Coupling microbial electrosynthesis to renewable energy sources can provide a promising future technology for carbon dioxide conversion. However, this technology suffers from a limited number of suitable biocatalysts, resulting in a narrow product range. Here, we present the characterization of the first thermoacidophilic electroautotrophic community using chronoamperometric, metagenomic, and 13C-labeling analyses. The cathodic biofilm showed current consumption of up to −80 µA cm−2 over a period of 90 days (−350 mV vs. SHE). Metagenomic analyses identified members of the genera Moorella, Desulfofundulus, Thermodesulfitimonas, Sulfolobus, and Acidianus as potential primary producers of the biofilm, potentially thriving via an interspecies sulfur cycle. Hydrogenases seem to be key for cathodic electron uptake. An isolation campaign led to a pure culture of a Knallgas bacterium from this community. Growth of this organism on cathodes led to increasing reductive currents over time. Transcriptomic analyses revealed a distinct gene expression profile of cells grown at a cathode. Moreover, pressurizable flow cells combined with optical coherence tomography allowed an in situ observation of cathodic biofilm growth. Autotrophic growth was confirmed via isotope analysis. As a natural polyhydroxybutyrate (PHB) producer, this novel species, Kyrpidia spormannii, coupled the production of PHB to CO2 fixation on cathode surfaces.

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BackgroundA future bioeconomy relies on the development of technologies to convert waste into valuable compounds. We present here an attempt to design a biotechnological cascade for the conversion of vegetable waste into acetoin and electrical energy.ResultsA vegetable waste dark fermentation effluent containing mainly acetate, butyrate and propionate was oxidized in a bioelectrochemical system. The achieved average current at a constant anode potential of 0 mV against standard hydrogen electrode was 177.5 ± 52.5 µA/cm2. During this step, acetate and butyrate were removed from the effluent while propionate was the major remaining component of the total organic carbon content comprising on average 75.6%. The key players with regard to carbon oxidation and electrode reduction were revealed using amplicon sequencing and metatranscriptomic analysis. Using nanofiltration, it was possible to concentrate the propionate in the effluent. The effluent was revealed to be a suitable medium for biotechnological production strains. As a proof of principle, the propionate in the effluent of the bioelectrochemical system was converted into the platform chemical acetoin with a carbon recovery of 86%.ConclusionsTo the best of our knowledge this is the first report on a full biotechnological production chain leading from vegetable waste to the production of a single valuable platform chemical that integrates carbon elimination steps leading to the production of the valuable side product electrical energy.Electronic supplementary materialThe online version of this article (10.1186/s12934-018-0937-4) contains supplementary material, which is available to authorized users.

Kyrpidia sp. strain EA-1 is a thermophilic hydrogen-oxidizing bacterium isolated from hydrothermal systems at São Miguel Island, Portugal. Here, we present the complete genome sequence of the strain assembled to a single circular chromosome. The genome spans 3,352,175 bp, with a GC content of 58.7%.

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