THE UTILIZATION OF BICARBONATE IONS BY THE MARINE MICROALGA NANNOCHLOROPSIS-OCULATA (DROOP) HIBBERD

HCO3- utilization by the marine microalga Nannochloropsis oculata was investigated using a pH drift technique in a closed system. Light-depe ndent alkalization of the medium resulted in a final pH of 10.5, confi rming substantial HCO3- use by this alga. Alkalinity remained constant throughout the pH drift, Measurement of dissolved inorganic carbon (D IC) or the uptake of (HCO3-)-C-14 showed that nearly 50% of the total DIC remained external to the plasma membrane on completion of a pH dri ft, The rate of light-driven alkalization was inhibited by 3-(3,4-dich lorophenyl)-1,1-dimethylurea (DCMU) and thus was dependent on photosyn thesis. Light-driven alkalization was not inhibited by a membrane-impe rmeable inhibitor of carbonic anhydrase (CA), dextran-bound sulphonami de (DBS), indicating that external CA was not involved in HCO3- utiliz ation. The anion-exchange inhibitor 4',4'-diisothiocyanostilbene-2,2-d isulphonic acid (DIDS) completely inhibited light-driven alkalization of the medium and (HCO3-)-C-14 uptake, providing unequivocal support f or a direct uptake of (HCO3-)-C-14. Chloride ions were essential for D IC-dependent photosynthetic oxygen evolution, suggesting that bicarbon ate transport occurs by MCO(3)(-)/Cl- exchange.