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.

随着蓝藻水华（cyanobacterial blooms）频繁暴发，微囊藻毒素（microcystins, MCs）污染已成为淡水水产养殖面临的全球性问题，其中微囊藻毒素-LR（MC-LR）因其强效肝毒性受到广泛关注。为探究野外蓝藻的MC-LR合成能力，本研究于2018年7月针对5处淡水鱼塘开展研究，分析了MC-LR合成能力与MC-LR含量、总氮（total nitrogen, TN）、总磷（total phosphate, TP）含量的相关性，并对宏基因组mcy基因的单核苷酸多态性（single nucleotide polymorphism, SNP）进行了考察。结果显示，MC-LR含量与宏基因组mcy基因A-J的丰度呈显著正相关；而宏基因组mcy基因A-J的MC-LR合成能力与该类基因的丰度及MC-LR含量呈不显著负相关。在氮磷充足的环境条件下，所有宏基因组mcy基因的MC-LR合成能力均与TN、TP含量呈不显著负相关。本研究共发现406个与MC-LR合成能力显著相关的宏基因组mcy基因单核苷酸多态性位点，其中18个位点与MC-LR合成能力呈显著正相关，388个位点呈显著负相关。在mcy C与mcy B基因中，未发现与MC-LR合成能力显著正相关的SNP位点。本研究预测，铜绿微囊藻（Microcystis aeruginosa）NIES-843的mcy C与mcy B基因具备最强的MC-LR合成能力。mcy C在非核糖体肽合成酶（NRPSs）区域中，与MC-LR合成能力显著相关的突变占比最低，同时在所有宏基因组mcy基因中，其协同突变比例也最低。推测mcy C可能存在某种调控机制，以规避因基因突变引发的底物特异性改变。