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 (HCO₃⁻) 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 <i>δ</i>¹³C signatures of dissolved
inorganic carbon (<i>δ</i>¹³C_DIC) and phytoplankton
particulate organic carbon (<i>δ</i>¹³C_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
<i>δ</i>¹³C signatures as well as a significant nonlinear negative
relationship between water column <i>ρ</i>CO₂ and isotopic composition of
phytoplankton, indicating a shift from diffusive uptake to active uptake by
phytoplankton of CO₂ or HCO₃⁻ 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 HCO₃⁻ 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.