Six CO₂ fixation pathways are known to operate in photoautotrophic and chemoautotrophic microorganisms. Here, we describe chemolithoautotrophic growth of the sulphate-reducing bacterium Desulfovibrio desulfuricans (strain G11) with hydrogen and sulphate as energy substrates. Genomic, transcriptomic, proteomic and metabolomic analyses reveal that D. desulfuricans assimilates CO₂ via the reductive glycine pathway, a seventh CO₂ fixation pathway. In this pathway, CO₂ is first reduced to formate, which is reduced and condensed with a second CO₂ to generate glycine. Glycine is further reduced in D. desulfuricans by glycine reductase to acetyl-P, and then to acetyl-CoA, which is condensed with another CO₂ to form pyruvate. Ammonia is involved in the operation of the pathway, which is reflected in the dependence of the autotrophic growth rate on the ammonia concentration. Our study demonstrates microbial autotrophic growth fully supported by this highly ATP-efficient CO₂ fixation pathway.

The Tinto River is an extreme environment located at the core of the Iberian Pyritic Belt (IPB). It is an unusual ecosystem due to its size (100 km long), constant acidic pH (mean pH, 2.3), and high concentration of heavy metals, iron, and sulfate in its waters, characteristics that make the Tinto River Basin comparable to acidic mine drainage (AMD) systems. In this paper we present an extensive survey of the Tinto River sediment microbiota using two culture-independent approaches: denaturing gradient gel electrophoresis and cloning of 16S rRNA genes. The taxonomic affiliation of the Bacteria showed a high degree of biodiversity, falling into 5 different phyla: Proteobacteria , Firmicutes , Bacteroidetes , Acidobacteria , and Actinobacteria ; meanwhile, all the Archaea were affiliated with the order Thermoplasmatales . Microorganisms involved in the iron ( Acidithiobacillus ferrooxidans , Sulfobacillus spp., Ferroplasma spp., etc.), sulfur ( Desulfurella spp., Desulfosporosinus spp., Thermodesulfobium spp., etc.), and carbon ( Acidiphilium spp., Bacillus spp., Clostridium spp., Acidobacterium spp., etc.) cycles were identified, and their distribution was correlated with physicochemical parameters of the sediments. Ferric iron was the main electron acceptor for the oxidation of organic matter in the most acid and oxidizing layers, so acidophilic facultative Fe(III)-reducing bacteria appeared widely in the clone libraries. With increasing pH, the solubility of iron decreases and sulfate-reducing bacteria become dominant, with the ecological role of methanogens being insignificant. Considering the identified microorganisms—which, according to the rarefaction curves and Good's coverage values, cover almost all of the diversity—and their corresponding metabolism, we suggest a model of the iron, sulfur, and organic matter cycles in AMD-related sediments.

Dysoxic marine waters (DMW, < 1 μM oxygen) are currently expanding in volume in the oceans, which has biogeochemical, ecological and societal consequences on a global scale. In these environments, distinct bacteria drive an active sulfur cycle, which has only recently been recognized for open‐ocean DMW. This review summarizes the current knowledge on these sulfur‐cycling bacteria. Critical bottlenecks and questions for future research are specifically addressed. Sulfate‐reducing bacteria (SRB) are core members of DMW. However, their roles are not entirely clear, and they remain largely uncultured. We found support for their remarkable diversity and taxonomic novelty by mining metagenome‐assembled genomes from the Black Sea as model ecosystem. We highlight recent insights into the metabolism of key sulfur‐oxidizing SUP05 and Sulfurimonas bacteria, and discuss the probable involvement of uncultivated SAR324 and BS‐GSO2 bacteria in sulfur oxidation. Uncultivated Marinimicrobia bacteria with a presumed organoheterotrophic metabolism are abundant in DMW. Like SRB, they may use specific molybdoenzymes to conserve energy from the oxidation, reduction or disproportionation of sulfur cycle intermediates such as S ⁰ and thiosulfate, produced from the oxidation of sulfide. We expect that tailored sampling methods and a renewed focus on cultivation will yield deeper insight into sulfur‐cycling bacteria in DMW.

Prokaryotes in natural environments respond rapidly to high concentrations of chemicals and physical stresses. Exposure to anthropogenic toxic substances—such as oil, chlorinated solvents, or antibiotics—favors the evolution of resistant phenotypes, some of which can use contaminants as an exclusive carbon source or as electron donors and acceptors. Microorganisms similarly adapt to extreme pH, metal, or osmotic stress. The metabolic plasticity of prokaryotes can thus be harnessed for bioremediation and can be exploited in a variety of ways, ranging from stimulated natural attenuation to bioaugmentation and from wastewater treatment to habitat restoration.