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.

Over the last 15 years, the advent of high-throughput "omics" techniques has revealed the multiple roles and interactions occurring among hosts, their microbial partners and their environment. This microbiome revolution has radically changed our views of biology, evolution, and individuality. Sitting at the interface between a host and its environment, the microbiome is a relevant yet understudied compartment for ecotoxicology research. Various recent works confirm that the microbiome reacts to and interacts with contaminants, with consequences for hosts and ecosystems. In this paper, we thus advocate for the development of a "microbiome-aware ecotoxicology" of organisms. We emphasize its relevance and discuss important conceptual and technical pitfalls associated with study design and interpretation. We identify topics such as functionality, quantification, temporality, resilience, interactions, and prediction as major challenges and promising venues for microbiome research applied to ecotoxicology.

Background: Cyanobacterial blooms are one of the most common stress encountered by metazoans living in freshwater lentic systems such as lakes and ponds. Blooms reportedly impair fish health, notably through oxygen depletion and production of bioactive compounds including cyanotoxins. However, in the times of the microbiome revolution, it is surprising that so little is still known regarding the influence of blooms on fish microbiota. In this study, an experimental approach is used to demonstrate that blooms affect fish microbiome composition and functions, as well as the metabolome of holobionts. To this end, the model teleost Oryzias latipes is exposed to simulated Microcystis aeruginosa blooms of various intensities in a microcosm setting, and the response of bacterial gut communities is evaluated in terms of composition, metagenome-encoded functions and metabolome profiling. Results: The gut bacterial community of O. latipes exhibits marked responses to the presence of M. aeruginosa blooms in a dose-dependent manner. Notably, abundant gut-associated Firmicutes almost disappear, while potential opportunists increase. The holobiont gut metabolome displays major changes, while functions encoded in the metagenome of bacterial partners are more marginally affected. Bacterial communities tend to return to original composition after the end of the bloom suggesting post-bloom resilience, and remain sensitive in case of a second bloom, reflecting a highly reactive gut community. Conclusion: In the context of increasingly frequent and intense blooms worldwide, results point to the relevance of accounting for short- and long-term microbiome-related effects in fish ecology, with potential outcomes relevant to conservation biology as well as aquaculture.

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In crustaceans, as in other animals, perception of environmental cues is of key importance for a wide range of interactions with the environment and congeners. Chemoreception involves mainly the antennae and antennules, which carry sensilla that detect water-borne chemicals. The functional importance of these as exchange surfaces in the shrimp's sensory perception requires them to remain free of any microorganism and deposit that could impair the fixation of odorant molecules on sensory neurons. We report here the occurrence of an unexpected dense bacterial colonization on surface of the antennae and antennules of four hydrothermal vent shrimp species. Microscopic observation, qPCR and 16S rRNA barcoding reveal the abundance, diversity and taxonomic composition of these bacterial communities, that are compared with those found on a related coastal shrimp. Bacterial abundances vary among species. Bacteria are almost absent in coastal shrimp, meanwhile they fully cover the antennal flagella in some hydrothermal vent species. Epsilon-and Gammaproteobacteria dominate the hydrothermal shrimp-associated communities, whereas Alphaproteobacteria and Bacteroidetes are dominant in the coastal ones. Bacteria associated with vent shrimp species are most similar to known chemoautotrophic sulfur-oxidizers. Potential roles of these bacteria on the hydrothermal shrimp antennae and antennules and on sensory functions are discussed.

Myctophids are among the most abundant mesopelagic teleost fishes worldwide. They are dominant in the Southern Ocean, an extreme environment where they are important both as consumers of zooplankton as well as food items for larger predators. Various studies have investigated myctophids diet, but no data is yet available regarding their associated microbiota, despite that the significance of bacterial communities to fish health and adaptation is increasingly acknowledged. In order to document microbiota in key fish groups from the Southern Ocean, the bacterial communities associated with the gut, fin, gills and light organs of members of six species within the three myctophid genera Electrona, Protomyctophum and Gymnoscopelus were characterized using a 16S rRNA-based metabarcoding approach. Gut communities display limited diversity of mostly fish-specific lineages likely involved in food processing. Fin and skin communities display diversity levels and compositions resembling more those found in surrounding seawater. Community compositions are similar between genera Electrona and Protomyctophum, that differ from those found in Gymnoscopelus and in water. Low abundances of potentially light-emitting bacteria in light organs support the hypothesis of host production of light. This first description of myctophid-associated microbiota, and among the first on fish from the Southern Ocean, emphasizes the need to extend microbiome research beyond economically-important species, and start addressing ecologically-relevant species.

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Microcystis aeruginosa is one of the major species that cause toxic cyanobacterial blooms in freshwater systems worldwide. Here, we report the draft genome sequence of M. aeruginosa PMC 728.11, a microcystin-producing cyanobacterium isolated from the freshwater reservoir of Juanon in Valence, France. The genome sequence contains 276 contigs, consisting of 5,536,025 bp and 5,594 putative protein-coding genes, among which are several biosynthetic gene clusters encoding enzyme complexes involved in the production of various bioactive and toxic metabolites.