Expanded Genomic Sampling Refines Current Understanding of the Distribution and Evolution of Sulfur Metabolisms in the Desulfobulbales.

The reconstruction of modern and paleo-sulfur cycling relies on understanding the long-term relative contribution of its main actors; these include microbial sulfate reduction (MSR) and microbial sulfur disproportionation (MSD). However, a unifying theory is lacking for how MSR and MSD, with the same enzyme machinery and intimately linked evolutionary histories, perform two drastically different metabolisms. Here, we aim at shedding some light on the distribution, diversity, and evolutionary histories of MSR and MSD, with a focus on the Desulfobulbales as a test case. The Desulfobulbales is a diverse and widespread order of bacteria in the Desulfobacterota (formerly Deltaproteobacteria) phylum primarily composed of sulfate reducing bacteria. Recent culture- and sequence-based approaches have revealed an expanded diversity of organisms and metabolisms within this clade, including the presence of obligate and facultative sulfur disproportionators. Here, we present draft genomes of previously unsequenced species of Desulfobulbales, substantially expanding the available genomic diversity of this clade. We leverage this expanded genomic sampling to perform phylogenetic analyses, revealing an evolutionary history defined by vertical inheritance of sulfur metabolism genes with numerous convergent instances of transition from sulfate reduction to sulfur disproportionation.

Genomic sequence analysis of Dissulfurirhabdus thermomarina SH388 and proposed reassignment to Dissulfurirhabdaceae fam. nov.

Here, we report the draft genome sequence of Dissulfurirhabdus thermomarina SH388. Improved phylogenetic and taxonomic analysis of this organism using genome-level analyses supports assignment of this organism to a novel family within the phylum Desulfobacterota. Additionally, comparative genomic and phylogenetic analyses contextualize the convergent evolution of sulfur disproportionation and potential extracellular electron transfer in this organism relative to other members of the Desulfobacterota.

Draft Genome Sequence of Desulfobacter hydrogenophilus DSM 3380, a Psychrotolerant Sulfate-Reducing Bacterium.

Here, we describe the genome of Desulfobacter hydrogenophilus DSM 3380, a bacterium that belongs to the Desulfobacterales The genome of this strictly anaerobic bacterium capable of sulfate reduction expands our understanding of microbial sulfate reduction in a wide range of environmental conditions.

Draft Genome Sequence of Desulfovibrio sulfodismutans ThAc01, a Heterotrophic Sulfur-Disproportionating Member of the Desulfobacterota.

Here, we describe the genome of Desulfovibrio sulfodismutans ThAc01, a Desulfobacterota member first isolated from freshwater mud and the first strain reported to be capable of growth via sulfur disproportionation. As such, this genome expands our understanding of the diversity of sulfur-disproportionating microorganisms.

Draft Genome Sequence of Acidianus ambivalens DSM 3772, an Aerobic Thermoacidophilic Sulfur Disproportionator.

Here, we describe the genome sequence of Acidianus ambivalens DSM 3772, an archaeon belonging to the Sulfolobales order that was first isolated from continental solfataric fields. This thermoacidophile was sequenced because it utilizes a unique sulfur disproportionation pathway that enables this metabolism under aerobic conditions, in contrast to obligately anaerobic bacterial sulfur disproportionators.

Draft Genome Sequence of Desulfofundulus thermobenzoicus subsp. thermosyntrophicus DSM 14055, a Moderately Thermophilic Sulfate Reducer.

Here, we describe the genome of Desulfofundulus thermobenzoicus subsp. thermosyntrophicus DSM 14055, a member of the Clostridiales that is capable of sulfate reduction coupled to the oxidation of propionate, lactate, pyruvate, and H2/CO2 This genome expands our understanding of microbial sulfate reduction (MSR) in anaerobic methanogenic environments.

Deconstructing the Dissimilatory Sulfate Reduction Pathway: Isotope Fractionation of a Mutant Unable of Growth on Sulfate.

The sulfur isotope record provides key insight into the history of Earth's redox conditions. A detailed understanding of the metabolisms driving this cycle, and specifically microbial sulfate reduction (MSR), is crucial for accurate paleoenvironmental reconstructions. This includes a precise knowledge of the step-specific sulfur isotope effects during MSR. In this study, we aim at resolving the cellular-level fractionation factor during dissimilatory sulfite reduction to sulfide within MSR, and use this measured isotope effect as a calibration to enhance our understanding of the biochemistry of sulfite reduction. For this, we merge measured isotope effects associated with dissimilatory sulfite reduction with a quantitative model that explicitly links net fractionation, reaction reversibility, and intracellular metabolite levels. The highly targeted experimental aspect of this study was possible by virtue of the availability of a deletion mutant strain of the model sulfate reducer Desulfovibrio vulgaris (strain Hildenborough), in which the sulfite reduction step is isolated from the rest of the metabolic pathway owing to the absence of its QmoABC complex (ΔQmo). This deletion disrupts electron flux and prevents the reduction of adenosine phosphosulfate (APS) to sulfite. When grown in open-system steady-state conditions at 10% maximum growth rate in the presence of sulfite and lactate as electron donor, sulfur isotope fractionation factors averaged -15.9‰ (1 σ = 0.4), which appeared to be statistically indistinguishable from a pure enzyme study with dissimilatory sulfite reductase. We coupled these measurements with an understanding of step-specific equilibrium and kinetic isotope effects, and furthered our mechanistic understanding of the biochemistry of sulfite uptake and ensuing reduction. Our metabolically informed isotope model identifies flavodoxin as the most likely electron carrier performing the transfer of electrons to dissimilatory sulfite reductase. This is in line with previous work on metabolic strategies adopted by sulfate reducers under different energy regimes, and has implications for our understanding of the plasticity of this metabolic pathway at the center of our interpretation of modern and palaeo-environmental records.