Carbon-concentrating mechanisms and beta-carboxylation: Their potential contribution to marine photosynthetic carbon isotope fractionation

The ability of the ocean to buffer the atmospheric CO₂ depends on the extent to which the photosynthetic rate of marine phytoplankton is limited by the CO₂ concentration in the water. If CO₂ is only available to phytoplankton by passive diffusion, then cell growth 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 (HCO ₃−) transport, and carbonic anhydrase activity.; 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.; Isotopic disequilibrium experiments were also performed in the Southern Ocean. While approximately half of the photosynthetic inorganic carbon uptake was direct HCO₃− uptake, the other half was direct CO₂ uptake. A low-CO₂ treatment induced an increase in uptake of CO₂ through extracellular carbonic anhydrase activity, which was at the expense of direct HCO₃− transport. Because of the presence of CCMs, biological productivity in the Southern Ocean is unlikely to be directly regulated by variations in atmospheric CO ₂. These results are consistent with stable isotope fractionation models.; A review and experimental study of the various factors that influence CCM activity revealed that, other than CCMs, several factors may also contribute to the isotopic signature of photosynthetic organic matter. Isotopic fractionation in P. tricornutum was found to be correlated to changes in Rubisco enzyme kinetics and to the molar organic carbon to nitrogen ratio (C/N). Contrary to the general scientific belief, the C/N proved to be dependent on the CO₂ concentration.; Carboxylases other than Rubisco, such as phosphoenolpyruvate carboxylase (PEPC), may also significantly contribute to photosynthetic stable carbon isotope fractionation. Changes in PEPC/Rubisco activity may explain some of the variations in stable isotope fractionation. The β-carboxylase activity in P. tricornutum increased with decreasing growth rates and increasing CO₂ concentrations.; Understanding the factors that influence the overall photosynthetic carbon isotope fractionation will be crucial to the use of isotopic analyses for paleo-CO₂ reconstruction.