Acetylenotrophic and Diazotrophic Bradyrhizobium sp. Strain I71 from TCE-Contaminated Soils.

Acetylene (C2H2) is a molecule rarely found in nature, with very few known natural sources, but acetylenotrophic microorganisms can use acetylene as their primary carbon and energy source. As of 2018 there were 15 known strains of aerobic and anaerobic acetylenotrophs; however, we hypothesize there may yet be unrecognized diversity of acetylenotrophs in nature. This study expands the known diversity of acetylenotrophs by isolating the aerobic acetylenotroph, Bradyrhizobium sp. strain I71, from trichloroethylene (TCE)-contaminated soils. Strain I71 is a member of the class Alphaproteobacteria and exhibits acetylenotrophic and diazotrophic activities, the only two enzymatic reactions known to transform acetylene. This unique capability in the isolated strain may increase the genus' economic impact beyond agriculture as acetylenotrophy is closely linked to bioremediation of chlorinated contaminants. Computational analyses indicate that the Bradyrhizobium sp. strain I71 genome contains 522 unique genes compared to close relatives. Moreover, applying a novel hidden Markov model of known acetylene hydratase (AH) enzymes identified a putative AH enzyme. Protein annotation with I-TASSER software predicted the AH from the microbe Syntrophotalea acetylenica as the closest structural and functional analog. Furthermore, the putative AH was flanked by horizontal gene transfer (HGT) elements, like that of AH in anaerobic acetylenotrophs, suggesting an unknown source of acetylene or acetylenic substrate in the environment that is selecting for the presence of AH. IMPORTANCE The isolation of Bradyrhizobium strain I71 expands the distribution of acetylene-consuming microbes to include a group of economically important microorganisms. Members of Bradyrhizobium are well studied for their abilities to improve plant health and increase crop yields by providing bioavailable nitrogen. Additionally, acetylene-consuming microbes have been shown to work in tandem with other microbes to degrade soil contaminants. Based on genome, cultivation, and protein prediction analysis, the ability to consume acetylene is likely not widespread within the genus Bradyrhizobium. These findings suggest that the suite of phenotypic capabilities of strain I71 may be unique and make it a good candidate for further study in several research avenues.

Acetylene-Fueled Trichloroethene Reductive Dechlorination in a Groundwater Enrichment Culture.

In aquifers, acetylene (C2H2) is a product of abiotic degradation of trichloroethene (TCE) catalyzed by in situ minerals. C2H2 can, in turn, inhibit multiple microbial processes including TCE dechlorination and metabolisms that commonly support dechlorination, in addition to supporting the growth of acetylenotrophic microorganisms. Previously, C2H2 was shown to support TCE reductive dechlorination in synthetic, laboratory-constructed cocultures containing the acetylenotroph Pelobacter sp. strain SFB93 and Dehalococcoides mccartyi strain 195 or strain BAV1. In this study, we demonstrate TCE and perchloroethene (PCE) reductive dechlorination by a microbial community enriched from contaminated groundwater and amended with C2H2 as the sole electron donor and organic carbon source. The metagenome of the stable, enriched community was analyzed to elucidate putative community functions. A novel anaerobic acetylenotroph in the phylum Actinobacteria was identified using metagenomic analysis. These results demonstrate that the coupling of acetylenotrophy and reductive dechlorination can occur in the environment with native bacteria and broaden our understanding of biotransformation at contaminated sites containing both TCE and C2H2IMPORTANCE Understanding the complex metabolisms of microbial communities in contaminated groundwaters is a challenge. PCE and TCE are among the most common groundwater contaminants in the United States that, when exposed to certain minerals, exhibit a unique abiotic degradation pathway in which C2H2 is a product. C2H2 can act as both an inhibitor of TCE dechlorination and of supporting metabolisms and an energy source for acetylenotrophic bacteria. Here, we combine laboratory microcosm studies with computational approaches to enrich and characterize an environmental microbial community that couples two uncommon metabolisms, demonstrating unique metabolic interactions only yet reported in synthetic, laboratory-constructed settings. Using this comprehensive approach, we have identified the first reported anaerobic acetylenotroph in the phylum Actinobacteria, demonstrating the yet-undescribed diversity of this metabolism that is widely considered to be uncommon.

