Combined regulation of CO2 and nutrients on carbon fixation and emission of Microcystis aeruginosa: physiological and transcriptomic insights.

Microcystis aeruginosa (M. aeruginosa), a dominant bloom-forming cyanobacterium, performs efficient photosynthetic carbon fixation and in situ aerobic methane (CH4) production simultaneously, playing a significant role in regulating the carbon source-sink in inland waters. However, how CO2 scenarios and nutrient levels interact to regulate the carbon fixation and emission of M. aeruginosa remains unclear. Here, we conducted a fully crossed experiment with three atmospheric CO2 concentrations (ambient 420 ppm, 650 ppm, and 1000 ppm) and three nutrient levels to explore growth, photosynthesis, and carbon metabolism responses in M. aeruginosa. Results showed that algal growth and carbon fixation exhibited nutrient-dependent positive responses to elevated atmospheric CO2. High-nutrient conditions amplified CO2 fertilization effects, whereas nutrient limitation eliminated such benefits and aggravated photoinhibition. Aerobic CH4 production peaked within meso-oligotrophic systems, and elevated atmospheric CO2 further enhanced CH4 emission efflux and dissolved CH4 concentration, weakening the net carbon sink efficiency. We demonstrated that nutrient availability acts as a critical threshold factor governing CO2-driven carbon partitioning and carbon sink contribution, and evaluating carbon sinks merely by carbon fixation flux will lead to overestimation. This study advanced the inland water carbon cycle theory and provided a scientific basis for greenhouse gas accounting, bloom control, and aquatic carbon sink enhancement under global change.