MEPS 137:243-250 (1996)  -  doi:10.3354/meps137243

We studied and compared properties of inorganic carbon fixation for 2 coccolithophorid strains that differ in their capacity to calcify; namely high calcifying (HC) Pleurochrysis sp. CCMP 299 and low calcifying (LC) Pleurochrysis elongata CCAP 961/3. Pleurochrysis species are unicellular algae that, in nature and in culture, produce intracellular CaCO₃ encrusted structures (coccoliths). Pleurochrysis, and other calcifying algae, are potential players in atmospheric CO₂ cycling and the maintenance of global carbon balances. In Pleurochrysis sp. and P. elongata (hereafter Pleurochrysis), photosynthesis was affected by increasing O₂ (from 1 to 21% in air), with 18% inhibition for LC cells and 9% for HC cells. The inhibition could be reversed by (1) decreasing the ambient O₂, (2) reducing the ambient pH (which rose in the medium, particularly for LC cells) and (3) by increasing the ambient inorganic carbon concentration. Carbonic anhydrase activity was detected in Pleurochrysis; HC cells having approximately 4 times more activity than LC cells. Inhibition of carbonic anhydrase by 0.25 mM acetazolamide (a non-membrane-permeating inhibitor of the enzyme) averaged 30% in HC cells and only 10% in LC cells. Calcium uptake measured for HC cells was 2.5 to 3.0 times higher in the light and 4 times higher in the dark than calcium uptake measured for LC cells. Rates of photosynthetic O₂ evolution were significantly higher for both strains at acidic pH (e.g. 5.0, containing about 90% CO₂) than at seawater pH (e.g. 8.0, having about 1% CO₂), while at a basic pH (e.g. 9.0, virtually no CO₂ and about 50% HCO₃^-) rates were still substantial for HC cells but extremely low for LC cells. These data indicate that HC cells in their natural environment are primarily HCO₃^- users. By comparing seawater CO₂ concentrations (i.e. 15 uM) with calculated K_0.5(CO₂), the CO₂ concentration required for a half-maximal rate of photosynthetic O₂ evolution, for Pleurochrysis (51 and 37 uM for HC and LC cells, respectively) it follows that CO₂ must be concentrated intracellularly for effective photosynthesis in both strains. Thus, an adequate CO₂ supply depends on HCO₃^- utilization and concomitant calcification, particularly in HC cells.

Calcium uptake . Carbon utilization . Coccolithophorid . Photosynthesis . Rubisco