# Halophyte Plants and Their Residues as Feedstock for Biogas Production--Chances and Challenges
## 2 Natural Habitats of Halophytes, Their Role and Adaptive Mechanisms
### Salt Adaptation Mechanisms  
Salt-tolerant plants are highly evolved and specialized organisms with specific physiological and morphological characteristics, allowing them to grow in saline soils. This remarkable tolerance is complex since it is determined by a number of factors, specific for each particular species [43]. Halophytes can be classified according to their mechanism to tolerate salts, that is, halophytes can be salt excluders having an ultrafiltration mechanism in their root system, salt excreting in which internal salinity levels are regulated via foliar glands and/or salt accumulators that permits the plant to accumulate high salt contents in their cells and tissues and subdue salt toxicity by developing succulence [14]. Some adaptation mechanisms can include blends of, but not limited to, salt exclusion, excretion, succulence, transport and compartmentalization [44]. Salt avoidance involves structural and physiological adaptations to minimize salt concentrations in the cells or physiological exclusion by root membranes [45]. Water potential plays an important role in the halophyte cells, which has to be lower within than outside the plasmalemma to retain cellular water and the necessary osmotic adjustment is achieved mainly by Na and C1 ions [46]. Na\({}^{+}\) and Cl\({}^{-}\) are largely compartmentalized in vacuoles in halophytic plants [47], which contain 90% or more of cell water. Na\({}^{+}\) must be actively pumped into the vacuole from the cytoplasm and it appears to be mediated by Na\({}^{+}\)/H\({}^{+}\)-antiporters in the tonoplast, working together with H\({}^{+}\)-ATPases and perhaps PP\({}_{i}\)ases, which provide the proton motive force [8]. Nevertheless, excessive ion content in the cytoplasm should be avoided and this is apparently achieved by mechanisms that regulate K\({}^{+}\)/Na\({}^{+}\) selectivity. Some genes may also be involved in salt tolerance in halophytes such as the plasma membrane H\({}^{+}\)-ATPase gene [48]. The water potential in the cytoplasm of halophytic plants is also adjusted by the accumulation of organic solutes [8]. Some halophytes accumulate proline as a nontoxic and protective osmolyte under saline conditions [49]; additionally, ascorbate, phenols, flavonoids and total antioxidant capacity can be increased under saline conditions [50].  
As stated before, ion accumulation is an important adaptation mechanism of several halophytes. According to [PERSON] and [PERSON] [4], halophytes have measurably high Na\({}^{+}\) intake and transport in order to sustain its water potential gradient and osmotic pressure needed for water intake. Commonly, in salt-tolerant plants, the measurable abundance of ions follows the order Na\({}^{+}\) >K\({}^{+}\) > Mg\({}^{2+}\) > Ca\({}^{2+}\) and Cl\({}^{-}\) > SO\({}_{4}\)\({}^{2-}\)[44]. These findings were similar to those reported by [PERSON] et al. [51], where salt accumulation in dry material varied from 14-49% in seven different halophytes. In fact, _Saliconia europaea_ agg. contained the largest amounts of mineral compounds mainly Na\({}^{+}\) and Cl\({}^{-}\). It was also observed that the accumulation of salts varied by season.