Iron or sulfur respiration-an adaptive choice determining the fitness of a natronophilic bacterium Dethiobacter alkaliphilus in geochemically contrasting environments.

Haloalkaliphilic microorganisms are double extremophiles functioning optimally at high salinity and pH. Their typical habitats are soda lakes, geologically ancient yet widespread ecosystems supposed to harbor relict microbial communities. We compared metabolic features and their determinants in two strains of the natronophilic species Dethiobacter alkaliphilus, the only cultured representative of the class "Dethiobacteria" (Bacillota). The strains of D. alkaliphilus were previously isolated from geographically remote Mongolian and Kenyan soda lakes. The type strain AHT1T was described as a facultative chemolithoautotrophic sulfidogen reducing or disproportionating sulfur or thiosulfate, while strain Z-1002 was isolated as a chemolithoautotrophic iron reducer. Here, we uncovered the iron reducing ability of strain AHT1T and the ability of strain Z-1002 for thiosulfate reduction and anaerobic Fe(II) oxidation. Key catabolic processes sustaining the growth of both D. alkaliphilus strains appeared to fit the geochemical settings of two contrasting natural alkaline environments, sulfur-enriched soda lakes and iron-enriched serpentinites. This hypothesis was supported by a meta-analysis of Dethiobacterial genomes and by the enrichment of a novel phylotype from a subsurface alkaline aquifer under Fe(III)-reducing conditions. Genome analysis revealed multiheme c-type cytochromes to be the most probable determinants of iron and sulfur redox transformations in D. alkaliphilus. Phylogeny reconstruction showed that all the respiratory processes in this organism are likely provided by evolutionarily related early forms of unconventional octaheme tetrathionate and sulfite reductases and their structural analogs, OmhA/OcwA Fe(III)-reductases. Several phylogenetically related determinants of anaerobic Fe(II) oxidation were identified in the Z-1002 genome, and the oxidation process was experimentally demonstrated. Proteomic profiling revealed two distinct sets of multiheme cytochromes upregulated in iron(III)- or thiosulfate-respiring cells and the cytochromes peculiar for Fe(II) oxidizing cells. We suggest that maintaining high variation in multiheme cytochromes is an effective adaptive strategy to occupy geochemically contrasting alkaline environments. We propose that sulfur-enriched soda lakes could be secondary habitats for D. alkaliphilus compared to Fe-rich serpentinites, and that the ongoing evolution of Dethiobacterales could retrace the evolutionary path that may have occurred in prokaryotes at a turning point in the biosphere's history, when the intensification of the sulfur cycle outweighed the global significance of the iron cycle.

The first cultivated representatives of the actinobacterial lineage OPB41 isolated from subsurface environments constitute a novel order Anaerosomatales.

The continental subsurface harbors microbial populations highly enriched in uncultured taxa. OPB41 is an uncultured order-level phylogenetic lineage within the actinobacterial class Coriobacteriia. OPB41 bacteria have a wide geographical distribution, but the physiology and metabolic traits of this cosmopolitan group remain elusive. From two contrasting subsurface environments, a terrestrial mud volcano and a deep subsurface aquifer, located in the central part of Eurasia, within the Caucasus petroleum region, we have isolated two pure cultures of anaerobic actinobacteria belonging to OPB41. The cells of both strains are small non-motile rods forming numerous pili-like appendages. Strain M08DHBT is mesophilic, while strain Es71-Z0120T is a true thermophile having a broad temperature range for growth (25-77°C). Strain M08DHBT anaerobically reduces sulfur compounds and utilizes an aromatic compound 3,4-dihydroxybenzoic acid. Strain Es71-Z0120T is an obligate dissimilatory Fe(III) reducer that is unable to utilize aromatic compounds. Both isolates grow lithotrophically and consume molecular hydrogen or formate using either thiosulfate, elemental sulfur, or Fe(III) as an electron acceptor. Genomes of the strains encode the putative reductive glycine pathway for autotrophic CO2 fixation, Ni-Fe hydrogenases, putative thiosulfate/polysulfide reductases, and multiheme c-type cytochromes presumably involved in dissimilatory Fe(III) reduction. We propose to assign the isolated strains to the novel taxa of the species-order levels and describe strain M08DHBT as Anaerosoma tenue gen. nov., sp. nov., and strain Es71-Z0120T as Parvivirga hydrogeniphila gen. nov., sp. nov., being members of Anaerosomatales ord. nov. This work expands the knowledge of the diversity, metabolic functions, and ecological role of the phylum Actinomycetota.