Two-phase partitioning bioreactors for treatment of volatile organic compounds

Two-phase partitioning bioreactors (TPPBs) allow the biological removal of volatile organic compounds (VOCs) from contaminated gas streams at unprecedented rates and concentrations. TPPBs are constructed by adding a non-aqueous phase (e.g. hexadecane, silicone oil) to an aqueous phase that contains the microorganisms responsible for degrading the VOCs. Presence of a water-immiscible phase improves the transfer of hydrophobic substrates (e.g. hexane, oxygen) or reduces the toxicity of inhibitory substances (e.g. benzene, toluene) to the microorganisms present in the aqueous phase. The non-aqueous phase is selected based on cost, safety, good partitioning properties towards the target pollutants, biocompatibility, and non-biodegradability. TPPBs have hitherto been designed as laboratory-scale well-mixed stirred-tank reactors or as biofilters that contain a non-aqueous phase. Scale-up and industrial use of TPPBs require elucidation and modeling of the mechanisms of substrate transfer and uptake; understanding of the mechanisms of microbial selection; identification or synthesis of new inexpensive and robust non-aqueous phases; and generation of suitable guidelines for process design and control.     

Hexane biodegradation in two-liquid phase bioreactors: High-performance operation based on the use of hydrophobic biomass

An innovative operation mode in two-liquid phase bioreactors (TLPB) for the treatment of volatile organic compounds (VOC) was investigated. This mode was based on confining the biocatalytic activity exclusively in the non-aqueous phase (NAP) by using hydrophobic microorganisms. The TLPB was implemented in a 2.5 L stirred tank reactor using 10% (v/v) of silicone oil as NAP and hexane as model VOC. A stable elimination capacity (EC) of 21.0 ± 2.5 g m⁻³ h⁻¹ (corresponding to a removal efficiency of 80%) was recorded for 26 days. The accumulation of inhibitory metabolites resulted in drastic drops in the elimination capacity (EC) and an unstable performance of the system, hexanol being identified as potential inhibitory metabolite. Aqueous culture broth exchange by fresh mineral salt medium at a dilution rate of 0.2 day⁻¹ allowed maintaining a high and sustained VOC removal performance. Dissolved oxygen concentration measurements revealed that the oxidative metabolism was strongly stimulated by the aqueous broth exchange. The temporary blockage of the gas/water/NAP transfer pathway for O₂ highlighted the paramount role of this pathway on the performance of the TLPB based on hydrophobic microorganisms.     

Membrane process for biological treatment of contaminated gas streams

A hollow fiber membrane bioreactor was investigated for control of air emissions of biodegradable volatile organic compounds (VOCs). In the membrane bioreactor, gases containing VOCs pass through the lumen of microporous hydrophobic hollow fiber membranes. Soluble compounds diffuse through the membrane pores and partition into a VOC degrading biofilm. The hollow fiber membranes serve as a support for the microbial population and provide a large surface area for VOC and oxygen mass transfer. Experiments were performed to investigate the effects of toluene loading rate, gas residence time, and liquid phase turbulence on toluene removal in a laboratory-scale membrane bioreactor. Initial acclimation of the microbial culture to toluene occurred over a period of nine days, after which a 70% removal efficiency was achieved at an inlet toluene concentration of 200 ppm and a gas residence time of 1.8 s (elimination capacity of 20 g m-3 min-1). At higher toluene loading rates, a maximum elimination capacity of 42 g m-3 min-1 was observed. In the absence of a biofilm (abiotic operation), mass transfer rates were found to increase with increasing liquid recirculation rates. Abiotic mass transfer coefficients could be estimated using a correlation of dimensionless parameters developed for heat transfer. Liquid phase recirculation rate had no effect on toluene removal when the biofilm was present, however. Three models of the reactor were created: a numeric model, a first-order flat sheet model, and a zero-order flat sheet model. Only the numeric model fit the data well, although removal predicted as a function of gas residence time disagreed slightly with that observed. A modification in the model to account for membrane phase resistance resulted in an underprediction of removal. Sensitivity analysis of the numeric model indicated that removal was a strong function of the liquid phase biomass density and biofilm diffusion coefficient, with diffusion rates below 10(-9) m2 s-1 resulting in decreased removal rates.     

