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.