| Summary: | xiv, 222 leaves The ability of the ocean to buffer the concentration of CO2 in the atmosphere through the so-called biological pump depends on the extent to which the photosynthetic rate of marine phytoplankton is limited by the concentration of CO2 in the water. If CO2 becomes available to phytoplankton by passive diffusion through the boundary layer around the cell, then the growth of large cells, which are believed to contribute disproportionately to the biological pump, could be limited by CO2 availability. However, many species appear to have the ability to circumvent diffusion control through the use of carbon-concentrating mechanisms (CCMs) such as active CO2 uptake, bicarbonate (HCO3-) transport, and carbonic anhydrase activity. These mechanisms are likely adaptations to the fact that the main carbon fixing enzyme, ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco), is less than half saturated at normal seawater CO2 concentrations. Using short-term 14CO2-disequilibrium experiments, a clone of the marine diatom Phaeodactylum tricornutum was shown to take up little or no HCO3- even under conditions of severe CO2 limitation. These results agree with predictions based on stable carbon isotopic fractionation data and demonstrate that combining isotopic disequilibrium experiments with continuous growth cultures and stable isotope fractionation experiments is a powerful tool for understanding the response of oceanic primary producers to anthropogenic CO2 emissions as well as for interpreting paleoceanographic carbon isotope data. Isotopic disequilibrium experiments were also performed in the field to estimate the extent of photosynthetic bicarbonate (HCO3-) uptake in the oceans. The experiments were conducted in the Southern Ocean during the Southern Ocean Iron Experiment (SOFeX). In contrast to the results with P. tricornutum, approximately half of the photosynthetic inorganic carbon uptake was direct HCO3- uptake, the other half being direct CO2 uptake (passive and/or active uptake). A low-CO2 ...
|
|---|