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ABSTRACT We examine the role of physical structure versus biotic interactions in structuring host‐associated microbial communities on a marine angiosperm,Zostera marina, eelgrass. Across several months and sites, we compared microbiomes on physical mimics of eelgrass roots and leaves to those on intact plants. We find large, consistent differences in the microbiome of mimics and plants, especially on roots, but also on leaves. Key taxa that are more abundant on leaves have been associated with microalgal and macroalgal disease and merit further investigation to determine their role in mediating plant–microalgal–pathogen interactions. Root associated taxa were associated with sulphur and nitrogen cycling, potentially ameliorating environmental stresses for the plant. Our work identifies targets for future work on the functional role of the seagrass microbiome in promoting the success of these angiosperms in the sea through identifying components of microbial communities that are specific to seagrasses. 

Abstract Considerable research describes the interactions between seagrasses and their sedimentary environment, but there is little information on how populations differ in their innate versus plastic responses to these differences. Here, we test whether sediment contributes to eelgrass population differentiation and the nature of plastic responses to different sediment environments. We do this via a 15-week, fully crossed common garden experiment with two populations and their native sediment types. Plants from the warmer-temperature, clay-dominated site (90% silt + clay, 10% sand) consistently maintained greater biomass than plants from the cooler, sand-dominated site (60% sand, 40% silt + clay). Plants from both populations were highly plastic for root length and clonal shoot size, with both increasing when planted in clay-dominated compared to sand-dominated sediment. Plants from the clay-dominated site grew longer rhizomes in foreign sediment while plants from the sand-dominated site had no change in this plant trait, indicating some measure of home site advantage with respect to sediment conditions. Porewater sulfide also exhibited this pattern where concentrations were very low in clay-dominated sediment for all plants, but in the sand-dominated treatment, only plants native to sand-dominated sediment maintained porewater sulfide concentrations below toxic levels. These patterns may be mediated by microbiome differences between populations as roots from plants native to clay-dominated sediment had more fixed microbiomes between treatments compared to plants native to sand-dominated sediment. These results support that sediment type partially mediates home site advantage in eelgrass populations and suggest differential population responses may be mediated by the associated microbiome.