The utilization of bicarbonate ions by the marine microalga Nannochloropsis oculata (Droop) Hibberd

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Abstract
HCO3− utilization by the marine microalga Nannochloropsis oculata was investigated using a pH drift technique in a closed system. Light‐dependent alkalization of the medium resulted in a final pH of 10.5, confirming substantial HCO3− use by this alga. Alkalinity remained constant throughout the pH drift. Measurement of dissolved inorganic carbon (DIC) or the uptake of H14CO3− showed that nearly 50% of the total DIC remained external to the plasma membrane on completion of a pH drift. The rate of light‐driven alkalization was inhibited by 3‐(3,4‐dichlorophenyl)‐1,1‐dimethylurea (DCMU) and thus was dependent on photosynthesis. Light‐driven alkalization was not inhibited by a membrane‐impermeable inhibitor of carbonic anhydrase (CA), dcxtran‐bound sulphonamide (DBS), indicating that external CA was not involved in HCO3− utilization. The anion‐cxchangc inhibitor 4′,4′‐diisothiocyanostilbene‐2,2‐disulphonic acid (DIDS) completely inhibited light‐driven alkalization of the medium and H14CO3− uptake, providing unequivocal support for a direct uptake of H14CO3−. Chloride ions were essential for DIC‐dependent photosynthetic oxygen evolution, suggesting that bicarbonate transport occurs by HCO3−/CI− exchange.