Methane Oxidation and the Competition for Oxygen in the Rice Rhizosphere

A mechanistic approach is presented to describe oxidation of the greenhouse gas methane in the rice rhizosphere of flooded paddies by obligate methanotrophic bacteria. In flooded rice paddies these methanotrophs compete for available O₂ with other types of bacteria. Soil incubation studies and most-probable-number (MPN) counts of oxygen consumers show that microbial oxygen consumption rates were dominated by heterotrophic and methanotrophic respiration. MPN counts of methanotrophs showed large spatial and temporal variability. The most abundant methanotrophs (a Methylocystis sp.) and heterotrophs (a Pseudomonas sp. and a Rhodococcus sp.) were isolated and characterized. Growth dynamics of these bacteria under carbon and oxygen limitations are presented. Theoretical calculations based on measured growth dynamics show that methanotrophs were only able to outcompete heterotrophs at low oxygen concentrations (frequently <5 μM). The oxygen concentration at which methanotrophs won the competition from heterotrophs did not depend on methane concentration, but it was highly affected by organic carbon concentrations in the paddy soil. Methane oxidation was severely inhibited at high acetate concentrations. This is in accordance with competition experiments between Pseudomonas spp. and Methylocystis spp. carried out at different oxygen and carbon concentrations. Likely, methane oxidation mainly occurs at microaerophilic and low-acetate conditions and thus not directly at the root surface. Acetate and oxygen concentrations in the rice rhizosphere are in the critical range for methane oxidation, and a high variability in methane oxidation rates is thus expected.     

Enrichment and characteristics of mixed methane-oxidizing bacteria from a Chinese coal mine

In methane-rich environments, methane-oxidizing bacteria usually occur predominantly among consortia including other types of microorganisms. In this study, artificial coal bed gas and methane gas were used to enrich mixed methanotrophic cultures from the soil of a coal mine in China, respectively. The changes in microbial community structure and function during the enrichment were examined. The microbial diversity was reduced as the enrichment proceeded, while the capacity for methane oxidation was significantly enhanced by the increased abundance of methanotrophs. The proportion of type II methanotrophs increased greatly from 7.84 % in the sampled soil to about 50 % in the enrichment cultures, due to the increase of methane concentration. After the microbial community of the cultures got stable, Methylomonas and Methylocystis became the dominant type I and type II methanotrophs, while Methylophilus was the prevailing methylotroph. The sequences affiliated with pigment-producing strains, Methylomonas rubra, Hydrogenophaga sp. AH-24, and Flavobacterium cucumis, could explain the orange appearance of the cultures. Comparing the two cultures, the multi-carbon sources in the artificial coal bed gas caused more variety of non-methanotrophic bacteria, but did not help to maintain the diversity or to increase the quantity and activity of methanotrophs. The results could help to understand the succession and interaction of microbial community in a methane-driven ecosystem.     

Comparison of RNA- and DNA-based bacterial communities in a lab-scale methane-degrading biocover

Methanotrophs must become established and active in a landfill biocover for successful methane oxidation. A lab-scale biocover with a soil mixture was operated for removal of methane and nonmethane volatile organic compounds, such as dimethyl sulfide (DMS), benzene (B), and toluene (T). The methane elimination capacity was 211?±?40 g?m^?2 d^?1 at inlet loads of 330–516 g?m^?2 d^?1. DMS, B, and T were completely removed at the bottom layer (40–50 cm) with inlet loads of 221.6?±?92.2, 99.6?±?19.5, and 23.4?±?4.9 mg m^?2 d^?1, respectively. The bacterial community was examined based on DNA and RNA using ribosomal tag pyrosequencing. Interestingly, methanotrophs comprised 80 % of the active community (RNA) while 29 % of the counterpart (DNA). Types I and II methanotrophs equally contributed to methane oxidation, and Methylobacter, Methylocaldum, and Methylocystis were dominant in both communities. The DNA vs. RNA comparison suggests that DNA-based analysis alone can lead to a significant underestimation of active members.     

