Formate and hydrogen in hydrothermal vents and their use by extremely thermophilic methanogens and heterotrophs.

Extremely thermophilic methanogens in the and heterotrophs in the are common in deep-sea hydrothermal vents. All use H as an electron donor, and a few species can also use formate. Most have a coenzyme F-reducing formate dehydrogenase. All reduce S but have hydrogenases and produce H in the absence of S. Some have formate hydrogenlyase (Fhl) that reversibly converts H and CO to formate or an NAD(P)-reducing formate dehydrogenase (Nfd). Questions remain if or use or produce formate in nature, why only certain species can grow on or produce formate, and what the physiological role of formate is? Formate forms abiotically in hydrothermal fluids through chemical equilibrium with primarily H, CO, and CO and is strongly dependent upon H concentration, pH, and temperature. Formate concentrations are highest in hydrothermal fluids where H concentrations are also high, such as in ultramafic systems where serpentinization reactions occur. In nature, are likely to use formate as an electron donor when H is limiting. with Fhl likely convert H and CO to formate when H concentrations become inhibitory for growth. They are unlikely to grow on formate in nature unless formate is more abundant than H in the environment. Nearly all and have a gene for at least one formate dehydrogenase catalytic subunit, which may be used to provide free formate for purine biosynthesis. However, only species with a membrane-bound formate transporter can grow on or secrete formate. Interspecies H transfer occurs between and . This and putative interspecies formate transfer may support in low H environments, which in turn may prevent growth inhibition of by its own H. Future research directions include understanding when, where, and how formate is used and produced by these organisms in nature, and how transcription of genes encoding formate-related enzymes are regulated.