Fluctuations in microcystin concentrations, potentially toxic Microcystis and genotype diversity in a cyanobacterial community from a tropical reservoir

In the study of cyanobacterial communities, molecular methods can be used to detect and quantify toxin-producing strains and assess genotype diversity. These are promising methods for estimating the risk of exposure to the cyanobacterial toxin microcystin (MC), assuming that the abundance of toxic genotypes is related to the MC concentration. This assumption has been tested in many studies, most of which were conducted in temperate and subtropical aquatic environments, and has not always been met. In this study, we collected cyanobacterial samples from a tropical reservoir and tested whether the MC concentration could be explained by variations in the number or ratio of toxin-producing genotypes or by the presence of certain Microcystis genotypes. The genetic diversity of cyanobacteria was determined by cpcBA sequencing, and the proportion of potential MC-producing genotypes was quantified by qPCR using mcyB as the target gene. Cyanobacteria dominated during the entire sampling period (88.4-99.8% of the total phytoplankton biomass), with Microcystis, Anabaena and Cylindrospermopsis occurring as the main genera. Three major Microcystis genotypes were recognized along with other rare or unique genotypes. MCs were detected in all samples (MC-LR, RR and YR), with most concentrations being less than 200 ng L−1. Microcystis was the most abundant group during all the study period and was the main MC-producing genus. Potentially toxic Microcystis cells were present in all samples, varying from 10% to 100% of the total Microcystis. MC concentrations were not correlated with the number or ratio of toxic cells of Microcystis determined by qPCR or with the predominance of certain genotypes. These results indicate the limitations of using molecular methods such as qPCR in MC risk assessment.