Abstract. Photosynthesis by many marine phytoplankton algae is saturated by the inorganic C concentration in air-equilibrated sea water. These organisms appear to use an active inorganic C transport process (CO2-concentrating mechanism) which increases the CO2 concentration around rubisco and saturates this enzyme with CO2 and suppresses its oxygenase activity. A minority of marine phytoplankton algae have photosynthetic characteristics more suggestive of diffusive CO2 entry; the inorganic C concentration present in sea water does not saturate photosynthesis by these organisms. Theoretical considerations, tested when possible against observation, suggest that the organisms with a CO2-concentrating mechanism could have a lower cost of photons, nitrogen, iron, manganese and molybdenum to achieve a given rate of carbon accumulation by the cells than is the case for the organisms with diffusive CO2 entry. Zinc and selenium costs may show the reverse effect. The increased sea-surface inorganic C, and CO2 concentrations which will result from anthropogenic increases in atmospheric CO2 content are predicted to increase the rate of photosynthesis, and of growth when other resources are abundant, and to reduce, or reverse, the higher resource (photons, nitrogen, iron, manganese and molybdenum) cost of a given rate of CO2 assimilation in organisms with CO2 diffusion relative to those which have CO2 concentrating mechanisms and do not repress them at higher inorganic C concentrations. These effects may well alter species composition, and overall resource cost of growth, of phytoplankton; any influence that these effects may have on CO2 removal from the atmosphere are severely constrained by other trophic levels and, especially, oceanic circulation patterns. Changed sea-surface temperatures are unlikely to qualitatively alter these conclusions.