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

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 the 17 species 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 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 could be other species, the high affinity in vivo for inorganic C 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 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 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