First report of microcystin-producing benthic cyanobacteria in Mexican mountain rivers: polyphasic characterization and HPLC toxin detection

The detection and characterization of cyanotoxins produced by benthic cyanobacteria in rivers is an understudied topic compared to cyanotoxins produced by planktonic cyanobacteria. Particularly, detection methods for cyanotoxin-producing species in Latin America and Mexico continue to focus primarily on microscopy, with relatively scarce molecular detection. Herein, we present the first study in Mexican rivers that identifies a population of the genus Tychonema as a producer of two variants of microcystins, MC-LR and MC-RR, through a polyphasic approach that includes the quantification of toxins by HPLC-UV. Likewise, two populations of Nostoc and Microcoleus were identified and, based on the identification of the anaC and mcyE genes via PCR, may be producers of anatoxins and microcystins, respectively; however, this production could not be confirmed in the HPLC-UV analysis. Active microcystins production was observed in eutrophic environments, while potential production was recorded in populations in oligotrophic environments. Therefore, the presence of producing strains in both types of environment cannot be ruled out. We propose that toxin production could be a mechanism of competition against other microorganisms that make up benthic mats, rather than a direct response to eutrophication. We also found that the percent coverage of toxic cyanobacterial mats is not always a reliable indicator of the amount of toxin produced. Microcoleus sp. showed a higher percent coverage in the river but was not toxic, while Tychonema sp. Produced significant amounts of MC-LR and MC-RR under the same culture conditions despite less coverage in the river. We strongly recommend the implementation of multiple techniques and methods for identifying populations of toxin-producing benthic cyanobacteria (CyanoHAMs) in Mexican rivers in order to validate the results and develop a prevention plan against their associated risks.