Another chemolithotrophic metabolism missing in nature: sulfur comproportionation.

Chemotrophic microorganisms gain energy for cellular functions by catalyzing oxidation-reduction (redox) reactions that are out of equilibrium. Calculations of the Gibbs energy ( ΔG r ) can identify whether a reaction is thermodynamically favourable and quantify the accompanying energy yield at the temperature, pressure and chemical composition in the system of interest. Based on carefully calculated values of ΔG r , we predict a novel microbial metabolism - sulfur comproportionation (3H2 S + SO 4 2 - + 2H+ ⇌ 4S0 + 4H2 O). We show that at elevated concentrations of sulfide and sulfate in acidic environments over a broad temperature range, this putative metabolism can be exergonic ( ΔG r <0), yielding ~30-50 kJ mol-1 . We suggest that this may be sufficient energy to support a chemolithotrophic metabolism currently missing from the literature. Other versions of this metabolism, comproportionation to thiosulfate (H2 S + SO 4 2 - ⇌ S 2 O 3 2 - + H2 O) and to sulfite (H2 S + 3 SO 4 2 - ⇌ 4 SO 3 2 - + 2H+ ), are only moderately exergonic or endergonic even at ideal geochemical conditions. Natural and impacted environments, including sulfidic karst systems, shallow-sea hydrothermal vents, sites of acid mine drainage, and acid-sulfate crater lakes, may be ideal hunting grounds for finding microbial sulfur comproportionators.

Sulfur disproportionating microbial communities in a dynamic, microoxic-sulfidic karst system.

Biogeochemical sulfur cycling in sulfidic karst systems is largely driven by abiotic and biological sulfide oxidation, but the fate of elemental sulfur (S0 ) that accumulates in these systems is not well understood. The Frasassi Cave system (Italy) is intersected by a sulfidic aquifer that mixes with small quantities of oxygen-rich meteoric water, creating Proterozoic-like conditions and supporting a prolific ecosystem driven by sulfur-based chemolithoautotrophy. To better understand the cycling of S0 in this environment, we examined the geochemistry and microbiology of sediments underlying widespread sulfide-oxidizing mats dominated by Beggiatoa. Sediment populations were dominated by uncultivated relatives of sulfur cycling chemolithoautotrophs related to Sulfurovum, Halothiobacillus, Thiofaba, Thiovirga, Thiobacillus, and Desulfocapsa, as well as diverse uncultivated anaerobic heterotrophs affiliated with Bacteroidota, Anaerolineaceae, Lentimicrobiaceae, and Prolixibacteraceae. Desulfocapsa and Sulfurovum populations accounted for 12%-26% of sediment 16S rRNA amplicon sequences and were closely related to isolates which carry out autotrophic S0 disproportionation in pure culture. Gibbs energy (∆Gr ) calculations revealed that S0 disproportionation under in situ conditions is energy yielding. Microsensor profiles through the mat-sediment interface showed that Beggiatoa mats consume dissolved sulfide and oxygen, but a net increase in acidity was only observed in the sediments below. Together, these findings suggest that disproportionation is an important sink for S0 generated by microbial sulfide oxidation in this oxygen-limited system and may contribute to the weathering of carbonate rocks and sediments in sulfur-rich environments.

Sulfur cycling at natural hydrocarbon and sulfur seeps in Santa Paula Creek, CA.

Biogeochemical cycling of sulfur is relatively understudied in terrestrial environments compared to marine environments. However, the comparative ease of access, observation, and sampling of terrestrial settings can expand our understanding of organisms and processes important in the modern sulfur cycle. Furthermore, these sites may allow for the discovery of useful process analogs for ancient sulfur-metabolizing microbial communities at times in Earth's past when atmospheric O2 concentrations were lower and sulfide was more prevalent in Earth surface environments. We identified a new site at Santa Paula Creek (SPC) in Ventura County, CA-a remarkable freshwater, gravel-bedded mountain stream charged with a range of oxidized and reduced sulfur species and heavy hydrocarbons from the emergence of subsurface fluids within the underlying sulfur- and organic-rich Miocene-age Monterey Formation. SPC hosts a suite of morphologically distinct microbial biofacies that form in association with the naturally occurring hydrocarbon seeps and sulfur springs. We characterized the geology, stream geochemistry, and microbial facies and diversity of the Santa Paula Creek ecosystem. Using geochemical analyses and 16S rRNA gene sequencing, we found that SPC supports a dynamic sulfur cycle that is largely driven by sulfide-oxidizing microbial taxa, with contributions from smaller populations of sulfate-reducing and sulfur-disproportionating taxa. This preliminary characterization of SPC revealed an intriguing site in which to study geological and geochemical controls on microbial community composition and to expand our understanding of sulfur cycling in terrestrial environments.

