## 3 Results and Discussion
### Digestate Main Characteristics
This could suggest that the substrate was completely solubilized, unlike the other two tests where some parts of the substrates may have remained intact or not biodegraded. Due to its complex structure, casein is less soluble in water and takes longer to be digested by microorganisms. In biological systems, microorganisms need to break down casein into simpler components before they can metabolize it. The co-digestion of complex substrates, such as casein proteins and lignocellulosic can create a more stable environment for anaerobic microorganisms, thereby optimizing biogas production. The hydrolysis of proteins, particularly complex ones, such as casein, is likely to be synchronized with the hydrolysis of complex lignocellulosic, allowing for a more balanced release of nitrogen and carbon compounds. This synchronized release can maintain an optimal soluble C/N ratio, which is crucial for the proper development of anaerobic microorganisms. The parallel hydrolysis of nitrogenous and carbonaceous compounds promotes a more efficient digestion process, supporting the microbial community's stability and increasing methane production. This is in line with the principles established in anaerobic digestion, where proper synchronization of substrate degradation is key to maximizing biogas yields. In any case and despite the apparent good performance, pH, TA, and TAN analyses of the digestates indicated that the systems were close to failure due to inhibition processes. In fact, when compared with the first group, the TAN values were around 2-fold higher, the pH was higher at least by 0.5 units, and the TA also increased up to 9000 mg CaCO\({}_{3}\) kg\({}^{-1}\) (Table 3). Although these values were still within the normal limits for good AD performance, they were closer to the upper limits which indicates inhibition due to the presence of ammonia. This may  
Figure 2: Experimental and accumulative methane of the different biomethanization (BMP) tests carried out versus time. P.o. : _P. ocanica_; Ur: urea; Cas. : casein; Pep. : gelatine peptone; Syn.Cas. : amino acids mixture simulating a synthetic casein; R.s. : _Raphidocelis subcapiata_. Values are given as means \(\pm\) standard deviations. be due to the higher degradability of nitrogenated compounds from pure proteins and amino acids in contrast with those derived from more complex biomasses, such as the marine plant _P. oceanica_ or the microalga _R. subcapitata._ This also suggests that in a continuous mode of operation, the system may collapse and could be inhibited due to the accumulation of ammonia. This has been also confirmed by [PERSON] et al. [42], who reported that the AD of nitrogen-rich substrates produced an anaerobic digesting with a pH higher than those obtained from lipid-rich substrates. Nevertheless, the co-AD of lignocellulosic biomasses with protein-rich substrates enhances the methane yield greatly. For example, [PERSON] et al. [42] reported a methane yield of 325.9 L CH\({}_{4}\) kg\({}^{-1}\) VS for casein. In the present study, the mixture of _P. oceanica_ and casein reported a higher methane yield of 420 \(\pm\) 30 L CH\({}_{4}\) kg\({}^{-1}\) VS, which suggests the importance of the C/N balance. However, the methane yield of the mixture of _P. oceanica_ with a casein-like amino acid mixture was somewhat lower (380 \(\pm\) 30 L CH\({}_{4}\) kg\({}^{-1}\) VS). This suggests that not only the C/N ratio, but also, the N source-type impact AD performance. A total inhibition due to a high ammonia concentration (TAN: 4000 mg N-NH\({}_{4}\)\({}^{+}\) kg\({}^{-1}\)) was observed from the P.o.:Ur. test. Moreover, the pH was above 9 and TA above 15,000 mg CaCO\({}_{3}\) kg\({}^{-1}\). In fact, as could be observed in Figure 2, the accumulative methane yield was negative. This is because the background production from blanks was higher than the methane production from the P.o.:Ur. test, which demonstrates that the methanogenetic microbial communities were completely inhibited.