Methane oxidation activity and diversity of aerobic methanotrophs in pH-neutral and semi-neutral thermal springs of the Kunashir Island,Russian Far East

Aerobic methane oxidation has been mostly studied in environments with moderate to low temperatures. However, the process also occurs in terrestrial thermal springs, where little research on the subject has been done to date. The potential activity of methane oxidation and diversity of aerobic methanotrophic bacteria were studied in sediments of thermal springs with various chemical and physical properties, sampled across the Kunashir Island, the Kuriles archipelago. Activity was measured by means of the radioisotope tracer technique utilizing ¹⁴C-labeled methane. Biodiversity assessments were based on the particulate methane monooxygenase (pmoA) gene, which is found in all known thermophilic and thermotolerant methanotrophs. We demonstrated the possibility of methane oxidation in springs with temperature exceeding 74 °C, and the most intensive methane uptake was shown in springs with temperatures about 46 °C. PmoA was detected in 19 out of 30 springs investigated and the number of pmoA gene copies varied between 10⁴ and 10⁶ copies per ml of sediment. Phylogenetic analysis of PmoA sequences revealed the presence of methanotrophs from both the Alpha- and Gammaproteobacteria. Our results suggest that methanotrophs inhabiting thermal springs with temperature exceeding 50 °C may represent novel thermophilic and thermotolerant species of the genera Methylocystis and Methylothermus, as well as previously undescribed Gammaproteobacteria.     

Effects of ammonium on the activity and community of methanotrophs in landfill biocover soils

The influence of NH₄⁺ on microbial CH₄ oxidation is still poorly understood in landfill cover soils. In this study, effects of NH₄⁺ addition on the activity and community structure of methanotrophs were investigated in waste biocover soil (WBS) treated by a series of NH₄⁺-N contents (0, 100, 300, 600 and 1200 mg kg⁻¹). The results showed that the addition of NH₄⁺-N ranging from 100 to 300 mg kg⁻¹ could stimulate CH₄ oxidation in the WBS samples at the first stage of activity, while the addition of an NH₄⁺-N content of 600 mg kg⁻¹ had an inhibitory effect on CH₄ oxidation in the first 4 days. The decrease of CH₄ oxidation rate observed in the last stage of activity could be caused by nitrogen limitation and/or exopolymeric substance accumulation. Type I methanotrophs Methylocaldum and Methylobacter, and type II methanotrophs (Methylocystis and Methylosinus) were abundant in the WBS samples. Of these, Methylocaldum was the main methanotroph in the original WBS. With incubation, a higher abundance of Methylobacter was observed in the treatments with NH₄⁺-N contents greater than 300 mg kg⁻¹, which suggested that NH₄⁺-N addition might lead to the dominance of Methylobacter in the WBS samples. Compared to type I methanotrophs, the abundance of type II methanotrophs Methylocystis and/or Methylosinus was lower in the original WBS sample. An increase in the abundance of Methylocystis and/or Methylosinus occurred in the last stage of activity, and was likely due to a nitrogen limitation condition. Redundancy analysis showed that NH₄⁺-N and the C/N ratio had a significant influence on the methanotrophic community in the WBS sample.     

Microbial community and function of enrichment cultures with methane and toluene

The interaction effect of co-existence of toluene and CH₄ on community and activity of methanotrophs and toluene-degrading bacteria was characterized in three consortia enriched with CH₄ and toluene (MT), toluene (T), and CH₄ (M), respectively, in this study. The CH₄ oxidation activity in the enrichment culture of MT was significantly lower than that of M at the end of the experiment (P?=?0.001). The toluene degradation rate could be enhanced by continuous addition of CH₄ and toluene in the initial days, but it was inhibited in the later days. Phylogenetic analysis of 16S rRNA genes showed that Proteobacteria and Bacteroidetes were dominant in the three enriched consortia, but the community of methanotrophs and toluene-degrading bacteria was significantly affected by the co-existence of CH₄ and toluene. Both Methylosinus (91.8 %) and Methylocystis (8.2 %) were detected in the enrichment culture of MT, while only Methylocystis species were detected in M. The toluene-degrading bacteria including Burkholderia, Flavobacteria, Microbacterium, and Azoarcus were all detected in the enrichment culture of T. However, only Azoarcus was found in the enrichment culture of MT. Significantly higher contents of extracellular polymeric substances polysaccharose and protein in the enrichment culture of MT than that of T and M suggested that a higher environmental stress occurred in the enrichment culture of MT.     

