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Abstract The complex interactions between epiphytic bacteria and marine macroalgae are still poorly understood, with limited knowledge about their community structure, interactions, and functions. This study focuses on comparing epiphytic prokaryotes community structure between three seaweed phyla; Chlorophyta, Rhodophyta, and Heterokontophyta in an easternmost rocky intertidal site of the Mediterranean Sea. By taking a snapshot approach and simultaneously collecting seaweed samples from the same habitat, we minimize environmental variations that could affect epiphytic bacterial assembly, thereby emphasizing host specificity. Through 16S rRNA gene amplicon sequencing, we identified that the microbial community composition was more similar within the same seaweed phylum host compared to seaweed host from other phyla. Furthermore, exclusive Amplicon Sequence Variants (ASVs) were identified for each algal phyla despite sharing higher taxonomic classifications across the other phyla. Analysis of niche breadth indices uncovers distinctive affinities and potential specialization among seaweed host phyla, with 39% of all ASVs identified as phylum specialists and 13% as generalists. Using taxonomy function prediction, we observed that the taxonomic variability does not significantly impact functional redundancy, suggesting resilience to disturbance. The study concludes that epiphytic bacteria composition is connected to host taxonomy, possibly influenced by shared morphological and chemical traits among genetically related hosts, implying a potential coevolutionary relationship between specific bacteria and their host seaweeds.

The use of synthetic chemical products in agriculture is causing severe damage to the environment and human health, but agrochemicals are still widely used to protect our crops. To counteract this trend, we have been looking for alternative strategies to control plant diseases without causing harm to the environment or damage to our health. However, these alternatives are still far from completely replacing chemical products. Microorganisms have been widely known as a biological tool to control plant diseases, but their use is still limited due to the high variability in their efficacy, together with issues in product registration. However, the metabolites produced by these microorganisms can represent a novel tool for the environment-friendly management of plant diseases, while reducing the issues mentioned above. In this study, we explore the soil microbial diversity in natural systems to look for microorganisms with the potential to be used in pre- and post-harvest protection against fungal plant pathogens. Using a simple workflow, we isolated 22 bacterial strains that were tested both in vitro and in vivo for their ability to counteract the growth of common plant pathogens. The three best isolates, identified as members of the bacterial genus Pseudomonas, were used to produce a series of alcoholic extracts, which were then tested for their action against plant pathogens in simulated real-world applications. Results show that extracts from these isolates have an exceptional biocontrol activity and can be successfully used to control plant pathogens in operational setups. Thus, this study shows that the environmental microbiome is an important source of microorganisms producing metabolites that might provide an alternative strategy to synthetic chemical products.