applied sciences Review Halophyte Plants and Their Residues as Feedstock for Biogas Production—Chances and Challenges Ariel E. Turcios 1 , Aadila Cayenne 2, Hinrich Uellendahl 2 and Jutta Papenbrock 1,* 1 Institute of Botany, Leibniz University Hannover, Herrenhäuserstr, 2, 30419 Hannover, Germany;
[email protected] 2 Faculty of Mechanical and Process Engineering and Maritime Technologies, Flensburg University of Applied Sciences, Kanzleistr, 91-93, 24943 Flensburg, Germany; aadila.cayenne@hs-flensburg.de (A.C.); hinrich.uellendahl@hs-flensburg.de (H.U.) * Correspondence:
[email protected]; Tel.: +49-511-762-3788 Abstract: The importance of green technologies is steadily growing. Salt-tolerant plants have been proposed as energy crops for cultivation on saline lands. Halophytes such as Salicornia europaea, Tripolium pannonicum, Crithmum maritimum and Chenopodium quinoa, among many other species, can be cultivated in saline lands, in coastal areas or for treating saline wastewater, and the biomass might be used for biogas production as an integrated process of biorefining. However, halophytes have different salt tolerance mechanisms, including compartmentalization of salt in the vacuole, leading to an increase of sodium in the plant tissues. The sodium content of halophytes may have an adverse effect on the anaerobic digestion process, which needs adjustments to achieve stable and efficient conversion of the halophytes into biogas. This review gives an overview of the specificities of halophytes that needs to be accounted for using their biomass as feedstocks for biogas plants in order to expand renewable energy production. First, the different physiological mechanisms of halophytes to grow under saline conditions are described, which lead to the characteristic composition of the Citation: Turcios, A.E.; Cayenne, A.; Uellendahl, H.; Papenbrock, J.