Molecular analysis of high-affinity methane-oxidizing enrichment cultures isolated from a forest biocenosis and agrocenoses

Methane oxidation by microorganisms inhabiting aerobic soils is a key process involved in the regulation of the concentration of this significant greenhouse gas in the atmosphere; however, the microorganisms responsible for this process remain unknown. Three stable methane-oxidizing cultures were isolated from samples of forest soils (FS) and agricultural soils (AS) of Moscow oblast, as well as from soil samples collected from a Belgian agrocenosis (BS). The obtained enrichment cultures exhibit a high affinity for methane; their k_m values range from 54.2 to 176.8 nM CH₄ and are comparable to those of aerobic soils. Analysis of the fragments of the ribosomal (16S rRNA) and functional (pmoA) genes of methanotrophs by PCR— DGGE and cloning demonstrated the presence of bacteria belonging to the genera Methylocystis in FS, Methylosinus in AS and BS, and Methylocella in BS. It was established that Methylocystis and Methylosinus detected in the enrichment cultures contain the genes encoding the synthesis of the active center of two membrane-bound particulate methane monooxygenases; it is likely that one of these genes (pmoA2) is responsible for the capacity of these microorganisms for oxidation of atmospheric methane.     

Tobermolite effects on methane removal activity and microbial community of a lab-scale soil biocover

Three identical lab-scale biocovers were packed with an engineered soil (BC 1), tobermolite only (BC 2), and a mixture of the soil and tobermolite (BC 3), and were operated at an inlet load of 338–400 g-CH₄ m^?2 d^?1 and a space velocity of 0.12 h^?1. The methane removal capacity was 293 ± 47 g-CH₄ m^?2 d^?1 in steady state in the BC 3, which was significantly higher than those in the BC 1 and BC 2 (106 ± 24 and 114 ± 48 g-CH₄ m^?2 d^?1, respectively). Quantitative PCR indicated that bacterial and methanotrophic densities (6.62–6.78 × 10⁷ 16S rDNA gene copy number g-dry sample^?1 and 1.37–2.23 × 10⁷ pmoA gene copy number g-dry sample^?1 in the BC 1 and BC 3, respectively) were significantly higher than those in the BC 2. Ribosomal tag pyrosequencing showed that methanotrophs comprised approximately 60 % of the bacterial community in the BC 2 and BC 3, while they only comprised 43 % in the BC 1. The engineered soil favored the growth of total bacteria including methanotrophs, while the presence of tobermolite enhanced the relative abundance of methanotrophs, resulting in an improved habitat for methanotrophs as well as greater methane mitigation performance in the mixture. Moreover, a batch experiment indicated that the soil and tobermolite mixture could display a stable methane oxidation level over wide temperature (20–40 °C, at least 38 μmol g-dry sample^?1 h^?1) and pH (5–8, at least 61 μmol g-dry sample^?1 h^?1) ranges. In conclusion, the soil and tobermolite mixture is promising for methane mitigation.     

