Electron transfer pathways of formate-driven H₂ production in Desulfovibrio

The potential of sulfate-reducing bacteria (SRB) as biocatalysts for H₂ production from formate was recently demonstrated, but the electron transfer pathways involved were not described. In the present work, we analyzed the H₂ production capacity of five Desulfovibrio strains: Desulfovibrio vulgaris, Desulfovibrio desulfuricans, Desulfovibrio alaskensis, Desulfovibrio fructosivorans, and Desulfovibrio gigas. D. vulgaris showed the highest H₂ productivity (865 mL L_medium ⁻¹), and D. gigas the lowest one (374 mL L_medium ⁻¹ of H₂). The electron transfer pathways involved in formate-driven H₂ production by these two organisms were further investigated through the study of deletion mutants of hydrogenases (Hases) and formate dehydrogenases (Fdhs). In D. vulgaris, the periplasmic FdhAB is the key enzyme for formate oxidation and two pathways are apparently involved in the production of H₂ from formate: a direct one only involving periplasmic enzymes and a second one that involves transmembrane electron transfer and may allow energy conservation. In the presence of selenium, the Hys [NiFeSe] Hase is the main periplasmic enzyme responsible for H₂ production, and the cytoplasmic Coo Hase is apparently involved in the ability of D. vulgaris to grow by converting formate to H₂, in sparging conditions. Contrary to D. vulgaris, H₂ production in D. gigas occurs exclusively by the direct periplasmic route and does not involve the single cytoplasmic Hase, Ech. This is the first report of the metabolic pathways involved in formate metabolism in the absence of sulfate in SRB, revealing that the electron transfer pathways are species-specific.