# Halophyte Plants and Their Residues as Feedstock for Biogas Production--Chances and Challenges
## 6 Conversion of Halophytes into Biogas
### Inhibiting Effects of Halophytes on the AD Process  
The efficiency of the AD process is steered by many pivotal parameters. Hence, it is essential to provide suitable conditions for anaerobes. Growth and activity of anaerobic microorganisms is considerably affected by parameters such as pH value, absence of oxygen, constant temperature, C/N ratio, ammonia, macro- and micronutrients and toxic compounds [81]. Particularly, the acid-producing bacteria and methanogens in the anaerobic digestion pathway vary significantly with respect to nutritional requirements, growth kinetics, physiology and sensitivity to environmental conditions. Inability to preserve the balance amongst these two groups of anaerobic microorganisms is the principal source of instability in reactors [89]. An array of inorganic and organic substances has been reported to be inhibitory or toxic in the process of AD. A substance could be deemed toxic when it creates an unfavorable shift in the population of microorganisms or inhibition of the growth of the microorganism. This is usually identified by a reduction of the steady-state rate of CH\({}_{4}\) yield and organic acid accumulation [89]. The application of the AD process to saline biomass will broaden the industrial use of haplotypes for biofuel production. The AD of halophytic biomass has many challenges that demands attention before the integration of this process can be valorized in the halophyte industry. One unambiguous challenge encountered in the AD of halophytic biomass is the associated high salinity within the cells and tissues of the biomass. The effects of salinity on AD are clearly established [89; 90; 91]; it is indicated that for sodium, which is considered the main inhibiting element of the light earth metals, with a concentration ranging between 3.5-5.5 g L\({}^{-1}\), AD can moderately inhibit methanogen microorganisms. [PERSON] [91] later investigated this inhibitory concentration of sodium and suggested an increase to 8.0 g L\({}^{-1}\) Na can cause a strong inhibitory effect on the methanogenic bacteria at mesophilic conditions. In light of their findings [92], the light earth metals are required in very minimal concentrations for the metabolism in bacterial cells, and on the other hand, above optimal concentrations, they can be severely toxic to methanogenic bacteria. Many researchers studied the effects of the cations Na\({}^{+}\), K\({}^{+}\), Ca\({}^{2+}\), Mg\({}^{2+}\) and NH\({}_{4}\)[92; 93]. These early research findings indicated the monovalent cations  
Figure 3: Biogas production using different plant biomass cultivated under different conditions. Corn: green corn stover [86]; Maize: green maize for ensiling [86]; Salicornia: fresh biomass of _Salicornia europaea_ cultivated in hydroponics with 10 g L\({}^{-1}\) NaCl [87]; Tripolium 1: fresh biomass of _Tripolium panonicum_ cultivated in hydroponics with 30 g L\({}^{-1}\) NaCl [73]; Quinoa: fresh biomass of _Chenopodium quinoa_ cultivated in hydroponics with 20 g L\({}^{-1}\) NaCl [74]; Quinoa stover: stover of _Chenopodium quinoa_ cultivated with 20 g L\({}^{-1}\) NaCl [74]; Tripolium 2: fresh biomass of _Tripolium panonicum_ cultivated in hydroponics with 5 mg L\({}^{-1}\) sulfidamidine [88]. were more toxic or the same in toxicity to divalent cations based on molarity. Furthermore, according to their results, the inhibition on biogas production due to sodium concentration depends of the antagonistic/synergistic effects between elements. Consequently, it is essential to establish the elemental content of the halophytic substrates and to analyze its effect on biogas/biomethane yield. According to several studies, increasing sodium concentration may have caused a decrease in specific biogas and biomethane yields [69, 73, 74, 94]. Nevertheless, it is uncertain whether or not this is solely due to the sodium concentration levels in the anaerobic reactor or an accumulation of other metal elements or organic compounds.