How rising CO2 may stimulate harmful cyanobacterial blooms

Climate change is likely to stimulate the development of harmful cyanobacterial blooms in eutrophic waters, with negative consequences for water quality of many lakes, reservoirs and brackish ecosystems across the globe. In addition to effects of temperature and eutrophication, recent research has shed new light on the possible implications of rising atmospheric CO2 concentrations. Depletion of dissolved CO2 by dense cyanobacterial blooms creates a concentration gradient across the air-water interface. A steeper gradient at elevated atmospheric CO2 concentrations will lead to a greater influx of CO2, which can be intercepted by surfacedwelling blooms, thus intensifying cyanobacterial blooms in eutrophic waters. Bloom-forming cyanobacteria display an unexpected diversity in CO2 responses, because different strains combine their uptake systems for CO2 and bicarbonate in different ways. The genetic composition of cyanobacterial blooms may therefore shift. In particular, strains with lowaffinity uptake systems may benefit from the anticipated rise in inorganic carbon availability. Furthermore, cyanobacteria differ from eukaryotic algae in that they can fix dinitrogen, and new insights show that the nitrogen-fixation activities of some, but not all, diazotrophic cyanobacteria are strongly stimulated at elevated CO2 levels. However, models and lake studies indicate that the response of cyanobacterial growth to rising CO2 concentrations can be suppressed by nutrient limitation. Hence, the greatest response of cyanobacterial blooms to climate change is expected to occur in eutrophic and hypertrophic lakes.