Abstract

Hydrogen is considered a promising alternative for replacing fossil fuels in the future because of its high energy content and non-polluting properties. Currently, biological production processes such as dark fermentation suffer from contamination and efficiency issues. Extremophilic anaerobes that produce hydrogen via anaerobic fermentation are promising organisms for the conversion of biomass and organic wastes to hydrogen in open (non-sterile) systems in which the extreme process conditions efficiently prevent the growth of H2-consuming microorganisms. Furthermore, extremophilic microorganisms are viable candidates for processing harsh (hot, highly saline, acidic or alkaline) organic waste streams to valuable products. Recent studies have mainly focused on thermophiles due to the high H2 production yields achieved at high temperature, while investigation on H2 production by other extremophilic organisms is still scarce. This review discusses various groups of anaerobic extremophiles, including thermophiles, halophiles, acidophiles and alkaliphiles, as hydrogen producers with emphasis on advantages, current challenges, production efficiencies, economical substrates and process design.

Keywords: Acidophile, alkaliphile, alkalithermophile, bioenergy, biofuel, dark fermentation, extremophile, haloalkaliphile, halophile, hydrogen, hydrogenase, light-independent, organic waste, plant biomass, renewable energy, thermophile.