Competition between cyanobacteria and green algae at low versus elevated CO(2): who will win, and why?

Traditionally, it has often been hypothesized that cyanobacteria are superior competitors at low CO(2) and high pH in comparison with eukaryotic algae, owing to their effective CO(2)-concentrating mechanism (CCM). However, recent work indicates that green algae can also have a sophisticated CCM tuned to low CO(2) levels. Conversely, cyanobacteria with the high-flux bicarbonate uptake system BicA appear well adapted to high inorganic carbon concentrations. To investigate these ideas we studied competition between three species of green algae and a bicA strain of the harmful cyanobacterium Microcystis aeruginosa at low (100 ppm) and high (2000 ppm) CO(2). Two of the green algae were competitively superior to the cyanobacterium at low CO(2), whereas the cyanobacterium increased its competitive ability with respect to the green algae at high CO(2). The experiments were supported by a resource competition model linking the population dynamics of the phytoplankton species with dynamic changes in carbon speciation, pH and light. Our results show (i) that competition between phytoplankton species at different CO(2) levels can be predicted from species traits in monoculture, (ii) that green algae can be strong competitors under CO(2)-depleted conditions, and (iii) that bloom-forming cyanobacteria with high-flux bicarbonate uptake systems will benefit from elevated CO(2) concentrations.