Temporal variations in carbon isotope ratio of phytoplankton in a eutrophic lake

Temporal variations in carbon isotope ratio of phytoplankton and dissolved inorganic carbon (DIC) in Lake Suwa were reported. In summer, blooming of Microcystis spp. resulted in low concentrations of DIC and high pH, and HCO3− was the prominent species of DIC. Chlorophyll-specific rates of photosynthesis were relatively constant irrespective of the algal biomass during summer. Carboxylation in photosynthesis of Microcystis spp. was mainly catalyzed by ribulose bisphosphate carboxylase (RuBPCase). Carbon isotope discrimination between δ13C of phytoplankton and DIC was considerably small in early summer and appeared to be negatively correlated to DIC concentration. We concluded that carbon fixation by phytoplankton in Lake Suwa is controlled not by the switch of photosynthetic pathways, but by low DIC concentration and high pH, suggesting that photosynthesis of Microcystis spp. in Lake Suwa is governed by uptake kinetics other than the carboxylation step.