Methane oxidation and methane driven redox process during sequential reduction of a flooded soil ecosystem

A laboratory incubation study conducted to assess the temporal variation of CH₄ oxidation during soil reduction processes in a flooded soil ecosystem. A classical sequence of microbial terminal electron accepting process observed following NO₃ ^? reduction, Fe³⁺ reduction, SO₄ ^2? reduction and CH₄ production in flooded soil incubated under initial aerobic and helium-flushed anaerobic conditions. CH₄ oxidation in the slurries was influenced by microbial redox process during slurry reduction. Under aerobic headspace condition, CH₄ oxidation rate (k) was stimulated by 29 % during 5 days (NO₃ ^? reduction) and 32 % during both 10 days (Fe³⁺) and 20 days (early SO₄ ^2? reduction) over unreduced slurry. CH₄ oxidation was inhibited at the later methanogenic period. Contrastingly, CH₄ oxidation activity in anaerobic incubated slurries was characterized with prolonged lag phase and lower CH₄ oxidation. Higher CH₄ oxidation rate in aerobically incubated flooded soil was related to high abundance of methanotrophs (r?=?0.994, p?<?0.01) and ammonium oxidizers population (r?=?0.184, p?<?0.05). Effect of electron donors NH₄ ⁺, Fe^2+, S^2? on CH₄ oxidation assayed to define the interaction between reduced inorganic species and methane oxidation. The electron donors stimulated CH₄ oxidation as well as increased the abundance of methanotrophic microbial population except S^2? which inhibited the methanotrophic activity by affecting methane oxidizing bacterial population. Our result confirmed the complex interaction between methane-oxidizing microbial groups and redox species during sequential reduction processes of a flooded soil ecosystem.     

Genome Analysis Coupled with Physiological Studies Reveals a Diverse Nitrogen Metabolism in Methylocystis sp. Strain SC2

Background

Methylocystis sp. strain SC2 can adapt to a wide range of methane concentrations. This is due to the presence of two isozymes of particulate methane monooxygenase exhibiting different methane oxidation kinetics. To gain insight into the underlying genetic information, its genome was sequenced and found to comprise a 3.77 Mb chromosome and two large plasmids.

Principal Findings

We report important features of the strain SC2 genome. Its sequence is compared with those of seven other methanotroph genomes, comprising members of the Alphaproteobacteria, Gammaproteobacteria, and Verrucomicrobia. While the pan-genome of all eight methanotroph genomes totals 19,358 CDS, only 154 CDS are shared. The number of core genes increased with phylogenetic relatedness: 328 CDS for proteobacterial methanotrophs and 1,853 CDS for the three alphaproteobacterial Methylocystaceae members, Methylocystis sp. strain SC2 and strain Rockwell, and Methylosinus trichosporium OB3b. The comparative study was coupled with physiological experiments to verify that strain SC2 has diverse nitrogen metabolism capabilities. In correspondence to a full complement of 34 genes involved in N₂ fixation, strain SC2 was found to grow with atmospheric N₂ as the sole nitrogen source, preferably at low oxygen concentrations. Denitrification-mediated accumulation of 0.7 nmol ³⁰N₂/hr/mg dry weight of cells under anoxic conditions was detected by tracer analysis. N₂ production is related to the activities of plasmid-borne nitric oxide and nitrous oxide reductases.

Conclusions/Perspectives

Presence of a complete denitrification pathway in strain SC2, including the plasmid-encoded nosRZDFYX operon, is unique among known methanotrophs. However, the exact ecophysiological role of this pathway still needs to be elucidated. Detoxification of toxic nitrogen compounds and energy conservation under oxygen-limiting conditions are among the possible roles. Relevant features that may stimulate further research are, for example, absence of CRISPR/Cas systems in strain SC2, high number of iron acquisition systems in strain OB3b, and large number of transposases in strain Rockwell.     

Contribution of Methanotrophic and Nitrifying Bacteria to CH4 and NH4+ Oxidation in the Rhizosphere of Rice Plants as Determined by New Methods of Discrimination

