The formation of aegagropiles from the Mediterranean seagrass Posidonia oceanica (L.) Delile (1813): plant tissue sources and colonisation by melanised fungal mycelium

Aegagropiles are conglomerations of fibre debris from Posidonia oceanica meadows that are frequently found along Medi- terranean beaches, but the plant organ from which these fibres arise remains unknown. In this study, a histological compari- son of P. oceanica organs from 3 shoots with the structure of aegagropile fibres showed that most of them arise from leaf sheaths and rhizomes, suggesting that they are degradation products from the “matte” rather than from the leaf litter, which is mainly composed of detached leaf blades. Moreover, fungal hyphae, micro-sclerotia and typical degradation traces were found in the peripheral tissues of living P. oceanica organs, as well as in degrading aegagropiles. We assume, by comparing Vohník’s observations and the observations made in this study, that these endophytic fungi and degradation traces might be attributed to a dark septate endophyte (DSE) in the Aigialaceae (Pleosporales), Posidoniomyces atricolor, which was recently described as an endosymbiont in P. oceanica roots. It constitutes one of the most important microorganisms by abundance that degrade P. oceanica tissues within the matte and give rise to the different fibre types in aegagropiles. This study shows that the proliferation of fungi causes organ degradation in Posidonia, starting early in living P. oceanica plants, continuing in the matte and, probably, in the leaf litter. The DSE plays a much more important role than that of a simple plant endosym- biont; its omnipresence within P. oceanica (and the degradation of the middle lamella and cell death during proliferation) causing the degradation of various Posidonia organs also contributes to the enrichment of the ‘matte’ compartment of this ecosystem, notably favouring nitrogen retention in its chitinous walls.