Physiological, ecological, and evolutionary aspects of inorganic carbon acquisition in marine phytoplankton

This thesis examines the physiological mechanisms of inorganic carbon acquisition in marine phytoplankton assemblages, and the effects of atmospheric CO₂ variations on oceanic primary production. A comparative analysis of the kinetic properties of RubisCO—the central C fixing enzyme in all autotrophs—indicates that significant taxonomic variability exists among phytoplankton in the CO₂ affinity of this enzyme. The biochemical diversity of RubisCO reflects an evolutionary adaptation of photosynthetic organisms to decreasing CO₂/O₂ ratios over geological time. Differences in the catalytic efficiency of RubisCO could, in principle, significantly influence the resource costs associated with C acquisition in phytoplankton and influence their ecological interactions.; To overcome the catalytic inefficiency of RubisCO, phytoplankton employ cellular C concentrating mechanisms which have been well characterized in several model species. The results of a multi-year field study demonstrated that coastal marine phytoplankton assemblages also employ carbon concentrating mechanisms based on the active uptake of HCO₃− and/or CO₂. In most phytoplankton populations, HCO₃ − appears to be the primary source of inorganic C for photosynthesis, but significant taxonomic, and habitat-specific variability exists in the physiological mechanisms of HCO₃− uptake. Incubation experiments conducted for 3–5 days showed that inorganic C acquisition in diatom-dominated phytoplankton assemblages is tightly regulated by ambient CO₂ concentrations. This CO₂-dependent regulation affects cellular C transport systems, the expression of carbonic anhydrase and RubisCO, and the photosynthetic fractionation of stable C isotopes. In contrast, the steady-state growth rates, and primary productivity of diatom assemblages were generally unaffected by CO₂ manipulations from 150 to 750 ppm (i.e. ∼40% to 220% of present day atmospheric levels).; Despite the lack of CO₂-dependent growth rate or productivity responses in three to five day experiments, longer-term CO₂ effects on the species composition of an Equatorial Pacific phytoplankton assemblage were observed. In particular, the relative biomass of diatoms and prymnesiophytes (Phaeocycstis) was sensitive to ambient CO₂ concentrations. This result suggests that CO₂ may influence competition among marine phytoplankton. Such ecological effects have important implications for future oceanic C cycles and the interpretation of glacial-interglacial variations of production and nutrient cycling.