Got acetylene: a personal research retrospective.

In research, sometimes sheer happenstance and serendipity make for an unexpected discovery. Once revealed and if interesting enough, such a finding and its follow-up investigations can lead to advances by others that leave its originators 'scooped' and mulling about what next to do with their unpublished data, specifically what journals could it still be published in and be perceived as original. This is what occurred with us nearly 40 years ago with regard to our follow-up observations of acetylene fermentation and led us to concoct a 'cock-and-bull' story. We hypothesized about a plausible role for acetylene metabolism in the primordial biogeochemistry of Earth and the possibility of acetylene serving as a key life-sustaining substrate for alien microbes dwelling in the orbs of the outer solar system. With the passage of time, advances were made in whole-genome sequencing coupled with major in silico progress in bioinformatics. In parallel came the results of explorations of the outer solar system (i.e. the Cassini mission to Saturn and its moons). It now appears that these somewhat harebrained ideas of ours, arisen at first out of a sense of desperation, actually ring true in fact, and particularly well in song: 'Tell a tale of cock and bull, Of convincing detail full Tale tremendous, Heav'n defend us! What a tale of cock and bull!' From 'The Yeoman of the Guard' by Gilbert & Sullivan.

Methane, arsenic, selenium and the origins of the DMSO reductase family.

Mononuclear molybdoenzymes of the dimethyl sulfoxide reductase (DMSOR) family catalyze a number of reactions essential to the carbon, nitrogen, sulfur, arsenic, and selenium biogeochemical cycles. These enzymes are also ancient, with many lineages likely predating the divergence of the last universal common ancestor into the Bacteria and Archaea domains. We have constructed rooted phylogenies for over 1,550 representatives of the DMSOR family using maximum likelihood methods to investigate the evolution of the arsenic biogeochemical cycle. The phylogenetic analysis provides compelling evidence that formylmethanofuran dehydrogenase B subunits, which catalyze the reduction of CO2 to formate during hydrogenotrophic methanogenesis, constitutes the most ancient lineage. Our analysis also provides robust support for selenocysteine as the ancestral ligand for the Mo/W atom. Finally, we demonstrate that anaerobic arsenite oxidase and respiratory arsenate reductase catalytic subunits represent a more ancient lineage of DMSORs compared to aerobic arsenite oxidase catalytic subunits, which evolved from the assimilatory nitrate reductase lineage. This provides substantial support for an active arsenic biogeochemical cycle on the anoxic Archean Earth. Our work emphasizes that the use of chalcophilic elements as substrates as well as the Mo/W ligand in DMSORs has indelibly shaped the diversification of these enzymes through deep time.

Arsenolipids in Cultured Picocystis Strain ML and Their Occurrence in Biota and Sediment from Mono Lake, California.

