Response of microalgae to elevated CO₂ and temperature: impact of climate change on freshwater ecosystems

To estimate the combined effects of elevated CO₂ and temperature on microalgae, three typical and worldwide freshwater species, the green alga Scenedesmus acuminatus, the diatom Cyclotella meneghiniana, and the cyanobacterium Microcystis aeruginosa, as well as mixes of these three species were continuously cultured in controlled environment chambers with CO₂ at 390 and 1000 ppm and temperatures of 20, 25, and 30 °C. CO₂ and temperature significantly affected the production of microalgae. The cell productivity increased under elevated CO₂ and temperature. Although the green alga dominated in the mixed culture within all CO₂ and temperature conditions, rising temperature and CO₂ intensified the competition of the cyanobacterium with other microalgae. CO₂ affected the extracellular polymeric substances (EPS) characteristics of the green alga and the cyanobacterium. Elevated CO₂ induced the generation of humic substances in the EPS fractions of the green alga, the cyanobacterium, and the mixed culture. The extracellular carbohydrates of the diatom and the extracellular proteins of the cyanobacterium increased with elevated CO₂ and temperature, while the extracellular carbohydrates and proteins of the green alga and the mixes increased under elevated CO₂ and temperature. There were synergistic effects of CO₂ and temperature on the productivity and the EPS of microalgae. Climate change related CO₂ and temperature increases will promote autochthonous organic carbon production in aquatic ecosystems and facilitate the proliferation of cyanobacteria, which potentially changes the carbon cycling and undermines the functioning of ecosystems.