Assessing the Reason Why Heterotrophic Bacteria Present in Aquatic Environments Are Not Affected by Microcystins and Unraveling Alternative Genes for Microcystin Degradation

Abstract

Cyanobacteria are a ubiquitous and diverse group of phototrophic prokaryotes, which mainly inhabit aquatic ecosystems. In certain optimal environmental conditions, there may be a rapid increase in cyanobacteria populations, leading to the formation of blooms, which are frequently associated with the presence of cyanotoxins. Microcystins (MCs) are the most frequent hepatotoxin produced by cyanobacteria. Scarce previous studies have shown that the growth of aquatic heterotrophic bacteria, which co-occur with cyanobacteria, may not be affected by the presence of MCs, or may present a reduction, never being totally inhibited by their presence. In this study, we examined the effects of three microcystin variants (MCLR, MCRR and MCYR) on a set of heterotrophic aquatic bacteria living in the same ecosystem as cyanobacteria. In particular, the impact of microcystins on the growth of heterotrophic bacteria was tested, and a PCR screening for the presence of microcystin-degrading genes (mlr) was performed. The growth assays supported the hypothesis from previous studies, where most heterotrophic bacteria were only slightly or not at all affected by exposure to MCs. Moreover, it seems that the behavior of the isolates when exposed to these cyanotoxins was strain specific. A new bacteria, mlr+, was identified, belonging to Flectobacillus sp. Furthermore, we decided to perform a genomic study of 14 isolates from a set of potentially interesting bacteria, including Flavobacterium spp. and Aeromonas spp., to search for xenobiotic-related genes that could be involved in MC degradation. The whole-genome sequencing analysis of these 14 isolates revealed that no COG genes (COG0625; COG0841; COG1566) were present; however, genes similar to CAAX genes were present in the Aeromonas spp. isolates analyzed. These results shed new light into alternative molecular mechanisms for microcystin degradation.