Microbial Life Inside Posidonia Seeds: Beneficial Endophytes and Implications for Marine Plant Health.

Plant-microbe interactions are key drivers of plant health and ecosystem functioning, yet their roles in marine environments remain poorly understood. The seagrass Posidonia oceanica, a foundation species in the Mediterranean Sea, forms complex associations with microbial communities that influence its development and stress tolerance. Here, we provide the first evidence of culturable bacterial and fungal endophytes inhabiting P. oceanica seeds collected from the central Mediterranean, a region representing a major center of the species' genetic diversity. Using two different marine culture media, we isolated a diverse assemblage of endophytes, predominantly affiliated with Marinomonas, Celerinatantimonas, Vibrio, Halomonas, Kocuria, Bacillus, Metabacillus, Lysobacter, and Aureimonas, along with the fungi Paecilomyces maximus and Halophytophthora sp. Most bacterial isolates displayed plant growth-promoting (PGP) traits such as indole-3-acetic acid production and nitrogen fixation, supporting their potential contribution to seed germination and early seedling establishment. The detection of Candidatus Celerinatantimonas neptuna, a nitrogen-fixing symbiont previously described in P. oceanica roots, suggests a possible route of vertical transmission. Although fungal endophytes were less frequent, their presence indicates that P. oceanica seeds may serve as a reservoir of both beneficial and potentially pathogenic taxa. These findings expand our understanding of the P. oceanica holobiont, highlight the role of seeds in the persistence and dissemination of endophytic communities and lay the groundwork for the biotechnological use of seed-associated microbes in marine plant restoration and conservation, and in crop stress tolerance.