC CARBON UTILIZATION AND PHOTOSYNTHETIC RESPONSES IN NANNOCHLORIS (CHLOROPHYCEAE) AND NANNOCHLOROPSIS (EUSTIGMATOPHYCEAE) SPECIES

Four species of marine microalgae with similar morphology and life cyc le, namely Nannochloris atomus Butcher, Nannochloris maculata Butcher, Nannochloropsis gaditana Lubian, and Nannochloropsis oculata (Droop) Hibberd, have been examined with respect to their affinity for differe nt sources of dissolved inorganic carbon. External carbonic anhydrase activity was not found in any of these species, but the cell affinity for dissolved inorganic carbon (DIC) in Nannochloris species was affec ted by the inhibitor acetazolamide at a concentration of 400 mu M. Mea surement of photosynthetic rates and CO2 compensation points at differ ent pH values showed that the Nannochloris species had a greater capac ity for CO2 rather than HCO3- utilization. In contrast, the observed r ates of photosynthetic oxygen evolution in Nannochloropsis species wer e greater than could be accounted for by the theoretical rate of CO2 s upply from the spontaneous dehydration of bicarbonate in the external medium. This indicates that these algae were able to transport bicarbo nate across the plasmalemma. Furthermore, the K-0.5 (DIC) value at aci dic PH showed that Nannochloropsis oculata could also use CO2 as an ex ogenous carbon source for photosynthesis. Although the species of mari ne phytoplankton used in this study possess similar morphological char acteristics and life cycle, there exist many differences in the mode o f inorganic carbon utilization between these microalgae.