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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 CaC03 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 02, (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 0, evolution were significantly higher for both strains at acidic pH (e.g. 5.0, containing about 90% COz) than at seawater pH (e.g. 8.0, having about 1 % CO,), while at a basic pH (e.g. 9.0, virtually no CO2 and about 50% HC03-) 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 HC03users. By comparing seawater COz concentrations (i.e. 15 FM) with calculated KO,(CO2), the CO2 concentration required for a halfmaximal rate of photosynthetic O2 evolution, for Pleurochrysis (51 and 37 pM 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 HC03utilization and concomitant calcification, particularly in HC cells.

Photosynthesis and inorganic carbon utilization in Pleurochrysis sp. (Haptophyta), a coccolithophorid alga