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Inorganic carbon acquisition by natural phytoplankton assemblages and marine diatoms Martin, Cheryl Lynne


Marine phytoplankton (microalgae and cyanobacteria) play a vital role in the global carbon cycle. It is therefore essential to understand the mechanisms involved in inorganic carbon (C) acquisition by these organisms. I examined C uptake strategies of natural phytoplankton assemblages in the Bering Sea and numerous individual marine diatom species in the laboratory. This study employs the isotope disequilibrium method to quantitatively determine the C uptake strategies of phytoplankton, and the degree to which they utilize carbon dioxide (CO₂) and/or bicarbonate (HCO₃) as their C source for photosynthesis. The first part of this study investigated C acquisition strategies of natural phytoplankton communities in the Bering Sea. HCO₃ seemed to be the predominant C source for all phytoplankton assemblages, and the results suggest that HCO₃ utilization occurred mainly through a direct transport system. Although species composition and ambient CO₂ concentration did not appear to influence direct HCO₃ transport, these parameters did appear to influence indirect HCO₃ utilization by the enzyme external carbonic anhydrase (eCA). Ship-board CO₂ manipulation incubations were performed and there was a statistically significant CO₂ effect on the pathways of C assimilation. The second part of this study examined C uptake strategies of several diatom species in the laboratory. To my knowledge, no studies have thoroughly investigated an array of marine diatom species in relation to their C acquisition strategies. The majority of the diatom species utilized HCO₃ as their key C source, and it appears that most of the HCO₃ utilization occurred through a direct transport process. The C acquisition strategies of the diatoms in this study appear to be independent of diatom size (i.e. surface area: volume ratio) and growth rate.

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