Geography, not food availability, reflects compositional differences in the bacterial communities associated with larval sea urchins

Animals acclimate to changes in their environment through diverse responses, including phenotypic plasticity and shifts in their microbiome. These microbial communities are also taxonomically distinct across the geographical distribution of the host. It is less known, however, whether taxonomic differences in host-associated bacterial communities between geographically distinct populations mask shifts due to environmental changes within a population. We tested for potential ecological masking using larvae of the echinoid Strongylocentrotus droebachiensis from three coastal locations in the Pacific and Atlantic Oceans that were exposed to four feeding regimes. When considering OTU membership and the relative proportion of those taxa, the composition of the larval-associated bacterial communities were best explained by location, not feeding regime. Similarly, predicted metagenomic gene profiles from these bacterial communities were congruent with population specificity and may suggest a role in metabolism. We hypothesize that, while much of the differences in the bacterial communities is related to the large geographic distances between these locations, the predicted overlapping functions of the microbiome may relate to responding to ecological variation experienced by these larvae. Taken together, these results suggest that differences in community composition between populations masks local variation, and that scaling should be considered in when studying microbiome dynamics.