The relationship between carbon isotope fractionation and carbon concentrating mechanism activity in marine phytoplankton
The ocean is the largest reservoir of carbon on the Earth's surface and hence ocean processes involving carbon are of great importance in light of the growing concern over global warming due to an increased greenhouse effect. Modem phytoplankton assemblages appear to record the CO₂ concentration in seawater through their carbon stable isotope content. Phytoplankton preserved in marine sediments could contain a record of atmospheric CO₂ concentrations dating back as much as 200 million years.
... ever, before using a relationship between CO₂ concentration and the stable isotopic content of phytoplankton, the mechanism(s) behind this apparent relationship must be examined further. Research on carbon isotope fractionation in marine phytoplankton has typically assumed that cells rely on CO₂ diffusion to supply photosynthetic carbon demand. The results of this thesis show that active carbon uptake has a strong influence on carbon isotope fractionation in six species of marine phytoplankton. A more detailed study of the marine diatom Thalassiosira pseudonana demonstrated that carbon concentrating mechanism (CCM) induction occurred over an ecologically relevant range of CO₂ concentrations (ca. 3 to 16.5 μM), and that fractionation decreased simultaneously with increased CCM induction. CCM induction in T. pseudonana appeared to be regulated by CO₂ concentration rather than DIC concentration. Based on the results of experiments and modeling, the CCM appears to involve extracellular carbonic anhydrase catalyzed CO₂ formation from bicarbonate and subsequent uptake of CO₂. A better understanding of factors that influence fractionation, including the involvement of CCM activity and species variability, must be gained before the isotopic composition of sedimentary organic matter can be used as a proxy for past atmospheric CO₂ concentrations.