Changes in the gastrointestinal microbial community of Lateolabrax maculatus in a naturally occurring Microcystis aeruginosa bloom environment

Microcystis aeruginosa bloom in aquatic ecosystems is a worldwide problem that threaten aquatic animals. Fish are an unavoidable link between toxic cyanobacteria and human health. A stable microbiota is an important barrier for fish in defence against pathogen invasion. Therefore, in this study, we investigated the microbiota characteristics in different parts of the gastrointestinal tract and faeces of Lateolabrax maculatus that were cultured in a pond with a naturally occurring M. aeruginosa bloom. Compared with those of the healthy fish, the cyanobacterial bloom decreased the serum immune indexes of the diseased fish, such as the immunoglobulin M content and lysozyme activity. Microbiome analysis showed that the M. aeruginosa bloom did not change the microbial richness and diversity of the diseased fish but significantly altered the community composition. Specifically, M. aeruginosa bloom increased the abundance of Proteobacteria but decreased the abundance of Firmicutes; some genera of pathogenic bacteria (Acinetobacter, Mycobacterium and Staphylococcus) were abundant, while some beneficial bacteria (Weissella and Lactococcus) were reduced. Our results reveal that M. aeruginosa bloom significantly impair the immunity and gastrointestinal microbiota composition of L. maculatus and that microbiota variation may be a factor causing almost all fishes exhibit unhealthy or diseased characteristics. These results will be helpful for the utilization of microbiome characteristics to facilitate healthy fish farming in cyanobacterial bloom environments.