Response of Fish Gut Microbiota to Toxin-Containing Cyanobacterial Extracts: A Microcosm Study on the Medaka (Oryzias latipes)

The effect of toxin-producing cyanobacterial blooms on fishes’ health has been investigated extensively, but no study to date has evaluated this effect on fish-associated microbiota. In this study, we test the effect of pure microcystins and of crude extracts of metabolites from Microcystis aeruginosa cultures on the composition of gut bacterial microbiota in Medaka fishes (Oryzias latipes) exposed for 28 days in a microcosm experiment. A 16S rRNA-based marker gene-based approach was used to investigate the composition of bacterial communities. Results show that fish gut community compositions differ from those occurring in the water and among individual fishes. Exposure to extracts, rather than pure microcystin, has a significant influence on gut community composition, with a marked increase in relative abundances of pathogen-related bacteria (genera Nocardia and Mycobacterium) in the presence of one extract, and of bacterial orders Sphingomonadales and Saprospirales in the other. We suggest that compounds identified in the cyanobacterial extracts, but not microcystin LR alone, alter the composition of bacterial communities, with possible consequences for various biological functions in fishes. This pioneer microcosm experiment indicates that cyanobacterial blooms probably have an effect on fish gut microbiota and associated functions, including toxin degradation and feed efficiency and should be further explored.