Methanotrophic and nitrifying bacteria are both able to oxidize CH₄ as well as NH₄⁺. To date it is not possible to estimate the relative contribution of methanotrophs to nitrification and that of nitrifiers to CH₄ oxidation and thus to assess their roles in N and C cycling in soils and sediments. This study presents new options for discrimination between the activities of methanotrophs and nitrifiers, based on the competitive inhibitor CH₃F and on recovery after inhibition with C₂H₂. By using rice plant soil as a model system, it was possible to selectively inactivate methanotrophs in soil slurries at a CH₄/CH₃F/NH₄⁺ molar ratio of 0.1:1:18. This ratio of CH₃F to NH₄⁺ did not affect ammonia oxidation, but methane oxidation was inhibited completely. By using the same model system, it could be shown that after 24 h of exposure to C₂H₂ (1,000 parts per million volume), methanotrophs recovered within 24 h while nitrifiers stayed inactive for at least 3 days. This gave an “assay window” of 48 h when only methanotrophs were active. Applying both assays to model microcosms planted with rice plants demonstrated a major contribution of methanotrophs to nitrification in the rhizosphere, while the contribution of nitrifiers to CH₄ oxidation was insignificant.     

Mevalonate production by engineered acetogen biocatalyst during continuous fermentation of syngas or CO2/H2 blend

Naturally mevalonate-resistant acetogen Clostridium sp. MT1243 produced only 425 mM acetate during syngas fermentation. Using Clostridium sp. MT1243 we engineered biocatalyst selectively producing mevalonate from synthesis gas or CO₂/H₂ blend. Acetate production and spore formation were eliminated from Clostridium sp. MT1243 using Cre-lox66/lox71-system. Cell energy released via elimination of phosphotransacetylase, acetate kinase and early stage sporulation genes powered mevalonate accumulation in fermentation broth due to expression of synthetic thiolase, HMG-synthase, and HMG-reductase, three copies of each, integrated using Tn7-approach. Recombinants produced 145 mM mevalonate in five independent single-step fermentation runs 25 days each in five repeats using syngas blend 60 % CO and 40 % H₂ (v/v) (p < 0.005). Mevalonate production was 97 mM if only CO₂/H₂ blend was fed instead of syngas (p < 0.005). Mevalonate from CO₂/H₂ blend might serve as a commercial route to mitigate global warming in proportion to CO₂ fermentation scale worldwide.     

Enrichment culture and identification of endophytic methanotrophs isolated from peatland plants

Aerobic methane-oxidizing bacteria (MOB) are an environmentally significant group of microorganisms due to their role in the global carbon cycle. Research conducted over the past few decades has increased the interest in discovering novel genera of methane-degrading bacteria, which efficiently utilize methane and decrease the global warming effect. Moreover, methanotrophs have more promising applications in environmental bioengineering, biotechnology, and pharmacy. The investigations were undertaken to recognize the variety of endophytic methanotrophic bacteria associated with Carex nigra, Vaccinium oxycoccus, and Eriophorum vaginatum originating from Moszne peatland (East Poland). Methanotrophic bacteria were isolated from plants by adding sterile fragments of different parts of plants (roots and stems) to agar mineral medium (nitrate mineral salts (NMS)) and incubated at different methane values (1–20% CH4). Single colonies were streaked on new NMS agar media and, after incubation, transferred to liquid NMS medium. Bacterial growth dynamics in the culture solution was studied by optical density—OD600 and methane consumption. Changes in the methane concentration during incubation were controlled by the gas chromatography technique. Characterization of methanotrophs was made by fluorescence in situ hybridization (FISH) with Mg705 and Mg84 for type I methanotrophs and Ma450 for type II methanotrophs. Identification of endophytes was performed after 16S ribosomal RNA (rRNA) and mmoX gene amplification. Our study confirmed the presence of both types of methanotrophic bacteria (types I and II) with the predominance of type I methanotrophs. Among cultivable methanotrophs, there were different strains of the genus Methylomonas and Methylosinus. Furthermore, we determined the potential of the examined bacteria for methane oxidation, which ranged from 0.463 ± 0.067 to 5.928 ± 0.169 μmol/L CH4/mL/day.     