Biodegradation of BTEX vapors in a silicone membrane bioreactor system

The biotreatment of complex mixtures of volatile organic compounds (VOCs) such as benzene, toluene, ethylbenzene, and xylene isomers (BTEX) has been investigated by many workers. However, the majority of the work has dealt with the treatment of aqueous or soil phase contamination. The biological treatment of gas and vapor phase sources of VOC wastes has recently received attention with increased usage of biofilters and bioscrubbers. Although these systems are relatively inexpensive, performance problems associated with biomass plugging, gas channeling, and support media acidification have limited their adoption. In this report we describe the development and evaluation of an alternative biotreatment system that allows rapid diffusion of both BTEX and oxygen through a silicone membrane to an active biofilm. The bioreactor system has a rapid liquid recycle, which facilitates nutrient medium mixing over the biofilm and allows for removal of sloughing cell mass. The system removed BTEX at rates up to 30 μg h⁻¹ cm⁻² of membrane area. BTEX removal efficiencies ranged from 75% to 99% depending on the BTEX concentration and vapor flowrate. Consequently, the system can be used for continuous removal and destruction of BTEX and other potential target VOCs in vapor phase streams. Journal of Industrial Microbiology & Biotechnology (2001) 26, 316–325. Received 14 August 2000/ Accepted in revised form 28 February 2001     

Immobilization materials mixed with activated sludge as column biofilters for the treatment of gaseous stream containing benzene and toluene

Activated sludge has been utilized for the treatment of volatile organic compounds (VOCs) which are emitted from industrial processes. Nevertheless, activated sludge systems often suffer from the problem caused by concentration gradients as well as pressure drops. Channeling is also a major problem in the treatment process. As the bed height of the packed activated sludge system increases, the pressure drop increases accordingly. To solve these problems, we proposed immobilized activated sludge column reactors for treating VOCs in air. The immobilization material used to mix with activated sludge was properly selected in this work. Elemental compositions of these materials were analyzed. In this study, we also proposed a VOC feed system so that more stable inlet concentrations could be achieved. Hence, the equipment and operating costs were reduced and the problem of VOCs leaking from peristaltic pumps was avoided. The moisture content of the system was well maintained and better VOC removal efficiency was achieved. With an operation condition of progressive VOC inlet concentrations, better removal efficiency of benzene and toluene was then obtained. In conclusion, by the utilization of immobilization materials selected from wastes as well as immobilized activated sludge column reactors, significant removal efficiency for both benzene and toluene was demonstrated.     

Performance and Microbial Diversity of a Trickle‐Bed Air Biofilter under Interchanging Contaminants

A trickle‐bed air biofilter (TBAB) was evaluated under conditions of interchanging the feed volatile organic compounds (VOCs) in the sequence methyl ethyl ketone (MEK), toluene, methyl isobutyl ketone (MIBK), styrene, and then back to MEK. The obtained performance results revealed that the biofilter provided high removal efficiency within the critical loading of each VOC, which was previously defined in the non‐interchanging VOC fed biofilter. The biofilter easily acclimated to the oxygenated compounds (MEK and MIBK), but re‐acclimation was delayed for the aromatic compounds (toluene and styrene). Ratios of the molar mass of CO₂ produced per molar mass of VOC removed were investigated. It has been found that the ratios for the aromatic compounds closely resembled the theoretical complete chemical oxidation based ratios while larger differences were encountered with the oxygenated compounds. Denaturing gradient gel electrophoresis (DGGE) analysis of 16S rRNA genes was used to assess the impact of interchanging VOCs on the bacterial community structure in the biofilter. The results from denaturing gradient gel electrophoresis (DGGE) showed that the structure of the microbial community in the biofilter was different after each interchange of VOCs.     

