Eelgrass leaf surface microbiomes are locally variable and highlycorrelated with epibiotic eukaryotes

Eelgrass (Zostera marina) is a marine foundation species essential for coastal ecosystem services around the northern hemisphere. Like all macroscopic organisms, it possesses a microbiome which may play critical roles in modulating the interaction of eelgrass with its environment. For example, its leaf surface microbiome could inhibit or attract eukaryotic epibionts which may overgrow the eelgrass leading to reduced primary productivity and subsequent eelgrass meadow decline. We used amplicon sequencing of the 16S and 18S rRNA genes of prokaryotes and eukaryotes to assess the leaf surface microbiome (prokaryotes) as well as eukaryotic epibionts in-and outside lagoons on the German Baltic Sea coast. Bacterial microbiomes varied substantially both between sites inside lagoons and between open coastal and lagoon sites. Water depth, leaf area and biofilm chlorophyll a concentration explained a large amount of variation in both bacterial and eukaryotic community composition. Communities of bacterial and eukaryotic epibionts were highly correlated, and network analysis revealed disproportionate co-occurrence between a limited number of eukaryotic taxa and several bacterial taxa. This suggests that eelgrass leaf surface biofilms are a mosaic of the microbiomes of several eukaryotes, in addition to that of the eelgrass itself, and underlines that eukaryotic microbial diversity should be taken into account in order to explain microbiome assembly and dynamics in aquatic environments.