Production of lignofuels and electrofuels by extremely thermophilic microbes

Extreme thermophiles are microorganisms that grow optimally at elevated temperatures (≥ 70°C). They could play an important role in the emerging renewable energy landscape by exploiting thermophily to produce liquid transportation fuels. For example, Caldicellulosiruptor species can grow on unpretreated plant biomass near 80°C utilizing novel multi-domain glycoside hydrolases. Through metabolic engineering, advanced biofuels compatible with existing infrastructure liquid biofuels, so-called lignofuels, could be produced to establish consolidated bioprocessing at high temperatures. In another case, a new paradigm, electrofuels, addresses the inefficiency of biofuel production through the direct synthesis of advanced fuels from carbon dioxide using hydrogen gas as the electron carrier. This requires coupling of biological electron utilization to carbon dioxide fixation and ultimately to fuel synthesis. Using a hyperthermophilic host Pyrococcus furiosus and synthetic metabolic pathways comprised of genes from less thermophilic sources, temperature-regulated biosynthesis of industrial organic chemicals and liquid fuel molecules are possible. Herein, we review recent progress towards the synthesis of lignofuels and electrofuels by extremely thermophilic microorganisms.