Extracellular electron transfer in a syntrophic coculture of Geobacter sulfurreducens and Wolinella succinogenes

Syntrophic cocultures of Geobacter sulfurreducens and Wolinella succinogenes oxidize acetate with nitrate as terminal electron acceptor. It has been postulated earlier that the electrons are transferred in these cocultures not via hydrogen, but via a different carrier, e.g., a small c-type cytochrome. Actual a small (9.6 kDa) periplasmic cytochrome with a standard redox potential of - 169 mV was released by G. sulfurreducens during growth in pure as well as in coculture into the supernatant. This cytochrome was reduced by G. sulfurreducens and reoxidized by W. succinogenes. However, the release of the Cytochrome depends to a large extend on the growth conditions and is artificially stimulated. During analysis of concentrated culture supernatants other proteins were found, including at least two different cytochromes. The addition of cytochrome did neither stimulate growth of the coculture nor acetate oxidation in coculture suspensions. Therefore a specific electron transfer via the 9.6 kDa cytochrome in the observed coculture could be excluded. A release of quinoid substances as a possible electron carrier could not be confirmed. The internal pattern of quinones did not change during growth in coculture. Thus, the production of specific quinones during growth in coculture seems to be unlikely. In the present study, L-cysteine, which was provided as a reducing agent, was found to mediate the electrons between the two partners. Low concentrations of L-cysteine or L-cystine (10 - 100 µM) supported syntrophic growth, and no acetate oxidation was observed in the absence of cysteine or cystine. Cell suspensions of G. sulfurreducens or coculture cell suspensions reduced cystine to cysteine, and suspensions of W. succinogenes or coculture cell suspensions oxidized cysteine with nitrate, as measured by the formation or depletion of free thiol groups. Added cysteine was rapidly oxidized by the coculture during growth but the formed cystine was not entirely rereduced even under acceptor-limited conditions. The redox potential prevealing in acetate-oxidizing cocultures was 170 to 230 mV. Sulfide at low concentration supported syntrophic growth as well and could replace cysteine. Neither growth nor acetate oxidation could be observed with D-cysteine, homocysteine, cysteamine, 3-mercaptopropionate, dithiothreithol, thioglycolate, glutathione, coenzyme M, dimethylsulfoxide, trimethylamine-N-oxide, anthraquinone-2,6-disulfonate, or ascorbate