Increase of PCO, during blooms of Emiliania huxleyi: Theoretical considerations on the asymmetry between acquisition of HCO,- and respiration of free CO,

The principal source of dissolved inorganic carbon (C,) of most marine phytoplankton is thought to be free CO,, but Emiliania huxleyi can utilize HCO,ion both for calcification (C’) and photosynthesis (P). We present a simple model that simulates continuous diurnal removal of C, as either free CO, or HCO,across the range of C, typically encountered in seawater (2,100-1,800 pmol kg,,-‘). Removal as free CO, forces a drawdown in partial pressure of dissolved carbon dioxide (Pco,), at given C,, of up to 170 patm greater than that forced by removal as HCO,by E. huxleyi (assuming C,,,, : P,,, = 1.0). Diurnal C,,, : P,,, must exceed 1.5 for Pco, to increase during removal of HCO,-. When nocturnal respiration of free CO, (Z?) is incorporated into the model, the respective drawdowns in Pco, forced by diurnal removal of free CO, and HCO; are balanced by diel ratios R : P,,,, = 1 .O and R : (P+ C),, = 0.15. Therefore, with diel R : (P+C),,, > 0.15 but diurnal Cnct : P,,, = 1 .O, E. huxleyi can theoretically grow with net diel increase in Pco,; however, diel C,,, : P,,, > 1 .O is forced by nocturnal loss of particulate organic carbon but not particulate inorganic carbon. Published values for C : P do not necessarily represent diel C,,, : P,,, and caution is required when extrapolating these ratios to predict changes in C, and Pco, during natural blooms of E. huxleyi. Our understanding of the biological contribution to exchange of CO, between atmosphere and ocean is principally based on empirical observations and models of the removal of free CO, from seawater. Blooms of the coccolithophorid Emiliania huxleyi have aroused considerable interest because of their potential impact upon this exchange. Several recent studies have demonstrated that the partial pressure of dissolved CO, (PcoJ in blooms of E. huxleyi is substantially higher than that encountered in blooms of similar biomass of other phytoplankton (Holligan et al. 1993; Robertson et al. 1994; Purdie and Finch 1994). Several mechanisms have been identified that could help explain this: a ratio of calcification to photosynthesis (C: P) of greater than unity could result in “leakage” of CO, from the cell (e.g. Paasche and Brubak 1994); trapping of solar radiation by cells and detached liths could cause localized influence of temperature on Pco, (e.g. Holligan et al. 1993); removal of HCO,could cause equilibration effects on the dissolved CO, system (Holligan et al. 1993; Purdie and Finch 1994; Robertson et al. 1994). However, it has not yet been possible to determine the relative contribution of each mechanism under natural conditions nor is it clear if the relatively high levels of Pco, observed within blooms represent a real increase in Pco, during growth of E. huxleyi or simply result from a reduced drawdown compared to “organic” phytoplankton. Purdie and Finch (1994) have in fact presented evidence for a transitory increase in Pco, (but below atmospheric equilibrium) during the growth phase of a bloom, but supplementary data were not available to confirm the mechanism responsible. Coccolithophorids such as E. huxleyi are members of the ’ Present address: Department of Oceanography, University of British Columbia, Vancouver, B.C. V6T 124. Acknowledgments This study was funded under the EC MAST II program (contract number MAS2-CT92-0038). This is EHUX contribution 45. prymnesiophyte phytoplankton and produce external liths of calcium carbonate in the form of calcite (CaCO,). In highly calcifying cells of E. huxleyi (Klaveness and Paasche 1971), HCO,seems to be the substrate for calcification (Paasche 1964; Sikes et al. 1980; Nimer and Merrett 1992) that can be schematically represented as follows: 2HCO,+ Ca2+ w H,O + CaCO, + CO,. This reaction results in intracellular production of C02, which appears to be tightly coupled (i.e. ratio of C: P = 1.0) to photosynthetic assimilation within the cell (Paasche 1964; Sikes et al. 1980; Dong et al. 1993). Despite some evidence of C : P > 1 under limiting phosphorus and irradiance (Fernandez et al. 1993; Paasche and Brubak 1994; Van Bleijswijk et al. 1994b), there is no evidence of a resulting increase in Pco,, although few such studies of Pco, have been made. A balance appears to be maintained between production of H+ during calcification and production of OHduring photosynthesis (Sikes et al. 1980; Nimer and Merrett 1993). Thus, there is no requirement for an exogenous source or sink of OHor H+ to balance intracellular production of H+ or OH-, respectively, and therefore no evidence to suggest any influence on extracellular pH or total alkalinity other than through the carbonate system itself. Low or noncalcifying strains of E. huxleyi have also been described (Klaveness and Paasche 1971), although in the context of open-water blooms it is the calcifying cells that seem to be significant. In contrast to high calcifying cells, these naked strains depend on diffusive entry of free CO, for photosynthesis and growth and do not seem to utilize HCO,(Nimer et al. 1992; Dong et al. 1993). Here, we attempt to distinguish the relative effects of two modes of inorganic carbon removal on the dissolved CO, equilibrium, with particular reference to Pco,. We use the well-established concepts of the carbonate equilibrium in a simple model to predict the relative effects on Pco, levels of removing either free CO, or HCO, ion from solution at