Investigating of the microbial communities in a red circle disease of sea cucumber based on metagenomic sequencing

Diseases in aquaculture restrict the survival and development of Apostichopus japonicus, one of the most important Chinese marine culture species. A novel disease, named "red circle" disease, is frequently observed in sea cucumber seedling ponds and results in the death of sea cucumbers. No information about this disease in A. japonicus is available. In this study, we investigated the whole microbial community structure and predicted microbial functional profiles in "red circle" disease affecting sea cucumbers through metagenomics. Metataxonomic analysis of "red circle" samples showed different domains, namely, bacteria (71.39%), virus (5.11%), archaea (0.15%), eukaryotes (0.06%), and some unclassified microbiota (23.29%). Proteobacteria was the most abundant phylum (42.88%), followed by Bacteroidetes (3.30%), Viruses_noname (3.12%), Verrucomicrobia (2.99%), Planctomycetes (2.67%), and Viruses_unclassified (1.99%). The species belong to genus Vibrio, which accounted for 4.51% of the microbiota. Vibrio alginolyticus was the most abundant species of genus Vibrio in the "red circle" samples, followed by Vibrio diabolicus, Vibrio parahaemolyticus, Vibrio cholerae, and Vibrio tubiashii. Functional analysis of "red circle" samples metagenome revealed the genes for the metabolism and environmental information processing of a wide range of bioactive compounds, including amino acids and carbohydrates. Results of our study provides insight into the "red circle" disease and microbial community in A. japonicus seedling ponds and widens our understanding of this new disease.