Metabolomics and Microbiomics Insights into Differential Surface Fouling of Three Macroalgal Species of (Fucales, Phaeophyceae) That Co-Exist in the German Baltic Sea.

The brown algal genus provides essential ecosystem services crucial for marine environments. Macroalgae (seaweeds) release dissolved organic matter, hence, are under strong settlement pressure from micro- and macrofoulers. Seaweeds are able to control surface epibionts directly by releasing antimicrobial compounds onto their surfaces, and indirectly by recruiting beneficial microorganisms that produce antimicrobial/antifouling metabolites. In the Kiel Fjord, in the German Baltic Sea, three distinct species coexist: , , and subsp. . Despite sharing the same habitat, they show varying fouling levels; subsp. is the least fouled, while is the most fouled. The present study explored the surface metabolomes and epiphytic microbiota of these three spp., aiming to uncover the factors that contribute to the differences in the fouling intensity on their surfaces. Towards this aim, algal surface metabolomes were analyzed using comparative untargeted LC-MS/MS-based metabolomics, to identify the marker metabolites influencing surface fouling. Their epiphytic microbial communities were also comparatively characterized using high-throughput amplicon sequencing, to pinpoint the differences in the surface microbiomes of the algae. Our results show that the surface of the least fouling species, subsp. , is enriched with bioactive compounds, such as betaine lipids MGTA, 4-pyridoxic acid, and ulvaline, which are absent from the other species. Additionally, it exhibits a high abundance of the fungal genera and , along with the bacterial genus . These taxa are known for producing antimicrobial/antifouling compounds, suggesting their potential role in the observed fouling resistance on the surface of the subsp. compared to and . These findings provide valuable clues on the differential surface fouling intensity of spp., and their importance in marine chemical defense and fouling dynamics.