Viable methanotrophic bacteria enriched from air and rain can oxidize methane at cloud-like conditions

Atmospheric methane is degraded by both photooxidation and, in topsoils, by methanotrophic bacteria, but this may not totally account for the global sink of this greenhouse gas. Topsoils are a prominent source of airborne bacteria, which can degrade some organic atmospheric compounds at rates similar to photooxidation. Although airborne methanotrophs would have direct access to atmospheric methane, their presence and activity in the atmosphere has not been investigated so far. We enriched airborne methanotrophs from air and rainwater and showed that they oxidized methane at atmospheric concentration. The majority of seven OTUs, detected using pmoA gene clone libraries, were affiliated to the type II methanotrophic genera Methylocystis and Methylosinus. Furthermore, 16S rRNA gene clone libraries revealed the presence of OTUs affiliated with the genera Hyphomicrobium and Variovorax, members of which can stimulate methane oxidation by yet unidentified mechanisms. Simulating cloud-like conditions revealed that although both low pH and the presence of common cloud-borne organics negatively affected methane oxidation, airborne methanotrophs were able to degrade atmospheric methane in most cases. We demonstrate here for the first time that viable methanotrophic bacteria are present in air and rain and thus expand our knowledge on the global distribution of methanotrophs to include the atmosphere. The fact that they can degrade methane to below atmospheric concentrations when inoculated into artificial cloud water leads to an important possible effect of these organisms: the atmosphere may not only function as a medium for microbial dissemination, but also as a site of active microbial methane turnover.     

Methanotrophic bacteria in cold seeps of the floodplains of northern rivers

Small mud volcanoes (cold seeps), which are common in the floodplains of northern rivers, are potentially important (although poorly studied) sources of atmospheric methane. Field research on the cold seeps of the Mukhrina River (Khanty-Mansiysk Autonomous okrug, Russia) revealed methane fluxes from these structures to be orders of magnitude higher than from equivalent areas of the mid-taiga bogs. Microbial communities developing around the seeps were formed under conditions of high methane concentrations, low temperatures (3–5°C), and near-neutral pH. Molecular identification of methane-oxidizing bacteria from this community by analysis of the pmoA gene encoding particulate methane monooxygenase revealed both type I and type II methanotrophs (classes Gammaproteobacteria and Alphaproteobacteria, respectively), with prevalence of type I methanotrophs. Among the latter, microorganisms related to Methylobacter psychrophilus and Methylobacter tundripaludum, Crenothrix polyspora (a stagnant water dweller), and a number of methanotrophs belonging to unknown taxa were detected. Growth characteristics of two methanotrophic isolates were determined. Methylobacter sp. CMS7 exhibited active growth at 4–10°C, while Methylocystis sp. SB12 grew better at 20°C. Experimental results confirmed the major role of methanotrophic gammaproteobacteria in controlling the methane emission from cold river seeps.     

Abundance and diversity of methanotrophic Gammaproteobacteria in northern wetlands

Numeric abundance, identity, and pH preferences of methanotrophic Gammaproteobacteria (type I methanotrophs) inhabiting the northern acidic wetlands were studied. The rates of methane oxidation by peat samples from six wetlands of European Northern Russia (pH 3.9–4.7) varied from 0.04 to 0.60 μg CH₄ g^?1 peat h^?1. The number of cells revealed by hybridization with fluorochrome labeled probes M84 + M705 specific for type I methanotrophs was 0.05–2.16 × 10⁵ cells g^?1 dry peat, i.e., 0.4–12.5% of the total number of methanotrophs and 0.004–0.39% of the total number of bacteria. Analysis of the fragments of the pmoA gene encoding particulate methane monooxygenase revealed predominance of the genus Methylocystis (92% of the clones) in the studied sample of acidic peat, while the proportion of the pmoA sequences of type I methanotrophs was insignificant (8%). PCR amplification of the 16S rRNA gene fragments of type I methanotrophs with TypeIF-Type IR primers had low specificity, since only three sequences out of 53 analyzed belonged to methanotrophs and exhibited 93–99% similarity to those of Methylovulum, Methylomonas, and Methylobacter species. Isolates of type I methanotrophs obtained from peat (strains SH10 and 83A5) were identified as members of the species Methylomonas paludis and Methylovulum miyakonense, respectively. Only Methylomonas paludis SH10 was capable of growth in acidic media (pH range for growth 3.8–7.2 with the optimum at pH 5.8–6.2), while Methylovulum miyakonense 83A5 exhibited the typical growth characteristics of neutrophilic methanotrophs (pH range for growth 5.5–8.0 with the optimum at pH 6.5–7.5).     

