## 3 Results and Discussion  
### Posidonia Oceanica Characterization  
The collected samples showed a normal distribution in terms of biovolume (Table 2), ranging from 10 to 850 cm\({}^{3}\), with an average value of 300 \(\pm\) 200 cm\({}^{3}\) and a mode of 50 cm\({}^{3}\). The aegagropiles were milled and the particle size distribution was measured. The results showed that due to the morphology of the material, all the fibers passed through the 3.5 mm mesh sieve. As suggested by the specific literature (Stein  
Regarding the elemental analysis, the C/N ratio (153 \(\pm\) 5) indicated a low N content (Table 1), which may unbalance AD performance by affecting the bacterial growth and subsequently their hydrolytic activity [29]. This result supported the idea of carrying out a co-AD process using N-rich co-substrates. Seagrasses are also characterized by their potential to absorb heavy metals present in the seawater and act as bioindicators [30]. However, the heavy metal uptake capacity is strongly linked to the species and the location [30; 31]. Regarding _P. oceanica_, strong relationships between the location, the plant part, the collection season, and the heavy metal composition have been previously reported [31; 32]. The results obtained in this study are within the range of those previously reported, with very few exceptions [31; 32; 33]. For example, Zn is commonly detected in the 100-1000 ppm range, while in the sample used in the present study this element could not be detected. Similarly, Fe and Pb have been reported in the 100-3000 ppm and 10-100 ppm ranges, respectively, however, in the present study, Fe was found at a concentration of 1.6 \(\pm\) 0.1 ppm and Pb at 3 \(\pm\) 2 ppm [31; 32; 33]. While the data reported in the literature is related to healthy and alive \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameters** & _Posidonia oceanica_ \\ \hline \multicolumn{2}{c}{**Physical Bio-Parameters**} \\ \hline
**Average weight (g)** & 9 \(\pm\) 8 \\
**Average width (cm)** & 4 \(\pm\) 2 \\
**Average length (cm)** & 5 \(\pm\) 2 \\
**Average height (cm)** & 3 \(\pm\) 1 \\
**Biovolume (cm\({}^{3}\))** & 300 \(\pm\) 200 \\
**Q1 (cm\({}^{3}\))** & 61 \\
**Q3 (cm\({}^{3}\))** & 450 \\
**Range (cm\({}^{3}\))** & 10–850 \\
**Mode (cm\({}^{3}\))** & 50 \\ \hline \multicolumn{2}{c}{**Elemental Analysis**} \\ \hline C (g kg\({}^{-1}\)) & 429 \(\pm\) 2 \\ N (g kg\({}^{-1}\)) & 2.8 \(\pm\) 0.4 \\ B (g kg\({}^{-1}\)) & 3.0 \(\pm\) 0.2 \\ Na (g kg\({}^{-1}\)) & 17.9 \(\pm\) 0.8 \\ Mg (g kg\({}^{-1}\)) & 4.3 \(\pm\) 0.2 \\ Al (g kg\({}^{-1}\)) & 0.63 \(\pm\) 0.03 \\ K (g kg\({}^{-1}\)) & 1.0 \(\pm\) 0.1 \\ Ca (g kg\({}^{-1}\)) & 8 \(\pm\) 1 \\ \(p\) (ppm) & \textless{LOD} \\ As (ppm) & 10.4 \(\pm\) 0.4 \\ Ti (ppm) & 12 \(\pm\) 1 \\ Cr (ppm) & 3 \(\pm\) 1 \\ Cu (ppm) & 3 \(\pm\) 2 \\
**Co (ppm)** & \textless{LOD} \\
**Mn (ppm)** & \textless{LOD} \\ Ni (ppm) & 0.5 \(\pm\) 0.1 \\
**Fe (ppm)** & 1.6 \(\pm\) 0.1 \\ Hg (ppm) & 2 \(\pm\) 1 \\ Pb (ppm) & 3 \(\pm\) 2 \\
**Mo (ppb)** & 310 \(\pm\) 70 \\
**Zn (ppb)** & \textless{LOD} \\
**Sn (ppb)** & \textless{LOD} \\
**Sb (ppb)** & \textless{LOD} \\ \hline \hline \end{tabular}  
**Q1**: 1 st quartile of biovolume normal distribution; **Q3**: 3 rd quartile of biovolume normal distribution; **LOD**: Limit of detection.  
\end{table} 
Table 2: Biometric and physicochemical characterization of the aegagropiles of _Posidonia oceanica_. Values are given by means \(\pm\) standard deviations. specimens of _P. oceanica_, in the present study, the _P. oceanica_ samples were collected ashere, sun-dried, and after a natural decomposition process. These differences in the biomass origin could significantly alter the elemental profile as it has been recently reported [28]. Furthermore, heavy metals are strongly linked to a good performance of the AD process. While it is well known that the presence of trace elements is essential for efficient AD performance, at certain concentrations, these may disrupt the process by inhibiting the microbiome activity [34].