Elevated CO₂-mediated climate warming favors protozoan's top-down effect on controlling toxic Microcystis

Under temperature and CO₂ level rising, the dominance of toxic cyanobacteria in primary producers is continuously increasing the risks of water safety and hindering functions of aquatic ecosystems. Thus, it is necessary to evaluate the efficiency of algal control measures under climate warming. Based on highly efficient control of cyanobacteria by protozoan reported in previous studies, this study aimed to investigate top-down effect of protozoan Paramecium on toxic Microcystis under CO₂-mediated climate warming. Results showed that Microcystis removal by Paramecium was mainly affected by Microcystis growth under climate warming based on path analysis. Growth rate and ingestion rate of Paramecium increased with Microcystis density, especially Paramecium growth was further promoted by >20 % under high-density Microcystis at elevated CO₂ and high temperature, however, Microcystis exhibited the contrary response, which indicated that there was a stronger sensitivity of Paramecium growth to increasing Microcystis relative to Microcystis itself under simulated conditions of "climate warming" such as elevated CO₂ and high temperature, thereby helping Paramecium control Microcystis. Furthermore, reduction ratio of Microcystis and degradation ratio of microcystins were about 100 % by Paramecium at the end of experiment. The time to Microcystis extinction and the time to microcystins degradation by Paramecium were reduced by about 10 % at high CO₂ and high temperature, and decrease rate of the ratio of Microcystis and Paramecium was enhanced by at least 25 % relative to that under current temperature, which further demonstrated that enhanced top-down effect of Paramecium on Microcystis. Consequently, these findings demonstrated that climate warming and enhanced cyanobacterial growth by elevated CO₂ and high temperature did not exacerbate the challenge for protozoans removing algae but promoted their top-down effect. Overall, this study provides new insights in protozoan-cyanobacteria interactions and strongly supports a practical application using protozoan in cyanobacteria-contaminated lake management under actual-future global warming.