ThermoElectroSynth

Hydrothermal vents are inhabited by electrotrophic microorganisms capable of oxidizing extracellular compounds, such as metals, to power their metabolisms. Electrotrophic bacteria and archaea can be used for Microbial Electrosynthesis, whereby electricity and carbon dioxide are converted into value-added products. However, their diversity is not fully known. Bioelectrochemical reactors allow for the specific enrichment of these microorganisms by providing an electrode as an electron donor. A total of 60 different reactors were set up and inoculated with either a microbial community coming from the acidic (ca. pH 5.5) and hot (ca. 120 C) hydrothermal system of Panarea, Aeolian islands, Italy, or the alkaline (pH 11) and mild (40 C) hydrothermal system of Prony Bay, New Caledonia. With the alkaline sample, no current increase was seen in any of the 15 reactors operated for 6 days under Prony hydrothermal conditions (pH 10, 30-75 C). By contrast, a 6-fold increase on average was observed in reactors operated under the Panarea hydrothermal conditions (pH 4.5-7, 75 C). A Multi-Factor Analysis revealed that these last reactors were the most distinct from the others, owing to their higher current production and archaea abundances (measured through qPCR). Most reactors produced organic acids (up to 2.9 mM in 6 days). Still, coulombic efficiencies indicated that this might have been due to the (electro)fermentation of traces of yeast extract in the medium rather than CO2 fixation. Finally, microbial communities were described by 16S metabarcoding and ordination methods, and potential electrotrophic taxa were identified. These results provide new insights into the distribution and diversity of electrotrophs in shallow-sea hydrothermal vents and allow the identification of potential novel biocatalysts for Microbial Electrosynthesis.