Clean room microbiome complexity impacts planetary protection bioburden.

BACKGROUND: The Spacecraft Assembly Facility (SAF) at the NASA's Jet Propulsion Laboratory is the primary cleanroom facility used in the construction of some of the planetary protection (PP)-sensitive missions developed by NASA, including the Mars 2020 Perseverance Rover that launched in July 2020. SAF floor samples (n=98) were collected, over a 6-month period in 2016 prior to the construction of the Mars rover subsystems, to better understand the temporal and spatial distribution of bacterial populations (total, viable, cultivable, and spore) in this unique cleanroom. RESULTS: Cleanroom samples were examined for total (living and dead) and viable (living only) microbial populations using molecular approaches and cultured isolates employing the traditional NASA standard spore assay (NSA), which predominantly isolated spores. The 130 NSA isolates were represented by 16 bacterial genera, of which 97% were identified as spore-formers via Sanger sequencing. The most spatially abundant isolate was Bacillus subtilis, and the most temporally abundant spore-former was Virgibacillus panthothenticus. The 16S rRNA gene-targeted amplicon sequencing detected 51 additional genera not found in the NSA method. The amplicon sequencing of the samples treated with propidium monoazide (PMA), which would differentiate between viable and dead organisms, revealed a total of 54 genera: 46 viable non-spore forming genera and 8 viable spore forming genera in these samples. The microbial diversity generated by the amplicon sequencing corresponded to ~86% non-spore-formers and ~14% spore-formers. The most common spatially distributed genera were Sphinigobium, Geobacillus, and Bacillus whereas temporally distributed common genera were Acinetobacter, Geobacilllus, and Bacillus. Single-cell genomics detected 6 genera in the sample analyzed, with the most prominent being Acinetobacter. CONCLUSION: This study clearly established that detecting spores via NSA does not provide a complete assessment for the cleanliness of spacecraft-associated environments since it failed to detect several PP-relevant genera that were only recovered via molecular methods. This highlights the importance of a methodological paradigm shift to appropriately monitor bioburden in cleanrooms for not only the aeronautical industry but also for pharmaceutical, medical industries, etc., and the need to employ molecular sequencing to complement traditional culture-based assays. Video abstract.

Draft Genome Sequences of Heat Shock-Tolerant Microbes Isolated from a Spacecraft Assembly Facility.

Heat shock-tolerant microorganisms belonging to the orders Bacillales and Micrococcales were isolated from the Spacecraft Assembly Facility at the Jet Propulsion Laboratory, and 63 draft genome sequences were assembled and identified. Further analyses of these genomes can provide insight into methods for preventing forward contamination.

Living at the Extremes: Extremophiles and the Limits of Life in a Planetary Context.

Prokaryotic life has dominated most of the evolutionary history of our planet, evolving to occupy virtually all available environmental niches. Extremophiles, especially those thriving under multiple extremes, represent a key area of research for multiple disciplines, spanning from the study of adaptations to harsh conditions, to the biogeochemical cycling of elements. Extremophile research also has implications for origin of life studies and the search for life on other planetary and celestial bodies. In this article, we will review the current state of knowledge for the biospace in which life operates on Earth and will discuss it in a planetary context, highlighting knowledge gaps and areas of opportunity.

Corrigendum: Living at the Extremes: Extremophiles and the Limits of Life in a Planetary Context.

[This corrects the article DOI: 10.3389/fmicb.2019.00780.].

Genomic Description of 'Candidatus Abyssubacteria,' a Novel Subsurface Lineage Within the Candidate Phylum Hydrogenedentes.

