EFFECT OF PHYTOPLANKTON CELL GEOMETRY ON CARBON ISOTOPIC FRACTIONATION

The carbon isotopic compositions of the marine diatom Porosira glacial is and the marine cyanobacterium Synechococcus sp. were measured over a series of growth rates (mu) in a continuous culture system in which the concentration and carbon isotopic composition of CO2(aq) were dete rmined. These data were compared with previously published isotopic re sults of growth rate experiments using the marine diatom Phaeodactylum tricornutum and the marine haptophyte Emiliania huxleyi. Systematic r elationships were found to exist between mu/[CO2(aq)] and carbon isoto pic fractionation (epsilon(p)) for each species. Maximum isotopic frac tionation (epsilon(f)) for P. glacialis, E. huxleyi, and P. tricornutu m was similar to 25 parts per thousand, suggesting that this value may be typical for maximum fractionation associated with Rubisco and beta -carboxylases for marine eukaryotic algae. By contrast, epsilon(f) det ermined for Synechococcus clone CCMP838 was similar to 7 parts per tho usand lower. The slopes of the lines describing the relationship betwe en epsilon(p) and mu/[CO2(aq)] for eukaryotic algal species were diffe rent 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 cel ls. Comparison of chemostat experimental results with calculated resul ts using a diffusion based model imply that the algae in the experimen ts were actively transporting inorganic carbon across the cell membran e. Our results suggest that accurate estimates of paleo-[CO2(aq)] from epsilon(p) measured in sediments will require knowledge of growth rat e as well as cell surface area and either cell carbon quota or cell vo lume. Given growth rate estimates, our empirical relationship permits reliable calculations of paleo-[CO2(aq)] using compound-specific isoto pic analyses of C-37 alkadienones (select haptophytes) or fossilized f rustules (diatoms). Copyright (C) 1998 Elsevier Science Ltd.