Uncoupling of seagrass host selection and succession for microbial guilds in meadow chronosequence

Succession describes the predictable assembly of communities and habitats towards increasingly complex ecosystems. In marine meadows, the relative roles of succession and host selection in structuring microbial communities and functions remain unclear. Using chronosequences of the pioneering seagrass Halophila ovalis , we examined bacterial community succession (16S rRNA gene) and functional genes linked to nitrogen fixation and sulfur cycling ( nifH , soxB , aprA , dsrA ) across meadow sediments and seagrass microbiomes (rhizoplane and phylloplane). Sediment communities diversified progressively with distance from the patch edge, whereas host-associated microbiomes were differentially impacted from the meadow sediments, indicating strong host selection. For sulfur-cycling genes, taxonomic succession was decoupled from gene-sequence diversity, while only nifH exhibited successional patterns. Nevertheless, all communities displayed directional assembly trajectories despite being shaped by contrasting ecological processes. Our findings show that both succession and host selection predictably drive microbial community assembly in developing seagrass meadows, while functional gene potential responded in a niche-dependent manner.