Investigation of iron reduction in bottom sediments of alkaline soda lakes resulted in the isolation of a new obligately anaerobic iron-reducing bacterium, strain Z-0531, from Lake Khadyn (Tuva, Russia) sediment samples. The cells of strain Z-0531 are short (1.0–1.5 by 0.3–0.5 µm), motile, non-spore-forming, gram-negative rods. The isolate is an obligate alkaliphile, developing in the pH range of 7.8–10.0, with an optimum at pH 8.6. It does not require NaCl but grows at NaCl concentrations of 0–50 g/l. It can oxidize acetate with such electron acceptors as amorphous Fe(III) hydroxide (AFH), EDTA-Fe(III), anthraquinone-2,6-disulfonate (quinone), Mn(IV), and S⁰. On medium with EDTA-Fe(III), the isolate can oxidize, apart from acetate, ethanol, pyruvate, oxalate, arginine, tartrate, lactate, propionate, and serine. H₂ is not utilized. The reduced products formed during growth with AFH are siderite or magnetite, depending on the growth conditions. The isolate is incapable of fermenting sugars, peptides, and amino acids. Yeast extract or vitamins are required as growth factors. The organism is capable of dinitrogen fixation and harbors the nifH gene. The DNA G+C content is 55.3 mol %. 16S rRNA analysis places strain Z-0531 into the family Geobacteraceae. Its closest relative (93% similarity) is Desulfuromonas palmitatis. Based on phenotypic distinctions and phylogenetic position, it is proposed that this strain be assigned to the new genus and species Geoalkalibacter ferrihydriticus gen. nov., sp. nov. (Z-0531^T-DSMZ-17813-VKMB-2401).

An anaerobic, spore-forming bacterium (strain Z-0521) was isolated from the iron-reducing microbial community enriched from sample of bottom sediments from low-mineralized soda lake Khadyn, Tuva...

Redox-active iron minerals can act as energy sources or electron-transferring mediators in microbial syntrophic associations, being important means of interspecies metabolic cooperation in sedimentary environments. Alkaline conditions alter the thermodynamic stability of iron minerals, influencing their availability for interspecies syntrophic interactions. We have modeled anaerobic alkaliphilic microbial associations in ethanol-oxidizing co-culture of an obligate syntroph Candidatus “Contubernalis alkalaceticum” and a facultative lithotroph Geoalkalibacter ferrihydriticus, which is capable of dissimilatory Fe(III) reduction and homoacetogenic oxidation of Fe(II) with CO₂. The co-cultures were cultivated with thermodynamically metastable ferric-containing ferrihydrite, or ferrous-containing siderite, or without minerals. Mössbauer spectral analysis revealed the transformation of both minerals to the stable magnetite. In the presence of ferrihydrite, G. ferrihydriticus firstly reduced Fe(III) with ethanol and then switched to syntrophic homoacetogenesis, providing the growth of obligate syntroph on ethanol. The ability of G. ferrihydriticus to accept hydrogen from its syntrophic partner and produce extra acetate from carbonate during ethanol oxidation was confirmed by co-culture growth without minerals. In the presence of siderite, G. ferrihydriticus performed homoacetogenesis using two electron donors simultaneously— siderite and hydrogen. Pieces of evidence for direct and indirect hydrogen-mediated electron exchange between partner organisms were obtained. Relative abundancies of partner organisms and the rate of acetate production by their co-cultures were strongly determined by thermodynamic benefits, which G. ferrihydriticus got from redox transformations of iron minerals. Even the minor growth of G. ferrihydriticus sustained the growth of the syntroph. Accordingly, microbe-to-mineral interactions could represent underestimated drivers of syntrophic interactions in alkaline sedimentary environments.

Chukotka is an arctic region located in the continuous permafrost zone, but thermal springs are abundant there. In this study, for the first time, the microbial communities of the Chukotka hot springs (CHS) biofilms and sediments with temperatures 54–94 °C were investigated and analyzed by NGS sequencing of 16S rRNA gene amplicons. In microbial mats (54–75 °C), phototrophic bacteria of genus Chloroflexus dominated (up to 89% of all prokaryotes), while Aquificae were the most numerous at higher temperatures in Fe-rich sediments and filamentous “streamers” (up to 92%). The electron donors typical for Aquificae, such as H₂S and H₂, are absent or present only in trace amounts, and the prevalence of Aquificae might be connected with their ability to oxidize the ferrous iron present in CHS sediments. Armatimonadetes, Proteobacteria, Deinococcus-Thermus, Dictyoglomi, and Thermotogae, as well as uncultured bacteria (candidate divisions Oct-Spa1-106, GAL15, and OPB56), were numerous, and Cyanobacteria were present in low numbers. Archaea (less than 8% of the total community of each tested spring) belonged to Bathyarchaeota, Aigarchaeota, and Thaumarchaeota. The geographical location and the predominantly autotrophic microbial community, built on mechanisms other than the sulfur cycle-based ones, make CHS a special and unique terrestrial geothermal ecosystem.

Using a sample from a terrestrial hot spring (pH 6.8, 60 °C), we enriched a thermophilic microbial consortium performing anaerobic autotrophic oxidation of hydrothermal siderite (FeCO₃), with CO₂/bicarbonate as the electron acceptor and the only carbon source, producing green rust and acetate. In order to reproduce Proterozoic environmental conditions during the deposition of banded iron formation (BIF), we incubated the microbial consortium in a bioreactor that contained an unmixed anoxic layer of siderite, perfectly mixed N₂/CO₂-saturated liquid medium and microoxic (2% O₂) headspace. Long-term incubation (56 days) led to the formation of magnetite (Fe₃O₄) instead of green rust as the main product of Fe(II) oxidation, the precipitation of newly formed metabolically induced siderite in the anoxic zone, and the deposition of hematite (Fe₂O₃) on bioreactor walls over the oxycline boundary. Acetate was the only metabolic product of CO₂/bicarbonate reduction. Thus, we have demonstrated the ability of autotrophic thermophilic microbial consortium to perform a short cycle of iron minerals transformation: siderite–magnetite–siderite, accompanied by magnetite and hematite accumulation. This cycle is believed to have driven the evolution of the early biosphere, leading to primary biomass production and deposition of the main iron mineral association of BIF.