The impact of some microcystins on the growth of heterotrophic bacteria from Portuguese freshwater reservoirs Microcystins (MCs) are hepatotoxins that are abundantly produced by cyanobacteria. Studies have shown that these toxins affect many multicellular organisms that inhabit aquatic ecosystems; however, their impact on bacteria that co-occur with freshwater cyanobacteria is still unclear. In this study, the impact of the three most common variants of MCs (MCLR, MCRR, and MCYR) on the growth of heterotrophic bacteria isolated from four Portuguese reservoirs was evaluated. Some isolates were derived from freshwater sources where blooms of cyanobacteria that often contain microcystin-producing strains are frequently observed and from a reservoir where these phenomena do not occur. Morphological and molecular characterisation of the bacterial isolates was performed, and these bacteria were exposed to three different concentrations of each MC variant. The effect of MCs on bacterial growth was then evaluated. This study showed that MCLR, MCRR and MCYR can reduce the growth of some heterotrophic bacteria isolated from freshwater sources. To our knowledge, this is the first study in which the impact of several variants of MCs was evaluated in a diverse group of freshwater heterotrophic bacteria.