H-dependent formate production by hyperthermophilic Thermococcales: an alternative to sulfur reduction for reducing-equivalents disposal.

Removal of reducing equivalents is an essential catabolic process for all microorganisms to maintain their internal redox balance. The electron disposal by chemoorganotrophic Thermococcales generates H by proton reduction or HS in presence of S. Although in the absence of S growth of these (hyper)thermopiles was previously described to be H-limited, it remains unclear how Thermococcales could be present in H-rich S-depleted habitats. Here, we report that 12 of the 47 strains tested, distributed among all three orders of Thermococcales, could grow without S at 0.8 mM dissolved H and that tolerance to H was always associated with formate production. Two conserved gene clusters coding for a formate hydrogenlyase (FHL) and a putative formate dehydrogenase-NAD(P)H-oxidoreductase were only present in H-dependent formate producers, and were both systematically associated with a formate dehydrogenase and a formate transporter. As the reaction involved in this alternative pathway for disposal of reducing equivalents was close to thermodynamic equilibrium, it was strongly controlled by the substrates-products concentration ratio even in the presence of S. Moreover, experimental data and thermodynamic modelling also demonstrated that H-dependent CO reduction to formate could occur within a large temperature range in contrasted hydrothermal systems, suggesting it could also provide an adaptive advantage.