Reversal in competitive dominance of a toxic versus nontoxic cyanobacterium in response to rising CO2

Background/Question/Methods

Climate change scenarios predict a doubling of the atmospheric CO₂ concentration by the end of this century. Yet, how rising CO₂ will affect the species composition of aquatic microbial communities is still largely an open question. Here, we develop a resource competition model to investigate competition for dissolved inorganic carbon in dense algal blooms. The model predicts how dynamic changes in carbon chemistry, pH and light conditions during bloom development feed back on competing phytoplankton species. We test the model predictions in chemostat experiments with monocultures and mixtures of a toxic and nontoxic strain of the freshwater cyanobacterium Microcystis aeruginosa.

Results/Conclusions

The toxic strain was able to reduce dissolved CO₂ to lower concentrations than the nontoxic strain, and became dominant in competition at low CO₂ levels. Conversely, the nontoxic strain could grow at lower light levels, and became dominant in competition at high CO₂ levels but low light availability. The model captured the observed reversal in competitive dominance, and was quantitatively in good agreement with the results of the competition experiments. To assess whether the toxin microcystin might play a role in this reversal of competitive dominance, we performed further competition experiments with the wildtype strain M. aeruginosa PCC 7806 and a mutant impaired in microcystin production. The toxic microcystin-producing wildtype had a strong selective advantage at low CO₂ levels but not at high CO₂ levels. Our results thus demonstrate both in theory and experiment that rising CO₂ levels can alter the community composition and toxicity of harmful algal blooms.