How does the endophytic fungal transform the Posidonia oceanica (L.) Delile (1813) meadow into the aegagropiles ?

While walking along the beaches of the Mediterranean Sea, you must have already met agglomerate of vegetable fibres more or less spherical. You must have wondered what it were made of? Is it a waste? How were it formed? Does this have a specific name? No? Well, we did it for you during my master thesis. The name of the conglomerations is Posidonia oceanca aegagropiles. It exists under two types of general shapes one was elongated and the other was more spherical it was respectively defined as ellipsoids and ovoids. These aggregations of meadow’s debris are more complicated than it sounds, in fact it’s composed of various kind of minerals whose the chemical composition is micas, carbonate exoskeleton, etc. This plant debris is structured. Indeed, the internal architecture of aegagropiles is a succession of 4 layers. Two morphotypes were observed and defined: homogeneous aegagropiles and heterogeneous. These two morphotypes are differentiated by their nucleus, fibres densely grouped in one zone for the first type and one piece of rhizome for the other type. To answer the question: “What it were they made of?” The identification of their components was carried out in order to understand the internal composition. Thereby we describe the external and internal morphology of those conglomerates. After the macroscopic description, we will try to describe the histology of the plant debris in the aegagropiles which was observed by light microscopy (LM), transmission electron microscopy (TEM) and scanning electron microscopy (SEM) on isolated debris as well as on polished thin slices. During our observations in microscopy, we followed the strange traces, this traces were very interesting because it’s the result of the biotic degradation. The biotic degradation found notably in the plant cell walls (PCWs) seemed to be in the great majority caused by a dark septate endophytic that specifically attack the middle lamella of PCWs of parenchyma cells leading to longitudinal separation of sclerenchyma resistant fibres from Posidonia leaves, scales and roots. So the combination of their biotic degradation and mechanical shake in the meadow litter appears to be the starting point of the aegagropile formation. It leads Posidonia organs to be fragmented down to resistant fibres, that accumulate in the litter and may intertwine together in pellets by rolling in the ripples marks.