A Helping Hand: Fungi, as Well as Bacteria, Support Ecophysiological Descriptors to Depict the Posidonia oceanica Conservation Status

The crucial role of plant–microbe interactions in seagrass growth and overall fitness is widely recognized and known to influence plant response to stress. Human-induced changes in coastal ecosystems necessitate efficient descriptors for seagrass monitoring. Recently, for Posidonia oceanica meadows, an integrative approach combining ecophysiological descriptors with bacterial communities has been successfully applied. Conversely, the mycobiota remains largely unexplored and fungal communities cannot be included yet as a putative descriptor. This study aims to evaluate the ecological status of two P. oceanica meadows in the Akrotiri Bay (Cyprus), located under different geomorphological features (depth and seabed type) and degrees of human pressure (port proximity vs. Marine Protected Area). A set of descriptors including morphometry, biochemical markers and bacterial communities collected in 2023 are compared with those collected, at the same sites, in 2017. Furthermore, the investigation of the leaf-associated microbial community included the underrepresented fungal communities, in addition to the bacterial ones, to evaluate their usefulness in evaluating the plant conservation status. Results indicated a good P. oceanica conservation status at both sites, showing an amelioration in the Limassol port meadow from 2017. In 2023, the biometrical/biochemical descriptors were found comparable across sites as the bacterial communities, differing from 2017 results. Noteworthy, fungal communities exhibited significant differences between sites, with a clear reduction, in the Limassol port meadow, of the dominant Posidoniomyces atricolor which is known as a specific colonizer of P. oceanica roots. These results confirm the strong relationship between P. atricolor and P. oceanica host, and suggest its sensitivity to environmental changes, able to keep track of ecological shifts.