Metabolism of H2 by Desulfovibrio alaskensis G20 during syntrophic growth on lactate

Abstract

Syntrophic growth involves the oxidation of organic compounds and subsequent transfer of electrons to an H2- or formate-consuming micro-organism. In order to identify genes involved specifically in syntrophic growth, a mutant library of Desulfovibrio alaskensis G20 was screened for loss of the ability to grow syntrophically with Methanospirillum hungatei JF-1. A collection of 20 mutants with an impaired ability to grow syntrophically was obtained. All 20 mutants grew in pure culture on lactate under sulfidogenic conditions at a rate and to a maximum OD600 similar to those of the parental strain. The largest number of mutations that affected syntrophic growth with lactate was in genes encoding proteins involved in H2 oxidation, electron transfer, hydrogenase post-translational modification, pyruvate degradation and signal transduction. The qrcB gene, encoding a quinone reductase complex (Qrc), and cycA, encoding the periplasmic tetrahaem cytochrome c 3 (TpIc3), were required by G20 to grow syntrophically with lactate. A mutant in the hydA gene, encoding an Fe-only hydrogenase (Hyd), is also impaired in syntrophic growth with lactate. The other mutants grew more slowly than the parental strain in syntrophic culture with M. hungatei JF-1. qrcB and cycA were shown previously to be required for growth of G20 pure cultures with H2 and sulfate. Washed cells of the parental strain produced H2 from either lactate or pyruvate, but washed cells of qrcB, cycA and hydA mutants produced H2 at rates similar to the parental strain from pyruvate and did not produce significant amounts of H2 from lactate. Real-time quantitative PCR assays showed increases in expression of the above three genes during syntrophic growth compared with pure-culture growth with lactate and sulfate. Our work shows that Hyd, Qrc and TpIc3 are involved in H2 production during syntrophic lactate metabolism by D. alaskensis G20 and emphasizes the importance of H2 production for syntrophic lactate metabolism in this strain.