A bacterial and viral genome catalogue from Atlantic salmon highlights diverse gut microbiome compositions at pre- and post-smolt life stages.

Resolving the microbiome of the Atlantic salmon Salmo salar gut is challenged by a low microbial diversity often dominated by one or two species of bacteria, and high levels of host contamination in sequencing data. Nevertheless, existing metabarcoding and metagenomic studies consistently resolve a putative beneficial Mycoplasma species as the most abundant organism in gut samples. The remaining microbiome is heavily influenced by factors such as developmental stage and water salinity. We profiled the salmon gut microbiome across 540 salmon samples in differing conditions with a view to capture the genomic diversity that can be resolved from the salmon gut. The salmon were exposed to 3 different nutritional additives: seaweed, blue mussel protein and silaged blue mussel protein, including both pre-smolts (30-60 g salmon reared in freshwater) as well as post-smolts (300-600 g salmon reared in saltwater). Using genome-resolved metagenomics, we generated a catalogue of 11 species-level bacterial MAGs from 188 input metagenome assembled genomes, with 5 species not found in other catalogues. This highlights that our understanding of salmon gut microbial diversity is still incomplete. A prevalent bacterial genome annotated as Mycoplasmoidaceae is present in adult fish, and a comparison of functions revealed significant sub-species variation. Juvenile fish have a different microbial diversity, dominated by a species of Pseudomonas aeruginosa. We also present the first viral catalogue for salmon including prophage sequences which can be linked to the bacterial MAGs.