Extracellular polymeric substances enhanced photosynthesis over respiration in Microcystis aeruginosa

Abstract

Extracellular polymeric substances (EPS) play a critical role in Microcystis-dominated freshwater cyanobacterial blooms. However, the mechanisms through which EPS affects Microcystis photosynthesis, respiration, and further affects its growth are not understood completely. To address this, we investigated the effects of varying EPS concentrations on the physiological processes of Microcystis aeruginosa. The results demonstrated that increasing EPS concentrations significantly enhanced both cell density and energy fixation efficiency, accompanied by a reduction in CO₂ emission flux. Specifically, compared with the control group, the addition of 20 mg·L^-¹ EPS increased respiratory rates by 2.14 μmol·mg·h^-¹ and photosynthetic rates by 2.48 μmol·mg·h^-¹, suggesting that EPS stimulated both respiration and photosynthesis, with a more pronounced effect on photosynthesis, thereby leading to a substantial increase in algal growth. Further analysis indicated that EPS enhanced respiration by retaining hydrolases capable of breaking down macromolecules into bioavailable micromolecular substrates, which elevated acetyl-CoA concentrations and citrate synthase activity, thus improving respiratory efficiency. In terms of photosynthesis, EPS enhanced light utilization, as indicated by an increase in F_V/F_M, and improved the efficiency of inorganic carbon supply by enriching CO₂ and creating extracellular inorganic carbon gradients. Moreover, EPS enhanced the activities of carbonic anhydrase and ribulose bisphosphate carboxylase/oxygenase. These findings emphasize the essential role of EPS in promoting algal growth and its potential impact on CO₂ fixation. Future research should incorporate the role of EPS in reducing carbon limitation into discussions of algal growth mechanisms and develop technologies that use algal blooms to harvest high-value carbon products such as ethanol, astaxanthin, lipids, and other valuable compounds.