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.