Mechanisms of inorganic-carbon acquisition in marine phytoplankton and their implications for the use of other resources

Abstract

Most of the marine phytoplankton species for which data are available are rate saturated for photosynthesis and probably for growth with inorganic C at normal seawater concentrations; 2 of the 17 species are not saturated. Photosynthesis in these two species can probably be explained by assuming that CO2 reaches the site of its reaction with RUBISCO (ribulose bisphosphate carboxylase-oxygenase) by passive diffusion. The kinetics of CO2 fixation by intact cells are explicable by RUBISCO kinetics typical of (eucaryotic) algae, and a CO2-saturated in vivo RUBISCO activity not more than twice the in vivo light- and inorganic-C-saturated rate of photosynthesis. For the other species, the high affinity in vivo for inorganic C (and several other attributes) could be explained by postulating active influx of inorganic C yielding a higher concentration of CO2 available to RUBISCO during steady state photosynthesis than in the medium. Although such a higher concentration of internal CO2 in cells with high affinity for inorganic C is found at low (subseawater) levels of external inorganic C, the situation is more equivocal at normal seawater concentrations. In theory, the occurrence of a CO2 concentrating mechanism rather than passive CO2 entry (with consequent glycolate synthesis and metabolism or excretion) could reduce the photon, N, Fe, Mn, and Mo costs of growth, but increase the Zn and Se costs. Thus far, data on costs are available only for photons and N; these data generally agree with the predicted lower costs for cells with high affinity for inorganic C. The ecological significance of these attributes is that most marine phytoplankters are not likely to have photosynthetic or growth rates reduced by the measured decreases in inorganic C in productive seawater, drawdown of inorganic C in productive seawater (or increase as atmospheric CO2 increases) might alter the competitive balance between cells with low and high affinity for inorganic C, and differences in the effectiveness of use of other resources between cells with high and low affinity could cause differences in the rate and extent of resource-limited growth for communities dominated by high-affinity or low-affinity cells.