Dynamics of toxic genotypes of Microcystis aeruginosa complex (MAC) through a wide freshwater to marine environmental gradient.

Bloom-forming species belonging to Microcystis aeruginosa complex (MAC) are the most commonly reported worldwide. MAC blooms are composed by toxic and non-toxic genotypes and the environmental conditions favouring the dominance of toxic genotypes are still a matter of debate among the scientific community. In this study, we evaluated the distribution of toxic MAC genotypes along a seasonal cycle and over an environmental gradient spanning 800km, from a eutrophic freshwater reservoir in Río Uruguay to marine water in the outer limit of Río de la Plata. Abundance of four mcy genes, mcyB, mcyD, mcyE and mcyJ was determined by qPCR and used as a proxy of abundance of toxic MAC genotypes. All the mcy genes were detected through the seasonal cycle at all sampling sites, being systematically higher in the freshwater reservoir and decreasing towards the marine site. The highest toxic genotype abundance was found during the austral summer months. According to generalized linear regressions and random forest models, temperature and conductivity were the most relevant explanatory variables. This suggests that although toxic MAC genotypes grow optimally in freshwater, they are also able to tolerate the high-salinity and low temperature conditions found in estuarine and marine waters. This ability to resist harsh conditions impose a health risk and a management challenge. To our knowledge, this is the first report addressing several mcy genes in a broad gradient that includes a wide array of different environmental conditions.