Natural habitats in littoral salt marshes and desert salt marshes along the coastline of the Red Sea are inhabited by several halophytes, which tolerate salinity stress in these areas. Two distinct halophytes inhabiting the Red Sea coast, Limonium axillare and Nitraria retusa, were studied for their physiological adaptations and energy sources. Results revealed the accumulation of the most abundant soil salts in their tissues. The soils in the study area recorded high concentrations of Na+ and Cl- compared with other ions such as Ca2+, Mg2+, and K+. To combat this stress, the two species absorb salts to adjust the osmotic potential and raise the concentrations of soluble organic compounds – such as proline, soluble sugars, and nitrogen components – in their leaves. In anaerobic digestion, both species yield a large amount of organic components that might act as a source for the production of biogas. L. axillare produced more biogas 277.58 ml g-1 TS (361.23 ml g-1 VS) compared with N. retusa which produced 204.2 ml g-1 TS (309.4 ml g-1 VS). The presence of soluble organic solutes represents easily breakable molecules in anaerobic digestion. Therefore, L. axillare produced more biogas with more soluble carbohydrates (22.15 mg g-1 dry wt), proline (13.47 mg g-1 dry wt), and soluble protein (33.06 mg g-1 dry wt) compared with N. retusa which contains less soluble carbohydrates (18.53 ml g-1 dry wt), proline (13.64 mg g-1 dry wt), and soluble protein (20.04 mg g-1 dry wt).