The subsurface biosphere is a massive repository of fixed carbon, harboring approximately 90% of Earth's microbial biomass. These microbial communities drive transformations central to Earth's biogeochemical cycles. However, there is still much we do not understand about how complex subterranean microbial communities survive and how they interact with these cycles. Recent metagenomic investigation of deeply circulating terrestrial subsurface fluids revealed the presence of several novel lineages of bacteria. In one particular example, phylogenomic analyses do not converge on any one previously identified taxon; here we describe the first full genomic sequences of a new bacterial lineage within the candidate phylum Hydrogenedentes, 'Candidatus Abyssubacteria.' A global survey revealed that members of this proposed lineage are widely distributed in both marine and terrestrial subsurface environments, but their physiological and ecological roles have remained unexplored. Two high quality metagenome assembled genomes (SURF_5: 97%, 4%; SURF_17: 91% and 4% completeness and contamination, respectively) were reconstructed from fluids collected 1.5 kilometers below surface in the former Homestake gold mine-now the Sanford Underground Research Facility (SURF)-in Lead, South Dakota, United States. Metabolic reconstruction suggests versatile metabolic capability, including possible nitrogen reduction, sulfite oxidation, sulfate reduction and homoacetogenesis. This first glimpse into the metabolic capabilities of these cosmopolitan bacteria suggests that they are involved in key geochemical processes, including sulfur, nitrogen, and carbon cycling, and that they are adapted to survival in the dark, often anoxic, subsurface biosphere.

Development of a Custom MALDI-TOF MS Database for Species-Level Identification of Bacterial Isolates Collected From Spacecraft and Associated Surfaces.

Since the 1970s, the Planetary Protection Group at the Jet Propulsion Laboratory (JPL) has maintained an archive of spacecraft-associated bacterial isolates. Identification of these isolates was routinely performed by sequencing the 16S rRNA gene. Although this technique is an industry standard, it is time consuming and has poor resolving power for some closely related taxa. Matrix-assisted laser desorption/ionization time of flight (MALDI-TOF) mass spectrometry is widely used in clinical diagnostics and is a promising method to substitute standard 16S rRNA sequencing. However, manufacturer-provided databases lack the bacterial diversity found in spacecraft-assembly cleanrooms. This study reports the development of the first custom database of MALDI-TOF MS profiles of bacterial isolates obtained from spacecraft and associated cleanroom environments. With the use of this in-house database, 454 bacterial isolates were successfully identified in concurrence with their 16S rRNA sequence-based classifications. Additionally, MALDI-TOF MS resolved strain-level variations, identified potential novel species and distinguished between members of taxonomic groups, which is not possible using conventional 16S rRNA sequencing. MALDI-TOF MS has proved to be an accurate, high-throughput approach for real-time identification of bacterial isolates during the spacecraft assembly process.

The specific and exclusive microbiome of the deep-sea bone-eating snail, Rubyspira osteovora.

Rubyspira osteovora is an unusual deep-sea snail from Monterey Canyon, California. This group has only been found on decomposing whales and is thought to use bone as a novel source of nutrition. This study characterized the gut microbiome of R. osteovora, compared to the surrounding environment, as well as to other deep-sea snails with more typical diets. Analysis of 16S rRNA gene sequences revealed that R. osteovora digestive tissues host a much lower bacterial diversity (average Shannon index of 1.9; n = 12), compared to environmental samples (average Shannon index of 4.4; n = 2) and are dominated by two bacterial genera: Mycoplasma and Psychromonas (comprising up to 56% and 42% average total recovered sequences, respectively). These two bacteria, along with Psychrilyobacter sp. (∼16% average recovered sequences), accounted for between 43% and 92% of the total recovered sequences in individual snail digestive systems, with other OTUs present at much lower proportions. The relative abundance of these three groups remained similar over 6 years of sampling (collection date was not shown to be a significant predictor of community structure), suggesting a long-term association. Furthermore, these bacterial genera were either not present (Mycoplasma and Psychromonas) or at very low abundance (<0.04% for Psychrilyobacter), in environmental samples and other deep-sea gastropods, supporting the uniqueness of the R. osteovora gut microbiome.