Mj. Merrett et al., THE UTILIZATION OF BICARBONATE IONS BY THE MARINE MICROALGA NANNOCHLOROPSIS-OCULATA (DROOP) HIBBERD, Plant, cell and environment, 19(4), 1996, pp. 478-484
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.