Influence of Acetylene on Growth of Sulfate-Respiring Bacteria

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.

Molybdenum-Based Diazotrophy in a Sphagnum Peatland in Northern Minnesota

Microbial N₂ fixation (diazotrophy) represents an important nitrogen source to oligotrophic peatland ecosystems, which are important sinks for atmospheric CO₂ and are susceptible to the changing climate. The objectives of this study were (i) to determine the active microbial group and type of nitrogenase mediating diazotrophy in an ombrotrophic Sphagnum-dominated peat bog (the S1 peat bog, Marcell Experimental Forest, Minnesota, USA); and (ii) to determine the effect of environmental parameters (light, O₂, CO₂, and CH₄) on potential rates of diazotrophy measured by acetylene (C₂H₂) reduction and ¹⁵N₂ incorporation. A molecular analysis of metabolically active microbial communities suggested that diazotrophy in surface peat was primarily mediated by Alphaproteobacteria (Bradyrhizobiaceae and Beijerinckiaceae). Despite higher concentrations of dissolved vanadium ([V] 11 nM) than molybdenum ([Mo] 3 nM) in surface peat, a combination of metagenomic, amplicon sequencing, and activity measurements indicated that Mo-containing nitrogenases dominate over the V-containing form. Acetylene reduction was only detected in surface peat exposed to light, with the highest rates observed in peat collected from hollows with the highest water contents. Incorporation of ¹⁵N₂ was suppressed 90% by O₂ and 55% by C₂H₂ and was unaffected by CH₄ and CO₂ amendments. These results suggest that peatland diazotrophy is mediated by a combination of C₂H₂-sensitive and C₂H₂-insensitive microbes that are more active at low concentrations of O₂ and show similar activity at high and low concentrations of CH₄ IMPORTANCE Previous studies indicate that diazotrophy provides an important nitrogen source and is linked to methanotrophy in Sphagnum-dominated peatlands. However, the environmental controls and enzymatic pathways of peatland diazotrophy, as well as the metabolically active microbial populations that catalyze this process, remain in question. Our findings indicate that oxygen levels and photosynthetic activity override low nutrient availability in limiting diazotrophy and that members of the Alphaproteobacteria (Rhizobiales) catalyze this process at the bog surface using the molybdenum-based form of the nitrogenase enzyme.

Examining the impact of acetylene on N-fixation and the active sediment microbial community

Here we examined the impact of a commonly employed method used to measure nitrogen fixation, the acetylene reduction assay (ARA), on a marine sediment community. Historically, the ARA technique has been broadly employed for its ease of use, in spite of numerous known artifacts. To gauge the severity of these effects in a natural environment, we employed high-throughput 16S rRNA gene sequencing to detect differences in acetylene-treated sediments vs. non-treated control sediments after a 7 h incubation. Within this short time period, significant differences were seen across all activity of microbes identified in the sediment, implying that the changes induced by acetylene occur quickly. The results have important implications for our understanding of marine nitrogen budgets. Moreover, because the ARA technique has been widely used in terrestrial and freshwater habitats, these results may be applicable to other ecosystems.

Acetylene as fast food: implications for development of life on anoxic primordial Earth and in the outer solar system

Acetylene occurs, by photolysis of methane, in the atmospheres of jovian planets and Titan. In contrast, acetylene is only a trace component of Earth's current atmosphere. Nonetheless, a methane-rich atmosphere has been hypothesized for early Earth; this atmosphere would also have been rich in acetylene. This poses a paradox, because acetylene is a potent inhibitor of many key anaerobic microbial processes, including methanogenesis, anaerobic methane oxidation, nitrogen fixation, and hydrogen oxidation. Fermentation of acetylene was discovered approximately 25 years ago, and Pelobacter acetylenicus was shown to grow on acetylene by virtue of acetylene hydratase, which results in the formation of acetaldehyde. Acetaldehyde subsequently dismutates to ethanol and acetate (plus some hydrogen). However, acetylene hydratase is specific for acetylene and does not react with any analogous compounds. We hypothesize that microbes with acetylene hydratase played a key role in the evolution of Earth's early biosphere by exploiting an available source of carbon from the atmosphere and in so doing formed protective niches that allowed for other microbial processes to flourish. Furthermore, the presence of acetylene in the atmosphere of a planet or planetoid could possibly represent evidence for an extraterrestrial anaerobic ecosystem.

Effect of Specific Inhibitors on the Anaerobic Reductive Dechlorination of 2,4,6-Trichlorophenol by a Stable Methanogenic Consortium

The transformation of 2,4,6-trichlorophenol (TCP) into 4-chlorophenol (4CP) was studied using a stable methanogenic enrichment culture derived from an anaerobic fixed bed reactor. Using acetate as a growth substrate, different inhibitors of methanogenesis exhibited distinct effects on TCP dechlorination. Whereas reductive dechlorination activity was not affected by 2% ethylene in the gas phase, 25 mM bromoethanesulfonic acid (BESA) had a direct inhibitory effect on this process. The choice of BESA as a specific inhibitor for identifying the subpopulations involved in reductive dechlorination of chloroaromatics is thus questionable. Inhibitors of sulfate reduction such as molybdate (20 mM) and selenate (20 mM) had a direct inhibitory effect on reductive dechlorination independently of the presence of sulfate in the medium supplemented with acetate as growth substrate. Consequently much more care must also be taken with these inhibitors to prove that reductive chlorination is coupled to sulfate reduction.

