In natural anoxic environments, photosynthetic anoxygenic bacteria fix CO2 by photoheterotrophy, photoautotrophy or syntrophic anaerobic photosynthesis. Here we describe electroautotrophy, a previously unidentified dark CO2 mode of fixation allowed by the electrosyntrophic interaction between Geobacter metallireducens and Rhodopseudomonas palustris. After the formation of an electrosyntrophic coculture, electrons are transferred either directly or indirectly (via electron shuttles) from G. metallireducens at R. palustris, thus providing reducing power and energy for dark CO2 fixation. Transcriptomic analyzes demonstrated the strong expression of genes encoding the extracellular electron transfer pathway in G. metallireducens and the Calvin-Benson-Bassham carbon fixation cycle in R. palustris. Since sediment is one of the most ubiquitous and abundant niches on Earth, and at depth most of the sedimentary niche is both anoxic and dark, dark carbon uptake provides a metabolic window. for the survival of anoxygenic phototrophs, as well as a still unrecognized contribution to the global carbon cycle.
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