Switching on the light: using metagenomic shotgun sequencing to characterize the intestinal microbiome of Atlantic cod

The biological roles of the intestinal microbiome and how it is impacted by environmental factors are yet to be determined in wild marine fish species. Atlantic cod (Gadus morhua) is an ecologically important species with a wide-spread distribution in the North Atlantic Ocean. 16S rRNA-based amplicon analyses found no geographical differentiation between the intestinal microbiome of Atlantic cod from different locations. Nevertheless, it is unclear if this lack of differentiation results from an insufficient resolution of this method to resolve fine-scaled biological complexity. Here, we take advantage of the increased resolution provided by metagenomic shotgun sequencing to investigate the intestinal microbiome of 19 adult Atlantic cod individuals from two coastal populations in Norway - located 470 km apart. Our results show that the intestinal microbiome is dominated by the Vibrionales order, consisting of varying abundances of Photobacterium, Aliivibrio and Vibrio species. Moreover, resolving the species community to unprecedented resolution, we identify two abundant species, P. iliopiscarium and P. kishitanii, which comprise over 50% of the classified reads. Interestingly, genomic data shows that the intestinal P. kishitanii strains have functionally intact lux genes, and its high abundance suggests that fish intestines form an important part of its ecological niche. These observations support a hypothesis that bioluminescence plays an ecological role in the marine food web. Despite our improved taxonomical resolution, we identify no geographical differences in bacterial community structure, indicating that the intestinal microbiome of these coastal cod is colonized by a limited number of closely related bacterial species with a broad geographical distribution that are well suited to thrive in this host-associated environment.