## 3 Results and Discussion
### Digestate Main Characteristics
The highest value (94 L CH\({}_{4}\) kg\({}^{-1}\) VS) was observed for the unwashed biomass thermally pretreated at 132 \({}^{\circ}\)C for 30 min after sample acidification with HCl. Again, in the present study, the co-AD of _P. oceanica_ with a N-rich co-substrate, such as the microcalga _R. subcapitata_, led to higher values (194 \(\pm\) 4 NL CH\({}_{4}\) kg\({}^{-1}\) VS, Figure 2). \begin{table}
\begin{tabular}{c c c c c c c} \hline  & _P.o._ & _P.o.:_Ur & _P.o.:_Cas. & _P.o.:_Pep. & _P.o.:_Syn.Cas. & _P.o.:R.s._ \\ \hline TS (g kg\({}^{-1}\)) & 15 \(\pm\) 5\({}^{\rm\,a}\) & 16 \(\pm\) 2\({}^{\rm\,b}\) & 19 \(\pm\) 1\({}^{\rm\,b}\) & 12 \(\pm\) 5\({}^{\rm\,a}\) & 19.0 \(\pm\) 0.6\({}^{\rm\,c}\) & 23.5 \(\pm\) 0.6\({}^{\rm\,d}\) \\ VS (g kg\({}^{-1}\)) & 8 \(\pm\) 3\({}^{\rm\,a}\) & 11 \(\pm\) 1\({}^{\rm\,b}\) & 12 \(\pm\) 1\({}^{\rm\,c}\) & 8 \(\pm\) 3\({}^{\rm\,a}\) & 12.4 \(\pm\) 0.8\({}^{\rm\,c}\) & 15.6 \(\pm\) 0.3\({}^{\rm\,d}\) \\ MS (g kg\({}^{-1}\)) & 7 \(\pm\) 2\({}^{\rm\,a}\) & 5 \(\pm\) 1\({}^{\rm\,a}\) & 6.5 \(\pm\) 0.6\({}^{\rm\,ab}\) & 4 \(\pm\) 2\({}^{\rm\,c}\) & 6.6 \(\pm\) 0.4\({}^{\rm\,a}\) & 7.9 \(\pm\) 0.6\({}^{\rm\,b}\) \\ PH & 7.82 \(\pm\) 0.07 \({}^{\rm\,a}\) & 9.2 \(\pm\) 0.2\({}^{\rm\,b}\) & 8.2 \(\pm\) 0.1\({}^{\rm\,c}\) & 8.22 \(\pm\) 0.03\({}^{\rm\,c}\) & 8.16 \(\pm\) 0.05\({}^{\rm\,c}\) & 7.77 \(\pm\) 0.08 \({}^{\rm\,a}\) \\ TA (mg CaCO\({}_{3}\) kg\({}^{-1}\)) & 5590 \(\pm\) 70\({}^{\rm\,a}\) & 15,100 \(\pm\) 200 \({}^{\rm\,b}\) & 7800 \(\pm\) 200 \({}^{\rm\,d}\) & 8960 \(\pm\) 90\({}^{\rm\,c}\) & 6850 \(\pm\) 30 \({}^{\rm\,a}\) & 4500 \(\pm\) 500 \({}^{\rm\,f}\) \\ CODs (mg \({}_{2}\) kg\({}^{-1}\)) & 2100 \(\pm\) 500 \({}^{\rm\,a}\) & 7700 \(\pm\) 500 \({}^{\rm\,b}\) & 1600 \(\pm\) 500 \({}^{\rm\,a,d}\) & 3100 \(\pm\) 300 \({}^{\rm\,c}\) & 1400 \(\pm\) 400 \({}^{\rm\,a}\) & 1500 \(\pm\) 200 \({}^{\rm\,a,d}\) \\ TAN (mg N-NH\({}_{4}\) kg\({}^{-1}\)) & 990 \(\pm\) 40 \({}^{\rm\,a}\) & 4000 \(\pm\) 200 \({}^{\rm\,b}\) & 1830 \(\pm\) 50\({}^{\rm\,c}\) & 2030 \(\pm\) 60\({}^{\rm\,c}\) & 1600 \(\pm\) 200 \({}^{\rm\,d}\) & 970 \(\pm\) 30 \({}^{\rm\,a}\) \\ BMP\({}_{\rm th}\) (NL CH\({}_{4}\) kg\({}^{-1}\) VS) & 526.2 & 83.0 & 455.8 & 399.0 & 498.1 & 537.9 \\ BMP\({}_{\rm exp}\) (NL CH\({}_{4}\) kg\({}^{-1}\) VS) & 79 \(\pm\) 3\({}^{\rm\,a}\) & 0\({}^{\rm\,b}\) & 420 \(\pm\) 30\({}^{\rm\,c}\) & 389 \(\pm\) 7\({}^{\rm\,c}\) & 380 \(\pm\) 30\({}^{\rm\,c}\) & 194 \(\pm\) 4 \({}^{\rm\,d}\) \\ BD (\%) & 15 & 0 & 93 & 97 & 75 & 36 \\ \hline  & _P.o._: _Posidonia oceanica_; **Ur.**: Urea; **Cas**: Casein; **Pep.**: Peptone; **Syn.Cas.**: Synthetic casein made of amino acids; _R.s.: Rapidiocks subcapitata_. **TS**: Total solids; **VS**: Volatile solids; **MS**: Mineral Solids; **TA**: Total alkalinity; **CODs**: Soluble chemical oxygen demand; **TAN**: Total Ammonium Nitrogen; **BMP\({}_{\rm th}\)**: Theoretical Biochemical Methane Potential; **BMP\({}_{\rm exp}\)**: Experimental Biochemical Methane Potential; **BD**: Biodegradability. \\ \hline \end{tabular}
\end{table} 
Table 3: Physicochemical parameters of the digestates derived from the biomethanization tests. Values are given as means \(\pm\) standard deviations. Values with different letters in the same row mean significant differences. The second group was characterized by the highest methane yield (\(380\pm 30-420\pm 30\) NL CH\({}_{4}\) kg\({}^{-1}\) VS), with no significant differences between the three results. Moreover, biodegradability above 75% may indicate an almost complete degradation of the substrate [41]. COD values similar to those found in the first group (Table 3) also indicate that there was not an accumulation of solubilized organic matter, suggesting that both the hydrolytic and the methanogenetic stages operated properly. Only the P.o.:Pep. test showed a significantly higher COD value; however, it also showed the highest BD (97%). Peptone is a partially degraded protein, which makes it much more easily digestible and bioavailable for microorganisms, providing a quick source of nitrogen.