Biodegradation of DDT by stimulation of indigenous microbial populations in soil with cosubstrates

Stimulation of native microbial populations in soil by the addition of small amounts of secondary carbon sources (cosubstrates) and its effect on the degradation and theoretical mineralization of DDT [l,l,l-trichloro-2,2-bis(p-chlorophenyl)ethane] and its main metabolites, DDD and DDE, were evaluated. Microbial activity in soil polluted with DDT, DDE and DDD was increased by the presence of phenol, hexane and toluene as cosubstrates. The consumption of DDT was increased from 23 % in a control (without cosubstrate) to 67, 59 and 56 % in the presence of phenol, hexane and toluene, respectively. DDE was completely removed in all cases, and DDD removal was enhanced from 67 % in the control to ~86 % with all substrates tested, except for acetic acid and glucose substrates. In the latter cases, DDD removal was either inhibited or unchanged from the control. The optimal amount of added cosubstrate was observed to be between 0.64 and 2.6 mg C $ {\text{g}}^{ - 1}_{\text{dry soil}} $ . The CO₂ produced was higher than the theoretical amount for complete cosubstrate mineralization indicating possible mineralization of DDT and its metabolites. Bacterial communities were evaluated by denaturing gradient gel electrophoresis, which indicated that native soil and the untreated control presented a low bacterial diversity. The detected bacteria were related to soil microorganisms and microorganisms with known biodegradative potential. In the presence of toluene a bacterium related to Azoarcus, a genus that includes species capable of growing at the expense of aromatic compounds such as toluene and halobenzoates under denitrifying conditions, was detected.     

Experiments and three phase modelling of a biofilter for the removal of toluene and trichloroethylene

Volatile organic compounds, namely, toluene, trichloroethylene, styrene, etc., disposed off by electronics and polymer industries, are very harmful. The treatment of VOC laden air through biochemical route is one of the potential options for reduction of their concentration in parts per million or parts per billion level. Under the present investigation, a 0.05-m diameter and 0.58-m high trickle bed biofilter has been studied for the removal of VOCs namely toluene and trichloroethylene from a simulated air–VOC mixture using pure strain of Pseudomonas putida (NCIM2650) in immobilized form. Inlet concentrations of VOCs have been varied in two ranges, the lower being 0.20–2.00 g/m³ and higher being 10–20 g/m³, respectively. The Monod type rate kinetics of removal of VOCs has been determined. A three-phase deterministic mathematical model has been developed taking the simultaneous reaction kinetics and interphase (gas to liquid to biofilm) mass transfer rate of VOCs into consideration. Experimentally determined kinetic parameters and mass transfer coefficients calculated using standard correlations have been used. Concentrations have been simulated for all the three phases. Simulated results based on the model have been compared with the experimental ones for both gas and liquid phases satisfactorily. The mathematical model validated through the successful comparison with experimental data may be utilized for the prediction of performance of biofilters undergoing removal of different VOCs in any further investigation and may be utilized for the scale-up of the system to industrial scale.     

Microbial community related to volatile organic compound (VOC) emission in household biowaste

Malodorous emissions and potentially pathogenic microorganisms which develop during domestic organic waste collection are not only a nuisance but may also pose health risks. The aim of the present study was to determine whether the presence of specific microorganisms in biowastes is directly related to the composition of the emitted volatile organic compounds (VOCs). The succession of microbial communities during 16 days of storage in organic waste collection bins was studied by denaturing gradient gel electrophoresis (DGGE) of amplified 16S ribosomal DNA in parallel with a classical cultivation and isolation approach. Approximately 60 different bacterial species and 20 different fungal species were isolated. Additionally, some bacterial species were identified through sequencing of excised DGGE bands. Proton transfer reaction mass spectrometry (PTR-MS) was used to detect VOCs over the sampling periods, and co-inertia analyses of VOC concentrations with DGGE band intensities were conducted. Positive correlations, indicating production of the respective VOC or enhancement of microbial growth, and negative correlations, indicating the use of, or microbial inhibition by the respective compound, were found for the different VOCs. Measurement of the VOC emission pattern from a pure culture of Lactococcus lactis confirmed the positive correlations for the protonated masses 89 (tentatively identified as butyric acid), 63 (tentatively identified as dimethylsulfide), 69 (likely isoprene) and 73 (likely butanone).     

