Analysis of microcystin-LR producing ability of metagenomic mcy genes in freshwater aquaculture ponds focusing on abundance of metagenomic mcy genes and SNPs

With the frequently outbreak of cyanobacterial blooms, microcystins (MCs) pollution have become a worldwide problem for freshwater aquaculture, in which MC-LR has attracted considerable attention due to its potent hepatotoxicity. To analyze MC-LR producing ability of cyanobacteria in field, the relationship between MC-LR producing ability and MC-LR content, total nitrogen (TN) and total phosphate (TP) content, and the single nucleotide polymorphism (SNP) of metagenomic mcy genes were investigated in five freshwater fishponds during July 2018. The results showed that MC-LR content was significantly positively correlated with abundance of metagenomic mcy gene A-J, while MC-LR producing ability of metagenomic mcy gene A-J was insignificantly negatively correlated with abundance of metagenomic mcy gene and MC-LR content. MC-LR producing ability of all metagenomic mcy genes was insignificantly negatively correlated with TN and TP contents in sufficient nitrogen and phosphate conditions. Total 406 single nucleotide polymorphisms (SNPs) in metagenomic mcy genes significantly correlated with MC-LR producing ability were reported, in which 18 SNPs were significantly positively correlated with MC-LR producing ability and 388 SNPs were significantly negatively correlated with MC-LR producing ability. No SNP significantly positively correlated with MC-LR producing ability was observed in both mcy C and mcy B. It was predicted that both mcy C and mcy B of M. aeruginosa NIES-843 had the strongest MC-LR producing ability in the present study. mcy C had the lowest proportion of mutation significantly correlated with MC-LR production ability in NPRSs and the lowest coherence mutation proportion in all metagenomic mcy genes. There might be some mechanism to avoid substrate specific changes due to genetic mutation of mcy C.