Effect of Phytoplankton Cell Geometry on Carbon Isotopic Fractionation

The carbon isotopic compositions of the marine diatom Porosira glacialis and the marine cyanobacterium Synechococcus sp. were measured over a series of growth rates (μ) in a continuous culture system in which the concentration and carbon isotopic composition of CO ₂(aq) were determined. These data were compared with previously published isotopic results of growth rate experiments using the marine diatom Phaeodactylum tricornutum and the marine haptophyte Emiliania huxleyi. Systematic relationships were found to exist between μ/[CO ₂(aq)] and carbon isotopic fractionation (ɛ _P) for each species. Maximum isotopic fractionation (ɛ _f) for P. glacialis, E. huxleyi, and P. tricornutum was ∼25‰, suggesting that this value may be typical for maximum fractionation associated with Rubisco and β-carboxylases for marine eukaryotic algae. By contrast, ɛ _f determined for Synechococcus clone CCMP838 was ∼7‰ lower. The slopes of the lines describing the relationship between ɛ _P and μ/[CO ₂(aq)] for eukaryotic algal species were different by a factor of more than 20. This result can be accounted for by differences in the surface area and cellular carbon content of the cells. Comparison of chemostat experimental results with calculated results using a diffusion based model imply that the algae in the experiments were actively transporting inorganic carbon across the cell membrane. Our results suggest that accurate estimates of paleo-[CO ₂(aq)] from ɛ _P measured in sediments will require knowledge of growth rate as well as cell surface area and either cell carbon quota or cell volume. Given growth rate estimates, our empirical relationship permits reliable calculations of paleo-[CO ₂(aq)] using compound-specific isotopic analyses of C ₃₇ alkadienones (select haptophytes) or fossilized frustules (diatoms).