The genus Thioalkalivibrio comprises sulfur-oxidizing bacteria thriving in soda lakes at high pH and salinity. Depending on the geographical location and the season, these lakes can strongly vary in temperature. To obtain a comprehensive understanding of the molecular and physiological adaptations to low temperature, we compared the responses of two Thioalkalivibrio strains to low (10 C) and high (30 C) temperatures. For this, the strains were grown under controlled conditions in chemostats and analyzed for their gene expression (RNA sequencing [RNA-Seq]), membrane lipid composition, and glycine betaine content. The strain Thioalkalivibrio versutus AL2T originated from a soda lake in southeast Siberia that is exposed to strong seasonal temperature differences, including freezing winters, whereas Thioalkalivibrio nitratis ALJ2 was isolated from an East African Rift Valley soda lake with a constant warm temperature the year round. The strain AL2T grew faster than ALJ2 at 10 C, likely due to its 3-fold-higher concentration of the osmolyte glycine betaine. Moreover, significant changes in the membrane lipid composition were observed for both strains, leading to an increase in their unsaturated fatty acid content via the Fab pathway to avoid membrane stiffness. Genes for the transcriptional and translational machinery, as well as for counteracting cold-induced hampering of nucleotides and proteins, were upregulated. Oxidative stress was reduced by induction of vitamin B12 biosynthesis genes, and growth at 10 C provoked downregulation of genes involved in the second half of the sulfur oxidation pathway. Genes for intracellular signal transduction were differentially expressed, and interestingly, AL2T upregulated flagellin expression, whereas ALJ2 downregulated it.

Dimethylsulfoxide (DMSO) has long been known to support anaerobic respiration in a few species of basically aerobic extremely halophilic euryarchaea living in hypersaline lakes. Recently, it has also been shown to be utilized as an additional electron acceptor in basically anaerobic sulfur-reducing haloarchaea. Here we investigated whether haloarchaea would be capable of anaerobic respiration with other two sulfoxides, methionine sulfoxide (MSO) and tetramethylene sulfoxide (TMSO). For this, anaerobic enrichment cultures were inoculated with sediments from hypersaline salt and soda lakes in southwestern Siberia and southern Russia. Positive enrichments were obtained for both MSO and TMSO with yeast extract but not with formate or acetate as the electron donor. Two pure cultures obtained from salt lakes, either with MSO or TMSO, were obligate anaerobes closely related to sulfur-reducing Halanaeroarchaeum sulfurireducens, although the type strain of this genus was unable to utilize any sulfoxides. Two pure cultures isolated from soda lakes were facultatively anaerobic alkaliphilic haloarchaea using O₂, sulfur and sulfoxides as the electron acceptors. One isolate was identical to the previously described sulfur-reducing Natrarchaeobaculum sulfurireducens, while another one, enriched at lower alkalinity, is forming a new species in the genus Halobiforma. Since all isolates enriched with either MSO or TMSO were able to respire all three sulfoxides including DMSO and the corresponding activities were cross-induced, it suggest that a single enzyme of the DMSO-reductase family with a broad substrate specificity is responsible for various sulfoxide-dependent respiration in haloarchaea.

This study provides important new insights on how to achieve high sulfur selectivities and stable gas biodesulfurization process operation in the presence of both methanethiol and H₂S in the feed gas. On the basis of previous research, we hypothesized that a dual bioreactor lineup (with an added anaerobic bioreactor) would favor sulfur-oxidizing bacteria (SOB) that yield a higher sulfur selectivity. Therefore, the focus of the present study was to enrich thiol-resistant SOB that can withstand methanethiol, the most prevalent and toxic thiol in sulfur-containing industrial off gases. In addition, the effect of process conditions on the SOB population dynamics was investigated. The results confirmed that thiol-resistant SOB became dominant with a concomitant increase of the sulfur selectivity from 75 mol% to 90 mol% at a loading rate of 2 mM S methanethiol day⁻¹. The abundant SOB in the inoculum – Thioalkalivibrio sulfidiphilus – was first outcompeted by Alkalilimnicola ehrlichii after which Thioalkalibacter halophilus eventually became the most abundant species. Furthermore, we found that the actual electron donor in our lab-scale biodesulfurization system was polysulfide, and not the primarily supplied sulfide.

Removal of organic and inorganic sulfur compounds from sour gases is required because of their toxicity and atmospheric pollution. The most common are hydrogen sulfide (H₂S) and methanethiol (MT). Under oxygen-limiting conditions about 92 mol% of sulfide is oxidized to sulfur by haloalkaliphilic sulfur-oxidizing bacteria (SOB), whilst the remainder is oxidized either biologically to sulfate or chemically to thiosulfate. MT is spontaneously oxidized to dimethyl disulfide (DMDS), which was found to inhibit the oxidation of sulfide to sulfate. Hence, we assessed the effect of DMDS on product formation in a lab-scale biodesulfurization setup. DMDS was quantified using a newly, in-house developed analytical method. Subsequently, a chemical reaction mechanism was proposed for the formation of methanethiol and dimethyl trisulfide from the reaction between sulfide and DMDS. Addition of DMDS resulted in significant inhibition of sulfate formation, leading to 96 mol% of sulfur formation. In addition, a reduction in the dominating haloalkaliphilic SOB species, Thioalkalivibrio sulfidiphilus, was observed in favor of Thioalkaibacter halophilus as a more DMDS-tolerant with the 50 % inhibition coefficient at 2.37 mM DMDS.

Hypersaline anoxic habitats harbour numerous novel uncultured archaea whose metabolic and ecological roles remain to be elucidated. Until recently, it was believed that energy generation via dissimilatory reduction of sulfur compounds is not functional at salt saturation conditions. Recent discovery of the strictly anaerobic acetotrophic Halanaeroarchaeum compels to change both this assumption and the traditional view on haloarchaea as aerobic heterotrophs. Here we report on isolation and characterization of a novel group of strictly anaerobic lithoheterotrophic haloarchaea, which we propose to classify as a new genus Halodesulfurarchaeum. Members of this previously unknown physiological group are capable of utilising formate or hydrogen as electron donors and elemental sulfur, thiosulfate or dimethylsulfoxide as electron acceptors. Using genome-wide proteomic analysis we have detected the full set of enzymes required for anaerobic respiration and analysed their substrate-specific expression. Such advanced metabolic plasticity and type of respiration, never seen before in haloarchaea, empower the wide distribution of Halodesulfurarchaeum in hypersaline inland lakes, solar salterns, lagoons and deep submarine anoxic brines. The discovery of this novel functional group of sulfur-respiring haloarchaea strengthens the evidence of their possible role in biogeochemical sulfur cycling linked to the terminal anaerobic carbon mineralisation in so far overlooked hypersaline anoxic habitats.