Metabolism of acetylene by Nocardia rhodochrous

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

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.

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can lead to advances by others that leave its originators 'scooped' and mulling about what next to do with their unpublished data, specifically what journals could it still be published in and be perceived as original.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.

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.

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.

Acetylene degradation by new isolates of aerobic bacteria and comparison of acetylene hydratase enzymes

Aerobic acetylene-degrading bacteria were isolated from soil samples. Two isolates were assigned to the species Rhodococcus opacus, two others to Rhodococcus ruber and Gordona sp. They were compared with known strains of aerobic acetylene-, cyanide-, or nitrile-utilizing bacteria. The acetylene hydratases of R opacus could be measured in cell-free extracts only in the presence of a strong reductant like titanium(III) citrate. Expression of these enzymes was molybdenum-dependent. Acetylene hydratases in cell-free extracts of R ruber and Gordona spp. did not require addition of reductants. No cross-reactivity could be found between cell-free extracts of any of these aerobic isolates and antibodies raised against the acetylene hydratase of the strictly anaerobic fermenting bacterium Pelobacter acetylenicus. These results show that acetylene hydratases are a biochemically heterogeneous group of enzymes.

Acetylene as a substrate in the development of primordial bacterial communities

The fermentation of atmospheric acetylene by anaerobic bacteria is proposed as the basis of a primordial heterotrophic food chain. The accumulation of fermentation products (acetaldehyde, ethanol, acetate and hydrogen) would create niches for sulfate-respiring bacteria as well as methanogens. Formation of acetylene-free environments in soils and sediments would also alter the function of nitrogenase from detoxification to nitrogen-fixation. The possibility of an acetylene-based anaerobic food chain in Jovian-type atmospheres is discussed.

Acetylene Metabolism and Stimulation of Denitrification in an Agricultural Soil

The effects of C 2 H 2 metabolism on N 2 O production were examined in soil slurries. Enrichment of C 2 H 2 consumption activity occurred only in aerobic incubations. Rapid disappearance of subsequent C 2 H 2 additions, stimulation of CO 2 production, and most-probable-number enumerations of C 2 H 2 utilizers indicated enrichment of the population responsible. During C 2 H 2 consumption in slurries incubated statically under air, maximal rates of N 2 O evolution were 19 times higher than those in anaerobic incubations. After 20 days of enrichment with C 2 H 2 , the production of N 2 O by slurries supplemented with C 2 H 2 and nitrate was 10 times higher than that in the unenriched controls. A Nocardia - or Arthrobacter -like bacterium was isolated that grew on C 2 H 2 but did not denitrify. The behavior of soil inoculated with this bacterium became similar to that of C 2 H 2 -enriched soil incubated aerobically. Ethanol, acetate, and acetaldehyde were identified in enrichment experiments, and denitrification in soil slurries was stimulated by addition of the supernatant from a pure culture grown on mineral medium with C 2 H 2 . These results indicate that denitrification can be stimulated by the actions of an aerobic, nondenitrifying C 2 H 2 -metabolizing population. Utilization of intermediate metabolites by denitrifiers and enhanced O 2 consumption are two possible mechanisms for this stimulation.