Methanotrophic community structure and activity under warming and grazing of alpine meadow on the Tibetan Plateau

Knowledge about methanotrophs and their activities is important to understand the microbial mediation of the greenhouse gas CH₄ under climate change and human activities in terrestrial ecosystems. The effects of simulated warming and sheep grazing on methanotrophic abundance, community composition, and activity were studied in an alpine meadow soil on the Tibetan Plateau. There was high abundance of methanotrophs (1.2–3.4 × 10⁸ pmoA gene copies per gram of dry weight soil) assessed by real-time PCR, and warming significantly increased the abundance regardless of grazing. A total of 64 methanotrophic operational taxonomic units (OTUs) were obtained from 1,439 clone sequences, of these OTUs; 63 OTUs (98.4%) belonged to type I methanotrophs, and only one OTU was Methylocystis of type II methanotrophs. The methanotroph community composition and diversity were not apparently affected by the treatments. Warming and grazing significantly enhanced the potential CH₄ oxidation activity. There were significantly negative correlations between methanotrophic abundance and soil moisture and between methanotrophic abundance and NH₄–N content. The study suggests that type I methanotrophs, as the dominance, may play a key role in CH₄ oxidation, and the alpine meadow has great potential to consume more CH₄ under future warmer and grazing conditions on the Tibetan Plateau.     

Coal-Packed Methane Biofilter for Mitigation of Green House Gas Emissions from Coal Mine Ventilation Air

Methane emitted by coal mine ventilation air (MVA) is a significant greenhouse gas. A mitigation strategy is the oxidation of methane to carbon dioxide, which is approximately twenty-one times less effective at global warming than methane on a mass-basis. The low non-combustible methane concentrations at high MVA flow rates call for a catalytic strategy of oxidation. A laboratory-scale coal-packed biofilter was designed and partially removed methane from humidified air at flow rates between 0.2 and 2.4 L min⁻¹ at 30°C with nutrient solution added every three days. Methane oxidation was catalysed by a complex community of naturally-occurring microorganisms, with the most abundant member being identified by 16S rRNA gene sequence as belonging to the methanotrophic genus Methylocystis. Additional inoculation with a laboratory-grown culture of Methylosinus sporium, as investigated in a parallel run, only enhanced methane consumption during the initial 12 weeks. The greatest level of methane removal of 27.2±0.66 g methane m⁻³ empty bed h⁻¹ was attained for the non-inoculated system, which was equivalent to removing 19.7±2.9% methane from an inlet concentration of 1% v/v at an inlet gas flow rate of 1.6 L min⁻¹ (2.4 min empty bed residence time). These results show that low-cost coal packing holds promising potential as a suitable growth surface and contains methanotrophic microorganisms for the catalytic oxidative removal of methane.     

Isolation and characterization of marine diesel oil-degrading Acinetobacter sp. strain Y2

The marine diesel oil-degrading bacterium Acinetobacter sp. strain Y2 was isolated from oil-polluted seawater sampled from Dinghai port, Zhoushan City, Zhejiang Province, China. The isolated bacterium was identified as Acinetobacter sp. based on its 16S rDNA gene sequence as well as various morphological and physiological characteristics. The degradation characteristics of strain Y2 were studied and its parameters for oil degradation optimized. These optimal conditions were determined to be an initial pH of 7.5, an incubation temperature of 30 °C, an initial diesel oil concentration of 2 % (v/v), and an initial inoculating bacteria concentration of 3?×?10⁷ cells/mL. The results from the gas chromatography–mass spectrometry analysis showed that strain Y2 could almost completely degrade all components of diesel oil, with a degradation ratio of up to 80 % after 10 days of incubation at the optimal conditions.