Potential biohythane production from Sargassum sp. was evaluated in a two stage process. In the first stage, hydrogen dark fermentation was performed by Caldicellulosiruptor saccharolyticus. Sargassum sp. concentrations (VS) of 2.5, 4.9 and 7.4 g L-1 and initial inoculum concentrations (CDW) of 0.04 and 0.09 g L-1 of C. saccharolyticus were used in substrate/ inoculum ratios ranging from 28 to 123. The end products from hydrogen production process were subsequently used for biogas production. The highest hydrogen and methane production yields, 91.3 +/- 3.3 L kg(-1) and 541 +/- 10 L kg(-1), respectively, were achieved with 2.5 g L-1 of Sargassum sp. (VS) and 0.09 g L(-1)of inoculum (CDW). The biogas produced contained 14-20% of hydrogen. Potential energy production from Sargassum sp. in two stage process was estimated in 242 GJ ha(-1) yr(-1). A maximum energy supply of 600 EJ yr(-1) could be obtained from the ocean potential area for macroalgae production. (C) 2015 Elsevier Ltd. All rights reserved.