The energy‐conserving electron transfer system used by Desulfovibrio alaskensis strain G20 during pyruvate fermentation involves reduction of endogenously formed fumarate and cytoplasmic and membrane‐bound complexes, Hdr‐Flox and Rnf

Abstract

The adaptation capability of Desulfovibrio to natural fluctuations in electron acceptor availability was evaluated by studying Desulfovibrio alaskensis strain G20 under varying respiratory, fermentative and methanogenic coculture conditions in chemostats. Transition from lactate to pyruvate in coculture resulted in a dramatic shift in the population structure and closer interspecies cell-to-cell interactions. Lower methane production rates in coculture than predicted from pyruvate input was attributed to redirection of electron flow to fumarate reduction. Without a methanogenic partner, accumulation of H₂and formate resulted in greater succinate production. Comparative transcript and gene fitness analysis in concert with physiological data of G20 wildtype and mutants demonstrated that pyruvate fermentation involves respiration of cytoplasmically formed fumarate using cytoplasmic and membrane-bound energy-conserving complexes, Rnf, Hdr-Flox-1 and Hmc. At the low H₂/formate levels maintained in coculture, Rnf likely functions as proton-pumping ferredoxin (Fd): type-I cytochrome c oxidoreductase, which transitions to a proton-pumping Fd(red): nicotinamide adenine dinucleotide (NAD⁺) oxidoreductase at high H₂/formate levels during fermentation in monoculture. Hdr-Flox-1 is postulated to recycle Fd(red) via a flavin-based electron bifurcation involving NADH, Fdox and the thiol/disulphide-containing DsrC. In a menaquinone (MQ)-based electron confurcation reaction, the high-molecular-weight cytochrome-c₃complex, Hmc, is proposed to then couple DsrC(red) and periplasmic H₂/formate oxidation using the MQ pool to fuel a membrane-bound fumarate reductase.

A Post-Genomic View of the Ecophysiology, Catabolism and Biotechnological Relevance of Sulphate-Reducing Prokaryotes

“…Also, mutants lacking a functional Rnf complex failed to grow with these substrates and sulphate as terminal electron acceptor. In contrast, results reported by Meyer et al (2014), with the same D. alaskensis G20 mutant pool, found both Rnf and Hdr/Flx were needed during growth by pyruvate fermentation and pyruvate supported methanogenic co-cultures where substrate-level phosphorylation would be expected to be occurring. Further analysis suggested that fumarate respiration was augmenting the pyruvate fermentation.…”

“…The adh1 gene is one of the most highly expressed genes in D. vulgaris and is the main enzyme responsible for ethanol oxidation (Haveman et al, 2003). The flx-hdr genes are often implicated in expression and proteomic studies of this organism energy metabolism (Caffrey et al, 2007;Haveman et al, 2003;Meyer, Kuehl, Deutschbauer, Price, et al, 2013;Meyer et al, 2014;Ramos et al, 2015;Walker, He, et al, 2009;Zhang, Culley, Scholten, et al, 2006). Very recently, the function of the FlxABCD-HdrABC complex was elucidated in D. vulgaris, which contains a single copy of these genes, where it was shown to be essential for growth with ethanol/sulphate and to be involved in the production of ethanol during fermentation of pyruvate (Ramos et al, 2015).…”

“…Under this condition, part of the pyruvate is converted to fumarate, which is disproportionated. Part of the formed fumarate is used as electron acceptor and reduced to succinate, and the other part is oxidized to acetate and CO 2 yielding ATP through substrate-level phosphorylation via the conversion of acetyl phosphate to acetate Meyer et al, 2014). Two transcriptional studies of growth by pyruvate fermentation compared to lactate/sulphate respiration (Pereira, He, Valente, et al, 2008) or pyruvate/sulphate respiration were reported.…”

“…Global transcriptional studies aiming to detect genes possibly involved during syntrophic growth (Li, McInerney, et al, 2011;Meyer, Kuehl, Deutschbauer, Price, et al, 2013;Plugge et al, 2010;Walker, He, et al, 2009) have often identified upregulation of many of the same genes that were upregulated during fermentative metabolism Meyer et al, 2014) (see above in the same section). This suggests that a major part of the gene response is controlled by the internal redox status of the cell, that is possibly more reducing than in the presence of sulphate, and so might not be a specific response to the syntrophic growth.…”

“…For example, confirmation of regulon predictions by transcriptomics analyses of strains mutated in transcriptional regulators has been reported . Genes altered in expression by or essential for use of specific electron acceptors or growth with syntrophic partners have also been identified Meyer, Kuehl, Deutschbauer, Price, et al, 2013;Meyer et al, 2014;Price et al, 2014). Functional gene phenotypes were also derived from the mutant fitness experiments.…”