# Halophyte Plants and Their Residues as Feedstock for Biogas Production--Chances and Challenges
## 3 The Potential Use of Halophytes  
### Valuable Ingredients and Products of Halophytes  
In addition to the marketable value of the whole plant, halophytes contain important ingredients, such as flavors, fragrances, essential oils, resins, gums, oils, pharmaceuticals and fibers [52; 53], that make them even more profitable. Halophytes such as _Tripolium pannomicum_ ([PERSON].) [PERSON], _Plantago coronopus_ L., _Lepidium latifolium_ L. and _Salicontia_ spp. have been reported as having high potential as functional foods or for the supply of nutraceuticals [50]. However, changes in the metabolic profile during exposure to salinity are dependent on the genus, species, cultivar, stress conditions and developmental stage of the plants [50; 54; 55]. Sub-optimal environmental conditions are likely to cause stress in plants, and thus, there is the potential to manipulate metabolic changes through the growing conditions. [PERSON] et al. [50] pointed out that among the species studied, the halophytes _Tripolium pannomicum_, _Plantago coronopus_, _Lepidium latifolium_ and _Salicontia europaea_ demonstrated the most potential as functional foods or nutraceuticals. Other species, such as _Tetraena alba_ (L.f.) [PERSON] (synonym _Zygophyllum album_), _Crithum maritium_ L. and _Cakile maritium_ Scop., have also been reported as potential oilseed halophytes [56]. Seeds of several halophytes contain appreciable quantities of edible oil ranging from 22% to 25% [57]. According to [PERSON] et al. [57] the average of the fatty acid composition inseed oil of several halophytes was 84% unsaturated and 16% saturated, which is a favorable percentage ratio. The oilseed halophyte _Salicornia bigelovii_ Torr., a highly salt-tolerant annual halophyte, yields 2 t/ha of seed containing 28% oil and 31% protein, similar to soybean yield and seed quality with unsaturation degree comparable to oils from conventional oil seeds [58]. Seeds of _Cakile maritima_ and _Crichum maritium_ are also rich in oil (up to 42% and 30% of the seed fresh weight (FW), respectively); similar to olive oil, the fatty acid composition of _Crichum maritium_ seeds is characterized by a high level of oleic acid (81%) [56]. Some studies have shown that some halophytes, when grown in saline environments, can increase the concentration of interesting compounds. For example, [PERSON] et al. [59] pointed out that the species _Descurainaia sophia_ (L.[PERSON] collected from saline soil (0.4% NaCl) contained higher amounts of linolenic acid in their seeds as compared to plants originating from non-saline soil (<0.1% salt) (53.7% and 36% linolenic acid, respectively). Environmental stresses such as increased salinity often trigger a significant oxidative stress in plants, generating the formation of reactive oxygen species (ROS), which may cause cellular damage. Plants overcome this by enhancing biosynthesis of osmotically active metabolites, specific proteins and antioxidant compounds [9; 53; 60]. In this regard, in many halophytes, a positive correlation among increasing salt concentration in the medium and increasing contents of secondary metabolite production has been observed [9]. For example, _Nitaria retusa_ (Forssk.) Asch showed an increasing phenol and flavonoid contents with increasing salinity (0 to 800 mM NaCl), whereas the leaf phenol content of _Atriplex halimus_ L. was maximal in presence of 100 mM NaCl [61]. In addition to their role as antioxidant, polyphenols exhibit a wide spectrum of physiological properties, such as anti-microbial, anti-artherogenic, anti-allergic, anti-thrombotic, anti-inflammatory, cardio-protective and vasodilatory effects [62]. For example, the halophytes _Eryngium maritium_ L., _Crichum maritium_ and _Cakile maritima_ have shown antioxidant and antimicrobial activities [63], which could be used as natural food or cosmetic preservatives. The authors found that _Crichum maritium_ exhibited the highest total phenol content and ABTS (2,2\({}^{\prime}\)-azinobis (3-ethylbenzothazoline-6-sulfonic acid)) radical scavenging activity.