Primary production in Mono Lake, a hypersaline soda lake rich in dissolved inorganic arsenic, is dominated by Picocystis strain ML. We set out to determine if this photoautotrophic picoplankter could metabolize inorganic arsenic and in doing so form unusual arsenolipids (e.g., arsenic bound to 2-O-methyl ribosides) as reported in other saline ecosystems and by halophilic algae. We cultivated Picocystis strain ML on a seawater-based medium with either low (37 µM) or high (1000 µM) phosphate in the presence of arsenite (400 µM), arsenate (800 µM), or without arsenic additions (ca 0.025 µM). Cultivars formed a variety of organoarsenic compounds, including a phytyl 2-O-methyl arsenosugar, depending upon the cultivation conditions and arsenic exposure. When the cells were grown at low P, the organoarsenicals they produced when exposed to both arsenite and arsenate were primarily arsenolipids (~88%) with only a modest content of water-soluble organoarsenic compounds (e.g., arsenosugars). When grown at high P, sequestration shifted to primarily water-soluble, simple methylated arsenicals such as dimethylarsinate; arsenolipids still constituted ~32% of organoarsenic incorporated into cells exposed to arsenate but < 1% when exposed to arsenite. Curiously, Picocystis strain ML grown at low P and exposed to arsenate sequestered huge amounts of arsenic into the cells accounting for 13.3% of the dry biomass; cells grown at low P and arsenite exposure sequestered much lower amounts, equivalent to 0.35% of dry biomass. Extraction of a resistant phase with trifluoroacetate recovered most of the sequestered arsenic in the form of arsenate. Uptake of arsenate into low P-cultivated cells was confirmed by X-ray fluorescence, while XANES/EXAFS spectra indicated the sequestered arsenic was retained as an inorganic iron precipitate, similar to scorodite, rather than as an As-containing macromolecule. Samples from Mono Lake demonstrated the presence of a wide variety of organoarsenic compounds, including arsenosugar phospholipids, most prevalent in zooplankton (Artemia) and phytoplankton samples, with much lower amounts detected in the bottom sediments. These observations suggest a trophic transfer of organoarsenicals from the phytoplankton (Picocystis) to the zooplankton (Artemia) community, with efficient bacterial mineralization of any lysis-released organoarsenicals back to inorganic oxyanions before they sink to the sediments.

Got Selenium?

'There's antimony, arsenic, aluminum, selenium, and hydrogen, and oxygen, and nitrogen and rhenium'-so begins 'The Elements' song (https://www.youtube.com/watch?v=AcS3NOQnsQM), whereby Tom Lehrer (Fig. 1) assiduously deconstructed the many painstaking decades of research effort by scores of scientists in assembling the Periodic Table as primarily based upon the atomic numbers of the elements. Lehrer instead opted for his imaginative rhyme, with its musical meter purloined from the patter song of Major General Stanley ("I am the Very Model of a Modern Major General') as in the Gilbert and Sullivan's operetta 'The Pirates of Penzance'. By some coincidence, however, three of the four named in the first stanza are Group 15 and 16 elements with which I have considerable microbiological research experience. Only one is missing (tellurium). Hence, by futzing with Lehrer's 'libretto' to suit my own needs for this issue of FEMS, I would pose the following introductory re-rearrangement: 'There's antimony, arsenic, selenium, tellurium, and cadmium, and chromium, and calcium and curium'. While this may (or may not) sit well with Mr Lehrer, who at the time of this writing is still living, I hope it does not cause further discomfiture to the collective eternal peace of Professor Dimitri Mendeleev, Sir William Schwenk Gilbert and Sir Arthur Sullivan. Nonetheless, I will use this preface to take departure for the primary subject of this manuscript, namely our efforts on selenium, which is where it all got started.

Arsenic and the gastrointestinal tract microbiome.

Arsenic is a toxin, ranking first on the Agency for Toxic Substances and Disease Registry and the Environmental Protection Agency Priority List of Hazardous Substances. Chronic exposure increases the risk of a broad range of human illnesses, most notably cancer; however, there is significant variability in arsenic-induced disease among exposed individuals. Human genetics is a known component, but it alone cannot account for the large inter-individual variability in the presentation of arsenicosis symptoms. Each part of the gastrointestinal tract (GIT) may be considered as a unique environment with characteristic pH, oxygen concentration, and microbiome. Given the well-established arsenic redox transformation activities of microorganisms, it is reasonable to imagine how the GIT microbiome composition variability among individuals could play a significant role in determining the fate, mobility and toxicity of arsenic, whether inhaled or ingested. This is a relatively new field of research that would benefit from early dialogue aimed at summarizing what is known and identifying reasonable research targets and concepts. Herein, we strive to initiate this dialogue by reviewing known aspects of microbe-arsenic interactions and placing it in the context of potential for influencing host exposure and health risks. We finish by considering future experimental approaches that might be of value.

