## 1 Introduction  
Around 100 million years ago, terrestrial monocotyledonous plants, already reproducing through a floral system, began to colonize marine ecosystems, forming complex and well-structured habitats. Among these ecosystems, seagrass have become one of the most productive and valuable areas on our planet, providing a wide range of essential services: vital habitat for thousands of fish and invertebrate species, food, nutrient cycling, carbon sink, etc. [1]. _Posidonia oceanica_ (_P. oceanica_) is a marine angiosperm species that is endemic to the Mediterranean Sea. It forms extensive seagrass in shallow water, covering about 1.5% of the Mediterranean basin and playing an important role in coastal ecosystem dynamics [2; 3]. These meadows provide oxygen, food, shelter, and protection for many marine species. Additionally, these extensive meadows form a natural barrier that reduces wave energy, protecting against erosion and stabilizing the coastal profile [4]. Currently, _P. oceanica_ beds are in danger of disappearance and have been listed as priority habitats by the European authorities [5]. _P. oceanica_ presents a stem, long leaves grouped in bunches, and a root system that enables the individual specimens to cling to the substrate. Its leaves are removed from the rhizome in autumn after losing their photosynthetic capacity. These loosed leaves are released in large quantities along the shore and once they reach the sand, they start to decompose, causing serious environmental problems (methane release to the atmosphere, proliferation of mosquitoes, problems with navigation and fishing, etc.) [6; 7]. Fresh deposits of _P. oceanica_ debris usually appear at the coast during autumn and winter due to the weather and sea conditions, making this marine plant a renewable resource. Some leaves keep their fibrous structure for long periods after the easily biodegradable components are completely decomposed. These fibers are highly resistant to the mechanical and chemical action of the seawater. They are rolled up by the wave movement around a rhizomic nucleus, as a starting point for the formation of spherical agglomerates, from 3 to 12 cm in diameter (Figure 1). These agglomerates, called aegagropiles, are eventually left ashore where they remain and are sun-dried [8; 9; 10]. Although in protected natural areas, the collection of _P. oceanica_ and its debris is forbidden due to its benefits on the conservation of the local habitat (Decree 132/2005, 2005), in certain locations, exceptionally, the responsible authorities allow and encourage the collection of this biomass. For instance, on beaches of Formentera (Spain), this waste is widely used as bedding for animals, crops, and as insulation in traditional houses. In Italy, the use as a co-substrate in composting is also allowed, provided that it does not exceed 20% of the final mixture in terms of wet matter [11]. The removal of tons of _P. oceanica_ remains is a common practice in Mediterranean coasts to allow for recreational use in the summer months. Due to the lack of alternative solutions, this biomass is usually disposed of in landfills, which provokes an environmental problem as a signifi  
Figure 1: Detail of the formation of the _Posidonia oceanica_ aegagropiles. matter is removed from the ecosystem. Moreover, it carries an economic cost due to the disposal process [4; 12]. In the context of the circular economy, anaerobic digestion (AD) can be a highly efficient biological alternative for the treatment and valorization of wastes with a high organic load [13]. The interest in AD facilities has grown over the past few years because of the high methane content (around 50-60%) of the generated biogas, making it a renewable energy alternative to fossil fuels. Furthermore, the digestate can be used as an organic soil amender [13; 14]. However, the viability of these processes is highly dependent on the type of substrate used.