Influence of gaseous VOC concentration on the diversity and biodegradation performance of microbial communities

In this work, the influence of toluene gas concentration on the isolation of toluene degrading microbial communities from activated sludge was studied. Toluene biodegradation at gas phase concentration of 10?g?m^?3 (R1) resulted in process instability with removal efficiencies (RE) lesser than 33?%, while operation at toluene gas phase concentrations of 300?mg?m^?3 (R2) and 11?mg?m^?3 (R3) was stable with RE ranging from 74 to 94?%. The consortium isolated in R1 exhibited the highest tolerance toward toluene but the lowest biodegradation performance at trace level VOC concentrations. Despite R2 and R3 showed a similar sensitivity toward toluene toxicity, the microbial community from R2 supported the most efficient toluene biodegradation at trace level VOC concentrations. The Shannon-Wiener index showed an initial biodiversity decrease from 3.2 to 2.0, 1.9 and 2.7 in R1, R2 and R3, respectively. However, while R2 and R3 were able to recover their initial diversity levels by day 48, this loss in diversity was permanent in R1. These results showed that traditional inoculum isolation/acclimation techniques based on the exposure of the inoculum to high VOC concentrations, where toxicity tolerance plays a key role, may result in a poor abatement performance when the off-gas stream is diluted.     

Gas-phase degradation of VOCs using supported bacteria biofilms

Herein we report the use of Pseudomonas putida F1 biofilms grown on carbonized cellulosic fibers to achieve biodegradation of airborne volatile organic compounds (VOCs) in the absence of any bulk aqueous-phase media. It is believed that direct exposure of gaseous VOC substrates to biomass may eliminate aqueous-phase mass transfer resistance and facilitate VOC capture and degradation. When tested with toluene vapor as a model VOC, the supported biofilm could grow optimally at 300 p.p.m. toluene and 80% relative humidity, with a specific growth rate of 0.425 day⁻¹. During long-term VOC biodegradation tests in a tubular packed bed reactor, biofilms achieved a toluene degradation rate of 2.5 mg g_DCW⁻¹ h⁻¹ during the initial growth phase. Interestingly, the P. putida F1 film kept biodegrading activity even at the stationary nongrowth phase. The supported biofilms with a biomass loading of 20% (wt) could degrade toluene at a rate of 1.9 mg g_DCW⁻¹ h⁻¹ during the stationary phase, releasing CO₂ at a rate of 6.4 mg g_DCW⁻¹ h⁻¹ at the same time (indicating 100% conversion of substrate carbon to CO₂). All of these observations promised a new type of “dry” biofilm reactors for efficient degradation of toxic VOCs without involving a large amount of water.     

Exploratory Cumulative Risk Assessment (CRA) Approaches Using Secondary Data

Cumulative risk assessments (CRAs) include the examination of risks posed by multiple stressors and include population-specific vulnerabilities and susceptibilities. In this case study, we assess potential hearing impairment hazard due to joint exposure from noise and volatile organic compounds (VOCs) in order to examine the strengths and limitations of using secondary data on exposure and health effects for a CRA. Block group-level noise categories were estimated using modeled street-level data. A quantile regression model of sociodemographic and personal predictors from the 1999–2000 U.S. National Health and Nutrition Examination Survey VOC dataset was used along with block group-level sociodemographic and personal variables to estimate VOC exposures. Hazard indices (HIs) for potential hearing impairment due to joint noise and VOC exposures were calculated. County-averaged HIs for hearing impairment ranged from 0.8 (10th total VOCs percentile and 45–60 dB) to 1.7 (90th total VOCs percentile and 71–75 dB). Limitations of the exposure and health effects data included issues combining heterogeneous data and a lack of established threshold levels for combined low-level exposures; yet, this case study illustrates that screening-level CRAs, including non-chemical stressors, can be accomplished with publicly available data and existing methods.     