Bacterially synthesized tellurium nanostructures for broadband ultrafast nonlinear optical applications.

Elementary tellurium is currently of great interest as an element with potential promise in nano-technology applications because of the recent discovery regarding its three two-dimensional phases and the existence of Weyl nodes around its Femi level. Here, we report on the unique nano-photonic properties of elemental tellurium particles [Te(0)], as harvest from a culture of a tellurium-oxyanion respiring bacteria. The bacterially-formed nano-crystals prove effective in the photonic applications tested compared to the chemically-formed nano-materials, suggesting a unique and environmentally friendly route of synthesis. Nonlinear optical measurements of this material reveal the strong saturable absorption and nonlinear optical extinctions induced by Mie scattering over broad temporal and wavelength ranges. In both cases, Te-nanoparticles exhibit superior optical nonlinearity compared to graphene. We demonstrate that biological tellurium can be used for a variety of photonic applications which include their proof-of-concept for employment as ultrafast mode-lockers and all-optical switches.

Why I never worked on anaerobic oxidation of methane (AOM) beyond the unsuccessful attempts of my NRC postdoc at NASA Ames Research Center (Sept. 1976-Sept. 1977).

Anaeobes that consumed methane (and me).

Draft Genome Sequence of Picocystis sp. Strain ML, Cultivated from Mono Lake, California.

The microscopic alga Picocystis sp. strain ML is responsible for recurrent algal blooms in Mono Lake, CA. This organism was characterized by only very little molecular data, despite its prominence as a primary producer in saline environments. Here, we report the draft genome sequence for Picocystis sp. strain ML based on long-read sequencing.

Respiratory Selenite Reductase from Bacillus selenitireducens Strain MLS10.

The putative respiratory selenite [Se(IV)] reductase (Srr) from Bacillus selenitireducens MLS10 has been identified through a polyphasic approach involving genomics, proteomics, and enzymology. Nondenaturing gel assays were used to identify Srr in cell fractions, and the active band was shown to contain a single protein of 80 kDa. The protein was identified through liquid chromatography-tandem mass spectrometry (LC-MS/MS) as a homolog of the catalytic subunit of polysulfide reductase (PsrA). It was found to be encoded as part of an operon that contains six genes that we designated srrE, srrA, srrB, srrC, srrD, and srrF SrrA is the catalytic subunit (80 kDa), with a twin-arginine translocation (TAT) leader sequence indicative of a periplasmic protein and one putative 4Fe-4S binding site. SrrB is a small subunit (17 kDa) with four putative 4Fe-4S binding sites, SrrC (43 kDa) is an anchoring subunit, and SrrD (24 kDa) is a chaperon protein. Both SrrE (38 kDa) and SrrF (45 kDa) were annotated as rhodanese domain-containing proteins. Phylogenetic analysis revealed that SrrA belonged to the PsrA/PhsA clade but that it did not define a distinct subgroup, based on the putative homologs that were subsequently identified from other known selenite-respiring bacteria (e.g., Desulfurispirillum indicum and Pyrobaculum aerophilum). The enzyme appeared to be specific for Se(IV), showing no activity with selenate, arsenate, or thiosulfate, with a Km of 145 ± 53 µM, a Vmax of 23 ± 2.5 µM min-1, and a kcat of 23 ± 2.68 s-1 These results further our understanding of the mechanisms of selenium biotransformation and its biogeochemical cycle.IMPORTANCE Selenium is an essential element for life, with Se(IV) reduction a key step in its biogeochemical cycle. This report identifies for the first time a dissimilatory Se(IV) reductase, Srr, from a known selenite-respiring bacterium, the haloalkalophilic Bacillus selenitireducens strain MLS10. The work extends the versatility of the complex iron-sulfur molybdoenzyme (CISM) superfamily in electron transfer involving chalcogen substrates with different redox potentials. Further, it underscores the importance of biochemical and enzymological approaches in establishing the functionality of these enzymes.

