Host dispersal relaxes selective pressures in rafting microbiomes and triggers successional changes.

What little we know about how microbiomes change over the course of host dispersal has been gleaned from simulations or snapshot sampling of microbiomes of hosts undertaking regular, cyclical migrations. These studies suggest that major changes in both microbiome richness and turnover occur in response to long-distance movements, but we do not yet know how rare or sporadic dispersal events for non-migratory organisms might affect the microbiomes of their hosts. Here we directly examine the microbiomes of rafting seaweed, leveraging host genomic analyses, amplicon sequencing, and oceanographic modelling to study the impacts of ecological dispersal of hosts on their microbiomes. We find that once dislodged from coastal shores and adrift, kelp-associated microbial communities change profoundly-the core microbes found on attached kelp give way to a few abundant taxa and many rare taxa. Changes in microbial species richness and composition are strongly linked to variability in sea surface temperature rather than length of time spent rafting. These changes are associated with increased contributions of neutral processes shaping community assembly. These findings highlight the role of environmental predictability in triggering major community successional changes and challenge the importance of host selection in determining the microbiome.