Effect of acetylene on nitrous oxide reduction and sulfide oxidation in batch and gradient cultures of Thiobacillus denitrificans

Anaerobic enrichment cultures with H2S and N2O as substrates which were inoculated with a biofilm sample showed rapid growth and gas formation after 2 to 3 days at 27 degrees C. By using the deep-agar dilution technique, a pure culture was obtained. The strain was tentatively identified as Thiobacillus denitrificans. The isolate was used for batch and gradient culture studies under denitrifying conditions, oxidizing H2S with concomitant reduction of N2O to N2. In batch culture, oxidation of H2S was stepwise, with transient accumulation of elemental sulfur; the final oxidation product was SO4(2-). In gradient culture, there was no notable accumulation of elemental sulfur and microsensor measurements of H2S and N2O showed that H2S was oxidized directly to SO4(2-). In the presence of C2H2, however, oxidation of H2S stopped at the level of elemental sulfur and no SO4(2-) was produced in either batch or gradient cultures. This is a hitherto unknown inhibitory effect of C2H2. The inhibition is suggested to occur at the level of sulfite reductase, which catalyzes the oxidation of elemental sulfur to SO3(2-) in T. denitrificans. However, reduction of N2O in this strain was, surprisingly, not affected by C2H2. The isolate is the first chemolithoautotrophic organism shown to reduce N2O in the presence of C2H2. Denitrification in natural ecosystems is often quantified as N2O accumulation after C2H2 addition. However, the presence of large numbers of similar organisms with C2H2-insensitive N2O reduction could lead to underestimation of in situ rates.

Relationship between nitrogen-fixing sulfate reducers and fermenters in salt marsh sediments and roots of Spartina alterniflora

A combination of inhibitors and carbon substrates was used to determine the relative contribution of sulfate-reducing bacteria (SRB) and fermenting bacteria to nitrogen fixation in a salt marsh sediment and on the roots of Spartina alterniflora. Because a lag period precedes acetylene-reducing activity (ARA) in amended sediments, an extensive analysis was done to be sure that this activity was due to the activation of dormant cells, not simply to cell proliferation. Since ARA was not affected by metabolic inhibitors such as rifampin, nalidixic acid, or methionine sulfoximine, it appeared that cell growth was not responsible for this activity. Instead, dormant cells were being activated by the added energy source. Molybdate inhibition studies with glucose-amended sediment slurries indicated that ARA in the upper 5 cm of the salt marsh was due primarily (70%) to SRB and that below that level (5 to 10 cm) it was due primarily (greater than 90%) to fermenting bacteria. ARA associated with washed roots of intact S. alterniflora plants was not inhibited by molybdate, which indicates that bacteria other than SRB were responsible. However, when the roots were excised from the plant, the activity (per unit of root mass) was 10-fold higher and was severely inhibited by molybdate. While this high activity is probably an artifact, due to the release of oxidizable substrates from the excised roots, it indicates that SRB are present in high numbers on Spartina roots.

Quantification and Removal of Some Contaminating Gases from Acetylene Used to Study Gas-Utilizing Enzymes and Microorganisms

Acetylene generated from various grades of calcium carbide and obtained from commercial- and purified-grade acetylene cylinders was shown to contain high concentrations of various contaminants. Dependent on the source of acetylene, these included, at maximal values, H 2 (0.023%), O 2 (0.779%), N 2 (3.78%), PH 3 (0.06%), CH 4 (0.073%), and acetone (1 to 10%). The concentration of the contaminants in cylinder acetylene was highly dependent on the extent of cylinder discharge. Several conventional methods used to partially purify cylinder acetylene were compared. A small-scale method for extensively purifying acetylene is described. An effect of acetylene quality on acetylene reduction assays conducted with purified nitrogenase from Azotobacter vinelandii was demonstrated.

Sulfide Alleviation of the Acetylene Inhibition of Nitrous Oxide Reduction in Soil

The alleviation of the acetylene blockage of nitrous oxide reduction by sulfide was studied in anaerobically incubated Brookston soil to better characterize the process. Removal of nitrate-derived nitrous oxide from soil amended with acetylene and sulfide occurred earlier in the presence of glucose than it did in its absence. This was attributed to the influence of glucose on nitrous oxide production rather than reduction during the early stages of the soil incubation. Glucose was found to have no effect on reduction of injected nitrous oxide in the presence of acetylene- and sulfide-amended soil, whereas carbon dioxide significantly stimulated reduction. It is suggested that the microorganism(s) involved may use carbon dioxide as a cellular carbon source. The sulfide added to the soil probably did not act solely as an electron donor, as the number of electrons required to reduce the added nitrous oxide in our systems was greater than the amount supplied by the sulfide. The soil pH at which the alleviation occurred was 6.7 and was not affected by the sulfide treatment.