The Calvin–Benson–Bassham (CBB) cycle assimilates CO ₂ for the primary production of organic matter in all plants and algae, as well as in some autotrophic bacteria. The key enzyme of the CBB cycle, ribulose-bisphosphate carboxylase/oxygenase (RubisCO), is a main determinant of de novo organic matter production on Earth. Of the three carboxylating forms of RubisCO, forms I and II participate in autotrophy, and form III so far has been associated only with nucleotide and nucleoside metabolism. Here, we report that form III RubisCO functions in the CBB cycle in the thermophilic chemolithoautotrophic bacterium Thermodesulfobium acidiphilum, a phylum-level lineage representative. We further show that autotrophic CO ₂ fixation in T. acidiphilum is accomplished via the transaldolase variant of the CBB cycle, which has not been previously demonstrated experimentally and has been considered unlikely to occur. Thus, this work reveals a distinct form of the key pathway of CO ₂ fixation.

A new obligately anaerobic sulfate-reducing microorganism, strain 3127-1T, was isolated from geothermally heated soil (Oil Site, Uzon Caldera, Kamchatka, Russia). The new isolate was a moderately thermoacidophilic anaerobe able to grow on H2 or formate by sulfate or thiosulfate respiration. The pH range for growth was 3.7 - 6.5, with an optimum at 4.8-5.0. The temperature range for growth was 37 - 65 °C, with an optimum at 55 °C. The G+C content of genomic DNA was 33.7%. The genome of strain 3127-1T contained two almost identical 16S rRNA genes, differing by a single nucleotide substitution. The closest 16S rRNA gene sequence of a validly published species belonged to Thermodesulfobium narugense Na82T (99.5% identity). However, the average nucleotide identity of genomes of strain 3127-1T and T. narugense Na82T and the predicted DNA-DNA hybridization value (GGDC 2.1 BLAST+, formula 2) were as low as 86% and 32.5 ± 2.5%, respectively. This, together with phenotypic data, showed the new isolate to belong to a new species, for which we propose the name Thermodesulfobium acidiphilum sp. nov., with the type strain 3127-1T (=DSM 102892 T =VKM B-3043 T).

One of the important current issues of bioenergetics is the establishment of the thermodynamic limits of life. There is still no final understanding of what is the minimum value of the energy yield of a reaction that is sufficient to be used by an organism (the so-called “biological quantum of energy”). A reasonable model for determination of the minimal energy yield would be microorganisms capable of living on low-energy substrates, such as acetogenic prokaryotes. The most prominent metabolic feature of acetogens is autotrophic growth with molecular hydrogen and carbon dioxide as the substrates, which is hardly competitive in environments. Most probably, that is why only facultative autotrophic acetogens have been known so far. Here, we describe the first obligately autotrophic acetogenic bacterium Aceticella autotrophica gen. nov., sp. nov., strain 3443-3Ac^T. Phylogenetically, the new genus falls into a monophyletic group of heterotrophic bacteria of the genera Thermoanaerobacterium, Thermoanaerobacter, and Caldanaerobacter (hereinafter referred to as TTC group), where the sole acetogenic representative has so far been the facultatively autotrophic Thermoanaerobacter kivui. A. autotrophica and T. kivui both are acetogens employing energy-converting hydrogenase (Ech-acetogens) that are likely to have inherited the acetogenesis capacity vertically from common ancestor. However, their acetogenic machineries have undergone different adjustments by gene replacements due to horizontal gene transfers from different donors. Obligate autotrophy of A. autotrophica is associated with the lack of many sugar transport systems and carbohydrate catabolism enzymes that are present in other TTC group representatives, including T. kivui.

Microbial communities of the Kamchatka Peninsula terrestrial hot springs were studied using radioisotopic and cultural approaches, as well as by the amplification and sequencing of dsrB and 16S rRNA genes fragments. Radioisotopic experiments with ³⁵S-labeled sulfate showed that microbial communities of the Kamchatka hot springs are actively reducing sulfate. Both the cultivation experiments and the results of dsrB and 16S rRNA genes fragments analyses indicated the presence of microorganisms participating in the reductive part of the sulfur cycle. It was found that sulfate-reducing prokaryotes (SRP) belonging to Desulfobacterota, Nitrospirota and Firmicutes phyla inhabited neutral and slightly acidic hot springs, while bacteria of phylum Thermodesulofobiota preferred moderately acidic hot springs. In high-temperature acidic springs sulfate reduction was mediated by archaea of the phylum Crenarchaeota, chemoorganoheterotrophic representatives of genus Vulcanisaeta being the most probable candidates. The 16S rRNA taxonomic profiling showed that in most of the studied communities SRP was present only as a minor component. Only in one microbial community, the representatives of genus Vulcanisaeta comprised a significant group. Thus, in spite of comparatively low sulfate concentrations in terrestrial hot springs of the Kamchatka, phylogenetically and metabolically diverse groups of sulfate-reducing prokaryotes are operating there coupling carbon and sulfur cycles in these habitats.

An anaerobic sulfate-reducing micro-organism, strain 3408-1T, was isolated from a terrestrial hot spring in Kamchatka peninsula (Russia). The cells were spore-forming rods with a Gram-positive type of cell wall. The new isolate was a moderately thermoacidophilic anaerobe able to grow either by sulfate or thiosulfate respiration with H2 or formate as substrates, or by fermenting yeast extract, maltose, sucrose, glucose and pyruvate. The fermentation products were acetate, CO2 and H2. The pH range for growth was 2.9–6.5, with an optimum at 4.5. The temperature range for growth was 42–70 °C, with an optimum at 55 °C. The G+C content of DNA was 58 mol%. Phylogenetic analysis of the 16S rRNA gene showed that strain 3408-1T belongs to the family Thermoanaerobacteraceae , order Thermoanaerobacterales and was distantly related to the species of the genus Ammonifex (93–94 % sequence similarity). On the basis of physiological properties and results of phylogenetic analysis, strain 3408-1T is considered to represent a novel species of a new genus, for which the name Desulfothermobacter acidiphilus gen. nov., sp. nov. is proposed. The type strain is 3408-1T (=DSM 105356T=VKM B-3183T).