Cyanobacterial carbon concentrating mechanisms facilitate sustained CO₂ depletion in eutrophic lakes

Phytoplankton blooms are increasing in frequency, intensity, and duration in aquatic ecosystems worldwide. In many eutrophic lakes, these high levels of primary productivity correspond to periods of CO₂ depletion in surface waters. Cyanobacteria and other groups of phytoplankton have the ability to actively transport bicarbonate (HCO^-₃ / across their cell membrane when CO2 concentrations are limiting, possibly giving them a competitive advantage over algae not using carbon concentrating mechanisms (CCMs). To investigate whether CCMs can maintain phytoplankton bloom biomass under CO₂ depletion, we measured the δ¹³C signatures of dissolved inorganic carbon (δ¹³C_DIC/ and phytoplankton particulate organic carbon (δ¹³C_phyto/ in 16 mesotrophic to hypereutrophic lakes during the ice-free season of 2012. We used mass-balance relationships to determine the dominant inorganic carbon species used by phytoplankton under CO₂ stress. We found a significant positive relationship between phytoplankton biomass and phytoplankton δ¹³C signatures as well as a significant nonlinear negative relationship between water column φCO₂ and isotopic composition of phytoplankton, indicating a shift from diffusive uptake to active uptake by phytoplankton of CO₂ or HCO^-₃ during blooms. Calculated photosynthetic fractionation factors indicated that this shift occurs specifically when surface water CO₂ drops below atmospheric equilibrium. Our results indicate that active HCO^-₃ uptake via CCMs may be an important mechanism in maintaining phytoplankton blooms when CO₂ is depleted. Further increases in anthropogenic pressure, eutrophication, and cyanobacteria blooms are therefore expected to contribute to increased bicarbonate uptake to sustain primary production.