Syntrophotalea acetylenivorans sp. nov., a diazotrophic, acetylenotrophic anaerobe isolated from intertidal sediments.

A Gram-stain-negative, strictly anaerobic, non-motile, rod-shaped bacterium, designated SFB93T, was isolated from the intertidal sediments of South San Francisco Bay, located near Palo Alto, CA, USA. SFB93T was capable of acetylenotrophic and diazotrophic growth, grew at 22-37 °C, pH 6.3-8.5 and in the presence of 10-45 g l-1 NaCl. Phylogenetic analyses based on 16S rRNA gene sequencing showed that SFB93T represented a member of the genus Syntrophotalea with highest 16S rRNA gene sequence similarities to Syntrophotalea acetylenica DSM 3246T (96.6 %), Syntrophotalea carbinolica DSM 2380T (96.5 %), and Syntrophotalea venetiana DSM 2394T (96.7 %). Genome sequencing revealed a genome size of 3.22 Mbp and a DNA G+C content of 53.4 %. SFB93T had low genome-wide average nucleotide identity (81-87.5 %) and <70 % digital DNA-DNA hybridization value with other members of the genus Syntrophotalea. The phylogenetic position of SFB93T within the family Syntrophotaleaceae and as a novel member of the genus Syntrophotalea was confirmed via phylogenetic reconstruction based on concatenated alignments of 92 bacterial core genes. On the basis of the results of phenotypic, genotypic and phylogenetic analyses, a novel species, Syntrophotalea acetylenivorans sp. nov., is proposed, with SFB93T (=DSM 106009T=JCM 33327T=ATCC TSD-118T) as the type strain.

Arsenate-dependent growth is independent of an ArrA mechanism of arsenate respiration in the termite hindgut isolate Citrobacter sp. strain TSA-1.

Citrobacter sp. strain TSA-1 is an enteric bacterium isolated from the hindgut of the termite. Strain TSA-1 displays anaerobic growth with selenite, fumarate, tetrathionate, nitrate, or arsenate serving as electron acceptors, and it also grows aerobically. In regards to arsenate, genome sequencing revealed that strain TSA-1 lacks a homolog for respiratory arsenate reductase, arrAB, and we were unable to obtain amplicons of arrA. This raises the question as to how strain TSA-1 achieves As(V)-dependent growth. We show that growth of strain TSA-1 on glycerol, which it cannot ferment, is linked to the electron acceptor arsenate. A series of transcriptomic experiments were conducted to discern which genes were upregulated during growth on arsenate, as opposed to those on fumarate or oxygen. For As(V), upregulation was noted for 1 of the 2 annotated arsC genes, while there was no clear upregulation for tetrathionate reductase (ttr), suggesting that this enzyme is not an alternative to arrAB as occurs in certain hyperthermophilic archaea. A gene-deletion mutant strain of TSA-1 deficient in arsC could not achieve anaerobic respiratory growth on As(V). Our results suggest that Citrobacter sp. strain TSA-1 has an unusual and as yet undefined means of achieving arsenate respiration, perhaps involving its ArsC as a respiratory reductase as well as a detoxifying agent.

Metabolic Capability and Phylogenetic Diversity of Mono Lake during a Bloom of the Eukaryotic Phototroph Picocystis sp. Strain ML.

