Toxic and non-toxic strains of Microcystis aeruginosa induce temperature dependent allelopathy toward growth and photosynthesis of Chlorella vulgaris.

Global warming was believed to accelerate the expansion of cyanobacterial blooms. However, the impact of changes due to the allelopathic effects of cyanobacterial blooms with or without algal toxin production on the ecophysiology of its coexisting phytoplankton species arising from global warming were unknown until recently. In this study, the allelopathic effects of toxic and non-toxic Microcystis aeruginosa strains on the growth of green alga Chlorella vulgaris and photosynthesis of the co-cultivations of C. vulgaris and toxic M. aeruginosa FACHB-905 or non-toxic M. aeruginosa FACHB-469 were investigated at different temperatures. The growth of C. vulgaris, co-cultured with the toxic or non-toxic M. aeruginosa strains, was promoted at 20°C but inhibited at temperatures ≥25°C. The inhibitory effects of the toxic and non-toxic M. aeruginosa strains on of the co-cultivations (C. vulgaris and non-toxic M. aeruginosa FACHB-469 or toxic M. aeruginosa FACHB-905) also linearly increased with elevated temperatures. Furthermore, toxic M. aeruginosa FACHB-905 induced more inhibition toward growth of C. vulgaris or Pmax and Rd of the mixtures than non-toxic M. aeruginosa FACHB-469. C. vulgaris dominated over non-toxic M. aeruginosa FACHB-469 but toxic M. aeruginosa FACHB-905 overcame C. vulgaris when they were co-cultured in mesocosms in water temperatures from 20 to 25°C. The results indicate that allelopathic effects of M. aeruginosa strains on C. vulgaris are both temperature- and species-dependent: it was stimulative for C. vulgaris at low temperatures such as 20°C, but inhibitory at high temperatures (≥25°C); the toxic strain was determined to be more harmful to C. vulgaris than the non-toxic one. This suggests that global warming may aggravate the ecological risk of cyanobacteria blooms, especially those with toxic species as the main contributors.