Syngas as Electron Donor for Sulfate and Thiosulfate Reducing Haloalkaliphilic Microorganisms in a Gas-Lift Bioreactor

Biodesulfurization processes remove toxic and corrosive hydrogen sulfide from gas streams (e.g., natural gas, biogas, or syngas). To improve the efficiency of these processes under haloalkaline conditions, a sulfate and thiosulfate reduction step can be included. The use of H2/CO mixtures (as in syngas) instead of pure H2 was tested to investigate the potential cost reduction of the electron donor required. Syngas is produced in the gas-reforming process and consists mainly of H2, carbon monoxide (CO), and carbon dioxide (CO2). Purification of syngas to obtain pure H2 implies higher costs because of additional post-treatment. Therefore, the use of syngas has merit in the biodesulfurization process. Initially, CO inhibited hydrogen-dependent sulfate reduction. However, after 30 days the biomass was adapted and both H2 and CO were used as electron donors. First, formate was produced, followed by sulfate and thiosulfate reduction, and later in the reactor run acetate and methane were detected. Sulfide production rates with sulfate and thiosulfate after adaptation were comparable with previously described rates with only hydrogen. The addition of CO marginally affected the microbial community in which Tindallia sp. was dominant. Over time, acetate production increased and acetogenesis became the dominant process in the bioreactor. Around 50% of H2/CO was converted to acetate. Acetate supported biomass growth and higher biomass concentrations were reached compared to bioreactors without CO feed. Finally, CO addition resulted in the formation of small, compact microbial aggregates. This suggests that CO or syngas can be used to stimulate aggregation in haloalkaline biodesulfurization systems.

Genomic analysis of the mesophilic Thermotogae genusMesotogareveals phylogeographic structure and genomic determinants of its distinct metabolism

SummaryThe genusMesotoga, the only described mesophilicThermotogaelineage, is common in mesothermic anaerobic hydrocarbon-rich environments. Besides mesophily,Mesotogadisplays lineage-specific phenotypes, such as no or little H2production and dependence on sulfur-compound reduction, which may influence its ecological role. We used comparative genomics of 18Mesotogastrains (pairwise 16S rRNA identity > 99%) and a transcriptome ofM. primato investigate how life at moderate temperatures affects phylogeography and to interrogate the genomic features of its lineage-specific metabolism. We propose thatMesotogaaccomplish H2oxidation and thiosulfate reduction using a sulfide dehydrogenase and a hydrogenase-complex and that a pyruvate:ferredoxin oxidoreductase acquired fromClostridiais responsible for oxidizing acetate. Phylogenetic analysis revealed three distinctMesotogalineages (89.6-99.9% average nucleotide identity [ANI] within lineages, 79.3-87.6% ANI between lineages) having different geographic distribution patterns and high levels of intra-lineage recombination but little geneflow between lineages. Including data from metagenomes, phylogeographic patterns suggest that geographical separation historically has been more important forMesotogathan hyperthermophilicThermotogaand we hypothesize that distribution ofMesotogais constrained by their anaerobic lifestyle. Our data also suggest that recent anthropogenic activities and environments (e.g., wastewater treatment, oil exploration) have expandedMesotogahabitats and dispersal capabilities.Originality-Significance StatementThis study comprises one of the first whole-genome-based phylogeographic analyses of anaerobic mesophiles, and our data suggest that such microbes are more restricted by geography than are thermophiles (and mesophilic aerobes). This is likely to be a general trait for similar anaerobic organisms – and therefore broadly relevant to and testable in other environments. Moreover,Mesotogabacteria are part of the largely understudied subsurface ecosystem that has relatively recently been recognized as a new and important biosphere. Understanding the forces responsible for the distribution of organisms in the subsurface, as well as the identification of genes responsible forMesotoga’s distinct metabolism, will contribute to the understanding of these communities.