Algal blooms in lakes are often associated with anthropogenic eutrophication; however, they can occur without the human introduction of nutrients to a lake. A rare bloom of the alga Picocystis sp. strain ML occurred in the spring of 2016 at Mono Lake, a hyperalkaline lake in California, which was also at the apex of a multiyear-long drought. These conditions presented a unique sampling opportunity to investigate microbiological dynamics and potential metabolic function during an intense natural algal bloom. We conducted a comprehensive molecular analysis along a depth transect near the center of the lake from the surface to a depth of 25 m in June 2016. Across sampled depths, rRNA gene sequencing revealed that Picocystis-associated chloroplasts were found at 40 to 50% relative abundance, greater than values recorded previously. Despite high relative abundances of the photosynthetic oxygenic algal genus Picocystis, oxygen declined below detectable limits below a depth of 15 m, corresponding with an increase in microorganisms known to be anaerobic. In contrast to previously sampled years, both metagenomic and metatranscriptomic data suggested a depletion of anaerobic sulfate-reducing microorganisms throughout the lake's water column. Transcripts associated with photosystem I and II were expressed at both 2 m and 25 m, suggesting that limited oxygen production could occur at extremely low light levels at depth within the lake. Blooms of Picocystis appear to correspond with a loss of microbial activity such as sulfate reduction within Mono Lake, yet microorganisms may survive within the sediment to repopulate the lake water column as the bloom subsides.IMPORTANCE Mono Lake, California, provides a habitat to a unique ecological community that is heavily stressed due to recent human water diversions and a period of extended drought. To date, no baseline information exists from Mono Lake to understand how the microbial community responds to human-influenced drought or algal bloom or what metabolisms are lost in the water column as a consequence of such environmental pressures. While previously identified anaerobic members of the microbial community disappear from the water column during drought and bloom, sediment samples suggest that these microorganisms survive at the lake bottom or in the subsurface. Thus, the sediments may represent a type of seed bank that could restore the microbial community as a bloom subsides. Our work sheds light on the potential photosynthetic activity of the halotolerant alga Picocystis sp. strain ML and how the function and activity of the remainder of the microbial community responds during a bloom at Mono Lake.

Improved ZnS nanoparticle properties through sequential NanoFermentation.

Sequential NanoFermentation (SNF) is a novel process which entails sparging microbially produced gas containing H2S from a primary reactor through a concentrated metal-acetate solution contained in a secondary reactor, thereby precipitating metallic sulfide nanoparticles (e.g., ZnS, CuS, or SnS). SNF holds an advantage over single reactor nanoparticle synthesis strategies, because it avoids exposing the microorganisms to high concentrations of toxic metal and sulfide ions. Also, by segregating the nanoparticle products from biological materials, SNF avoids coating nanoparticles with bioproducts that alter their desired properties. Herein, we report the properties of ZnS nanoparticles formed from SNF as compared with ones produced directly in a primary reactor (i.e., conventional NanoFermentation, or "CNF"), commercially available ZnS, and ZnS chemically synthesized by bubbling H2S gas through a Zn-acetate solution. The ZnS nanoparticles produced by SNF provided improved optical properties due to their smaller crystallite size, smaller overall particle sizes, reduced biotic surface coatings, and reduced structural defects. SNF still maintained the advantages of NanoFermentation technology over chemical synthesis including scalability, reproducibility, and lower hazardous waste burden.

Acetylenotrophy: a hidden but ubiquitous microbial metabolism?

Acetylene (IUPAC name: ethyne) is a colorless, gaseous hydrocarbon, composed of two triple bonded carbon atoms attached to hydrogens (C2H2). When microbiologists and biogeochemists think of acetylene, they immediately think of its use as an inhibitory compound of certain microbial processes and a tracer for nitrogen fixation. However, what is less widely known is that anaerobic and aerobic microorganisms can degrade acetylene, using it as a sole carbon and energy source and providing the basis of a microbial food web. Here, we review what is known about acetylene degrading organisms and introduce the term 'acetylenotrophs' to refer to the microorganisms that carry out this metabolic pathway. In addition, we review the known environmental sources of acetylene and postulate the presence of an hidden acetylene cycle. The abundance of bacteria capable of using acetylene and other alkynes as an energy and carbon source suggests that there are energy cycles present in the environment that are driven by acetylene and alkyne production and consumption that are isolated from atmospheric exchange. Acetylenotrophs may have developed to leverage the relatively high concentrations of acetylene in the pre-Cambrian atmosphere, evolving later to survive in specialized niches where acetylene and other alkynes were produced.

