Abstract
BACKGROUND: Shallow coastal habitats are characterized by diverse macrophytes and often feature steep abiotic gradients, including salinity variations, which can shape the leaf- surface epi-microbiome (phyllosphere). To elucidate the effect of salinity and host identity on the phyllosphere of aquatic macrophytes in shallow water, we sampled the leaf surface microbiota across a salinity range of 6-15. Samples included the eelgrass Zostera marina, as well as the Eurasian water milfoil (Myriophyllum spicatum), muskgrass (Chara spp.), and sago pondweed (Stuckenia pectinata) in the brackish Baltic Sea during the summer of 2022. Microbial communities were characterized using 16S and 18S rRNA gene amplicon sequencing. RESULT: As hypothesized, the phyllosphere bacterial and protist community composition was distinct from the surrounding seawater microbiome. Typically associated taxa included the genera Loktanella, Pseudorhodobacter, the methylotrophic genus Methylotenera, unclassified Synechococcales, and Rhodobacteriaceae. Protist genera such as Picochlorum were consistently detected across all macrophyte hosts, while Cocconeis, Cyclotella, Mondous and unclassified Bacillariophyceae were present in all phyllospheres except Chara spp. Both, salinity and host species significantly influenced the composition and prevalence of the microbiota, primarily through shifts in the abundance of typical phyllosphere taxa. However, only 4-11% of phyllosphere taxa were uniquely associated with a specific salinity or macrophyte host. CONCLUSIONS: Our results demonstrate that aquatic macrophytes harbor a distinct and characteristic phyllosphere microbiome. The low proportion of host- or salinity specific taxa suggests that the most abundant members of this community are generalists, broadly adapted to the phyllosphere niche rather than being narrowly specialized. This implies that the presence of the macrophyte itself, providing a stable, nutrient-rich surface, exerts a stronger deterministic influence on the microbial community than the host identity or salinity fluctuations. Consequently, the phyllosphere appears relatively resilient to environmental variability, particularly salinity fluctuations. This highlights the robust nature of host-microbiome interactions and their importance for conservation of aquatic macrophyte ecosystems.