Rising temperature more strongly promotes low-abundance Paramecium to remove Microcystis and degrade microcystins

Driven by global warming and eutrophication, Microcystis blooms have posed a severe threat to freshwater ecosystems, especially their derived pollutants cause serious harm to aquatic organisms, thus it is urgent to develop an effective strategy to eliminate nuisance Microcystis. Some protozoa can efficiently graze on toxic Microcystis aeruginosa and degrade cyanotoxins, and play a vital role in regulating harmful cyanobacteria. In the process of protozoa feeding on harmful algae, both temperature and protozoa population density are critical factors that affect the consequences of harmful M. aeruginosa population dynamics. In this study, we first found that Paramecium multimicronucleatum has strong ability to feed on M. aeruginosa, and then studied the interactive effects between temperature and initial density of P. multimicronucleatum on controlling M. aeruginosa. Results showed that increasing temperature accelerated the elimination of M. aeruginosa by P. multimicronucleatum, e.g. the time for M. aeruginosa elimination at 32 °C was shortened to 3.5-4 days. The higher temperatures (26, 29, and 32 °C) were more conducive to improve the efficiency of controlling M. aeruginosa by P. multimicronucleatum with low initial density (10 inds mL^-1). Furthermore, P. multimicronucleatum can rapidly degrade microcystins, and the degradation ratio approximately 100% at 32 °C after 6 days. This is the first study to discover that P. multimicronucleatum can high efficiently graze on M. aeruginosa and has a much higher grazing rate (3.5-5.5 × 10⁴Microcystis Paramecium^-1 d^-1) than other protozoa. These findings contribute to the establishment of a new feasible method for the biological control of M. aeruginosa, and provide a theoretical guidance for the practical application of P. multimicronucleatum in the removal of M. aeruginosa.