Reciprocal transplantation experiments reveal local adaptation of seaweed-associated bacteria

Seaweed microbiomes are diverse and frequently species-specific. By actively attracting and repelling settling bacteria through exuded metabolites, seaweeds are thought to exert a strong selective pressure on their microbiomes. However, to what degree seaweed-associated bacteria are adapted to their host has received little attention. Here, we retrieve cultivable seaweed bacterial communities from Palmaria palmata (Dulse) and Fucus serratus (Serrated Wrack) and use reciprocal transplant experiments to test whether bacterial isolates have the greatest fitness on their host seaweed species. We used agar derived from host seaweed extracts for bacterial isolation, which was found to be superior to a generic marine agar formulation based on both 16S rRNA gene amplicon alpha- and beta-diversity comparisons to uncultured samples. We then demonstrate that bacterial isolates from both seaweed species exhibit higher fitness in media derived from their native host compared to a non-native host. Although epibacterial fitness varied between hosts, bacterial isolates on average outperformed non-native counterparts in their native environment. By integrating amplicon sequencing with laboratory experiments, we demonstrate that bacteria are locally adapted to their seaweed host species. These findings contribute to the growing body of research exploring the evolutionary and ecological drivers that shape bacterial communities, with implications for ecosystem management, disease control, and microbial biotechnology.