Posidonia oceanica is a long-living and very slow-growing marine seagrass endemic to the Mediterranean Sea. It produces large amounts of leaf material and rhizomes, which can reach the shore and build important banks known as "banquettes." In recent years, interest in the potential uses of these P. oceanica banquettes has increased, and it was demonstrated that biomass extracts showed antioxidant, antifungal, and antiviral activities. The discovery of new compounds through the culture of microorganisms is limited, and to overcome this limitation, we performed a metagenomic study to investigate the microbial community associated with P. oceanica banquettes. Our results showed that the microbial community associated with P. oceanica banquettes was dominated by Alphaproteobacteria, Gammaproteobacteria, Bacteroidetes, and Cyanobacteria. Pseudoalteromonas was the dominant genus, followed by Alteromonas, Labrenzia, and Aquimarina. The metagenome reads were binned and assembled into 23 nearly complete metagenome-assembled genomes (MAGs), which belonged to new families of Cyanobacteria, Myxococcota, and Granulosicoccaceae and also to the novel genus recently described as Gammaproteobacteria family UBA10353. A comparative analysis with 60 published metagenomes from different environments, including seawater, marine biofilms, soils, corals, sponges, and hydrothermal vents, indicated that banquettes have numbers of natural products and carbohydrate active enzymes (CAZymes) similar to those found for soils and were only surpassed by marine biofilms. New proteins assigned to cellulosome modules and lignocellulose-degrading enzymes were also found. These results unveiled the diverse microbial composition of P. oceanica banquettes and determined that banquettes are a potential source of bioactive compounds and novel enzymes. IMPORTANCE Posidonia oceanica is a long-living and very slow-growing marine seagrass endemic to the Mediterranean Sea that forms large amounts of leaf material and rhizomes, which can reach the shore and build important banks known as "banquettes." These banquettes accumulate on the shore, where they can prevent erosion, although they also cause social concern due to their impact on beach use. Furthermore, Posidonia dry material has been considered a source of traditional remedies in several areas of the Mediterranean, and a few studies have been carried out to explore pharmacological activities of Posidonia extracts. The work presented here provides the first characterization of the microbiome associated with Posidonia banquettes. We carried out a metagenomic analysis together with an in-depth comparison of the banquette metagenome with 60 published metagenomes from different environments. This comparative analysis has unveiled the potential that Posidonia banquettes have for the synthesis of natural products, both in abundance (only surpassed by marine biofilms) and novelty. These products include mainly nonribosomal peptides and carbohydrate active enzymes. Thus, the interest of our work lies in the interest of Posidonia "waste" material as a source of new bioactive compounds and CAZymes.
Keywords: BGC; CAZyme; Myxococcota; NRPS; Posidonia oceanica; Pseudoalteromonas; banquettes; bioactive compound.