Autotrophic microbial arsenotrophy in arsenic-rich soda lakes.

A number of prokaryotes are capable of employing arsenic oxy-anions as either electron acceptors [arsenate; As(V)] or electron donors [arsenite; As(III)] to sustain arsenic-dependent growth ('arsenotrophy'). A subset of these microorganisms function as either chemoautotrophs or photoautotrophs, whereby they gain sufficient energy from their redox metabolism of arsenic to completely satisfy their carbon needs for growth by autotrophy, that is the fixation of inorganic carbon (e.g. HCO3-) into their biomass. Here we review what has been learned of these processes by investigations we have undertaken in three soda lakes of the western USA and from the physiological characterizations of the relevant bacteria, which include the critical genes involved, such as respiratory arsenate reductase (arrA) and the discovery of its arsenite-oxidizing counterpart (arxA). When possible, we refer to instances of similar process occurring in other, less extreme ecosystems and by microbes other than haloalkaliphiles.

Detection of Diazotrophy in the Acetylene-Fermenting Anaerobe Pelobacter sp. Strain SFB93.

Acetylene (C2H2) is a trace constituent of the present Earth's oxidizing atmosphere, reflecting a mixture of terrestrial and marine emissions from anthropogenic, biomass-burning, and unidentified biogenic sources. Fermentation of acetylene was serendipitously discovered during C2H2 block assays of N2O reductase, and Pelobacter acetylenicus was shown to grow on C2H2 via acetylene hydratase (AH). AH is a W-containing, catabolic, low-redox-potential enzyme that, unlike nitrogenase (N2ase), is specific for acetylene. Acetylene fermentation is a rare metabolic process that is well characterized only in P. acetylenicus DSM3246 and DSM3247 and Pelobacter sp. strain SFB93. To better understand the genetic controls for AH activity, we sequenced the genomes of the three acetylene-fermenting Pelobacter strains. Genome assembly and annotation produced three novel genomes containing gene sequences for AH, with two copies being present in SFB93. In addition, gene sequences for all five compulsory genes for iron-molybdenum N2ase were also present in the three genomes, indicating the cooccurrence of two acetylene transformation pathways. Nitrogen fixation growth assays showed that DSM3426 could ferment acetylene in the absence of ammonium, but no ethylene was produced. However, SFB93 degraded acetylene and, in the absence of ammonium, produced ethylene, indicating an active N2ase. Diazotrophic growth was observed under N2 but not in experimental controls incubated under argon. SFB93 exhibits acetylene fermentation and nitrogen fixation, the only known biochemical mechanisms for acetylene transformation. Our results indicate complex interactions between N2ase and AH and suggest novel evolutionary pathways for these relic enzymes from early Earth to modern days.IMPORTANCE Here we show that a single Pelobacter strain can grow via acetylene fermentation and carry out nitrogen fixation, using the only two enzymes known to transform acetylene. These findings provide new insights into acetylene transformations and adaptations for nutrient (C and N) and energy acquisition by microorganisms. Enhanced understanding of acetylene transformations (i.e., extent, occurrence, and rates) in modern environments is important for the use of acetylene as a potential biomarker for extraterrestrial life and for degradation of anthropogenic contaminants.

Complete Genome Sequences of Two Acetylene-Fermenting Pelobacter acetylenicus Strains.

Acetylene fermentation is a rare metabolism that was serendipitously discovered during C2H2-block assays of N2O reductase. Here, we report the genome sequences of two type strains of acetylene-fermenting Pelobacter acetylenicus, the freshwater bacterium DSM 3246 and the estuarine bacterium DSM 3247.

Complete Genome Sequence of the Acetylene-Fermenting Pelobacter sp. Strain SFB93.

Acetylene fermentation is a rare metabolism that was previously reported as being unique to Pelobacter acetylenicus Here, we report the genome sequence of Pelobacter sp. strain SFB93, an acetylene-fermenting bacterium isolated from sediments collected in San Francisco Bay, CA.