# Halophyte Plants and Their Residues as Feedstock for Biogas Production--Chances and Challenges
## 4 Cultivation of Halophytes for Biogas Production  
The increased demand and scarcity of food, which inevitably means an increase in food prices, arises partially due to new demands on agriculture for biomass as feedstock in biogas production. Scarcity of these resources is exacerbated as demand increases mainly due to industrial uses, an increase in population and use in the production of biofuels [70]. Besides this, there are other causes that can decrease the availability of these resources such as climate change. This brings us to exploit renewable energy using sustainable systems, and thus, the preservation of resources for future generations is guaranteed. In order to use a sustainable system, avoiding the competition between food crops and energy crops, biomass from halophyte plants has a high potential for biogas production. In this context, marginal land and saline water resources could be used to cultivate plants for biogas production.  
The development of AD of biomass greatly increased as biogas continues to be perceived as a renewable and viable energy source. The feedstock used in conventional technologies were first-generation food crop biofuels derived from starchy grains, wheat and maize, though other sources are also possible. Therefore, to mitigate this food vs. biofuel dilemma, researchers have started to focus on agricultural residues, organic waste and biomass that grows on inedible food or lignocellulosic biomass such as halophytes or non-halophytes to make second-generation biofuels [3]. In addition, the increase in salinization of arable lands and continued depletion of limited freshwater resources is a major constraint for the sustainable production of agriculture as stated above. This vast amount of unused degraded niche saline-affected lands can be used for cultivating feedstocks that can grow under saline conditions [71], which will in turn spare arable lands and waste resources for traditional agriculture. For this reason, halophytes are an ideal candidate for biofuel production due to its ability to grow, reproduce and thrive under high saline conditions, with desirable lignocellulosic composition, mostly perennial and probable rehabilitation of the salt-affected lands.  
There are a number of annual and perennial species among halophytes that are able to grow in highly saline environments, such as _Chenopodium quinao_, _Tripolium pannonicum_ and _Salicornia_ spp. (Figure 1), among many other species, as described above (Table 1).  
These halophytes can also be cultivated in Constructed Wetlands (CWs) to treat wastewater, in hydroponics or in aquaponics [72], and the biomass has a great potential for the production of biogas (Figure 2). It was experimentally shown that some halophytes have a great potential for extracting inorganic contaminants from wastewater, such as nitrates and phosphates [9]. Some other species, such as _Tripolium panonicum_ and _Chenopodium quinoa_, among others, have been used for biodegradation xenobiotics in hydroponics and their biomass was used to produce biogas [73,74]. However, for the production of biogas, it is important to take into account the composition of the biomass, which can vary depending on the species, cultivation method and climatic conditions, among others.  
Figure 1: Halophyte plants grown under controlled conditions in hydroponic cultures, (**a**) _Chenopodium quinoa_, (**b**) _Tripolium panonicum_, (**c**) _Salicornia europaea_ and (**d**) _Crilumum_. Source: Own pictures, [PERSON].