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

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.

The role of extracellular carbonic anhydrase activity in inorganic carbon utilization of Phaeocystis globosa (Prymnesiophyceae): A comparison with other marine algae using the isotopic disequilibrium technique

“…Several lines of evidence indicate that the C i species crossing the membrane in the eustigmatophyte Nannochloropsis is HCO 3 Ϫ (Merrett et al 1996;Sukenik et al 1997). This species does not have extracellular CA (Sukenik et al 1997).…”

Dissolved inorganic carbon utilization and the development of extracellular carbonic anhydrase by the marine diatom Phaeodactylum tricornutum

“…Under conditions when CO,^ is rate-limiting for photosynthesis, marine phytoplankton species able to use HCO3~ might have a competitive advantage over those species unable to use HCO3", Two mechanisms of HCO3" utilization have been demonstrated in niarine phytoplankton. Some species employ direct HCO,," utilization, which depends on the transport of the bicarbonate ion across the plasma membrane into the cytosol (Colman & Gehl, 1983;Rees, 1984;Dixon et al, 1987;Nimer & Merrett, 1992;Colman & Rotatore, 1995;Merrett, Nimer & Dong, 1996), The other mechanism is the indirect utilization of HCO3 , in which CO,^ is produced by the dehydration of HCO3^ in the unstirred layer outside the plasma membrane, a process facilitated by extracellular CA (Tsuzuki, 1983;Tsuzuki & Miyachi, 1989), The inhibition of extracellular CA by DBS (Haglund et al, 1992;Nimer, Merrett & Brownlee, 1996) demonstrates the importance of this enzyme in the production of COj in the unstirred layer (Fig, 3), The proportion of photosynthetic '^CO,, fixation inhibited by DBS shows the contribution made by the catalytic production of CO.j in the unstirred layer to the total flux of CO,, into the cell and subsequent photosynthetic fixation.…”

Extra and intracelular activities of carbonic anhydrase of the marine microalga Tetraselmis gracilis (Chlorophyta)

“…Internally, CA may be located in the cytoplasm, mitochondria and mainly in the chloroplast (1). In the cytoplasm, the internal enzyme acts converting CO2 into HCO3 -to prevent the leaking of CO2 from the cell (21,22). In the chloroplast HCO3 -is converted to CO2 by another CA, concentrating CO2 around RUBISCO, helping to overcome the low affinity that this enzyme has for CO2 (5) and thus, being part of the Carbon Concentrating Mechanism that is found mainly in microalgae and cyanobacteria (7,16,22).…”