Experimental evidence for the selection of salinity-tolerant ecotypes of the Microcystis aeruginosa complex

Microcystis aeruginosa complex (MAC) generate harmful blooms in multiple aquatic ecosystems, from freshwater to estuaries. The intra-complex genetic diversity and its phenotypic plasticity emerge as the main hypothesis of ecological success of the group in eutrophic ecosystems. This intra-complex diversity is composed of ecotypes, phylogenetically closely related organisms differing in their ecological preferences that could be considered species. Under the hypothesis of a rapid selection of ecotypes caused by salinity, here, we experimentally evaluated the role of salinity (0, 5, 10 and 25 ppt) in the ecotype community configuration and toxicity of MAC. We assessed the response to different salinities by analyzing ecotype composition (mcyJ based genotyping) combined with mcyJ amplicon sequencing for ecotypes identification, MC-producing cells abundance and their toxin-production activity (mcy gene copies and transcripts abundance, and MC variants concentration), and machine learning methods for data analysis. We found that, although the abundance of MC-producing cells, the mcyE transcription and MC concentration were negatively affected by increasing salinity, the treatments at 10 and 25 ppt selected MAC ecotypes showing different profiles of MC variants. The results indicate that when freshwater MAC species are transported to brackish waters, the salt-induced stress (salinities between 5 and 10 ppt) selects salt-tolerant ecotypes, generating a shift in the MC variants produced. At higher salinities (25 ppt), the selected ecotypes maintain their abundance but MC production decreases. The existence of MC-producing ecotypes adapted to different salinity conditions is relevant in determining the success of MAC organisms in a wide range of ecosystems and poses a risk to estuarine health, justifying further research.