Effects of elevated atmospheric carbon dioxide to Microcystis aeruginosa under different forms of phosphorus sources.

Human activities have led to an increase in atmospheric carbon dioxide (CO2) concentration, which can enhance the flux of CO2 from air to water, thus impacting algal growth. Phosphorus (P) is a key factor influencing the formation of cyanobacteria blooms. Nutrient utilization is closely related to carbon (C) metabolism, but the effects of elevated CO2 on microalgae under different P sources are rarely studied. In this study, we investigated the growth and physiological and biochemical responses of Microcystis aeruginosa (M. aeruginosa) under ambient (400 ppm) and elevated (550 ppm) CO2 levels in P-free, dissolved inorganic P (DIP, 1 mg P/L), and dissolved organic P (DOP, 1 mg P/L) groups. The bioavailability of DIP to M. aeruginosa was greater than that of DOP, and elevated CO2 increased both the uptake of DIP and DOP. Elevated CO2 promoted the growth (increasing by 9.0%-14.2%), photosynthesis, and CO2 fixation of M. aeruginosa under different P sources (P-free, DIP, DOP), and increased total microcystin-LR content (increasing by 5.4%-12.6%), which increased the risk of microcystin-LR release into the environment. Furthermore, elevated CO2 aggravated the stress effect of DOP, leading to an increase in protein content and proportion of humic acid substances in the extracellular polymeric substances. Our study provides a theoretical basis for understanding the impact of elevated CO2 on cyanobacteria bloom under different P sources, and provides a new insight for the control of eutrophic waters under the background of climate change.