Cyanobacterial carbon concentrating mechanisms facilitate sustained CO2depletion in eutrophic lakes

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<p><strong>Abstract.</strong> 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 CO<sub>2</sub> depletion in surface waters. Cyanobacteria and other groups of phytoplankton have the ability to actively transport bicarbonate (HCO<sub>3</sub><sup>−</sup>) across their cell membrane when CO<sub>2</sub> 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 CO<sub>2</sub> depletion, we measured the <i>δ</i><sup>13</sup>C signatures of dissolved inorganic carbon (<i>δ</i><sup>13</sup>C<sub>DIC</sub>) and phytoplankton particulate organic carbon (<i>δ</i><sup>13</sup>C<sub>phyto</sub>) 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 CO<sub>2</sub> stress. We found a significant positive relationship between phytoplankton biomass and phytoplankton <i>δ</i><sup>13</sup>C signatures as well as a significant nonlinear negative relationship between water column <i>ρ</i>CO<sub>2</sub> and isotopic composition of phytoplankton, indicating a shift from diffusive uptake to active uptake by phytoplankton of CO<sub>2</sub> or HCO<sub>3</sub><sup>−</sup> during blooms. Calculated photosynthetic fractionation factors indicated that this shift occurs specifically when surface water CO<sub>2</sub> drops below atmospheric equilibrium. Our results indicate that active HCO<sub>3</sub><sup>−</sup> uptake via CCMs may be an important mechanism in maintaining phytoplankton blooms when CO<sub>2</sub> is depleted. Further increases in anthropogenic pressure, eutrophication, and cyanobacteria blooms are therefore expected to contribute to increased bicarbonate uptake to sustain primary production.</p>




Morales-Williams, A. M., Wanamaker, A. D., & Downing, J. A. (2017). Cyanobacterial carbon concentrating mechanisms facilitate sustained CO2depletion in eutrophic lakes. Biogeosciences, 14(11), 2865–2875.

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