Recent advances in two-phase partitioning bioreactors for the treatment of volatile organic compounds

Biological processes are considered to be the most cost-effective technology for the off-gas treatment of volatile organic compounds (VOC) at low concentrations. Two-phase partitioning bioreactors (TPPBs) emerged in the early 1990s as innovative multiphase systems capable of overcoming some of the key limitations of traditional biological technologies such as the low mass transfer rates of hydrophobic VOCs and microbial inhibition at high VOC loading rates. Intensive research carried out in the last 5 years has helped to provide a better understanding of the mass transfer phenomena and VOC uptake mechanisms in TPPBs, which has significantly improved the VOC biodegradation processes utilizing this technology platform. This work presents an updated state-of-the-art review on the advances of TPPB technology for air pollution control. The most recent insights regarding non-aqueous phase (NAP) selection, microbiology, reactor design, mathematical modeling and case studies are critically reviewed and discussed. Finally, the key research issues required to move towards the development of efficient and stable full-scale VOC biodegradation processes in TPPBs are identified.     

Long-term influence of the presence of a non-aqueous phase on the cell surface hydrophobicity of Pseudomonas in two-phase partitioning bioreactors

The long-term influence of silicone oil 200 cSt (SO200) and 2, 2, 4, 4, 6, 8, 8-heptamethylnonane (HMN) on the cell surface hydrophobicity (CSH) of a hexane-degrading Pseudomonas aeruginosa strain and a toluene-degrading Pseudomonas putida strain was assessed in two-phase partitioning bioreactors under batch and continuous operation. CSH was evaluated using a modified BATH method based on optical density (CSH_OD) and colony-forming unit (CSH_CFU) measurements. In the presence of HMN, P. aeruginosa turned hydrophobic over the time course as shown by the gradual increase in CSH_OD (61 ± 1%) and CSH_CFU (53 ± 3%) under batch degradation and in CSH_OD (49 ± 0%) under continuous operation. However, P. putida turned hydrophobic only under continuous operation (CSH_OD = 28 ±2% {\hbox{CS}}{{\hbox{H}}_{\rm{OD}}} = 28 \pm 2\% ). On the other hand, no significant CSH enhancement was observed in both Pseudomonas strains in the presence of SO200. These results suggested that CSH is species, non-aqueous phase, and cultivation mode dependant, and an inducible property of bacteria. Maximum hexane elimination capacities increased by 2 and 3 in the presence of SO200 and HMN, respectively. Based on the absence of CSH in P. aeruginosa in the presence of SO200, the higher elimination capacities recorded were likely due to an improved hexane mass transfer (physical effect). However, in the presence of HMN, a direct hexane uptake from the non-aqueous phase (biological effect) might have also contributed to this enhancement.     

Spatial/Temporal Variations and Source Apportionment of VOCs Monitored at Community Scale in an Urban Area

This study aimed to characterize spatial/temporal variations of ambient volatile organic compounds (VOCs) using a community-scale monitoring approach and identify the main sources of concern in Paterson, NJ, an urban area with mixed sources of VOCs. VOC samples were simultaneously collected from three local source-dominated (i.e., commercial, industrial, and mobile) sites in Paterson and one background site in Chester, NJ (located ∼58 km southwest of Paterson). Samples were collected using the EPA TO-15 method from midnight to midnight, one in every sixth day over one year. Among the 60 analyzed VOCs, ten VOCs (acetylene, benzene, dichloromethane, ethylbenzene, methyl ethyl ketone, styrene, toluene, m,p-xylene, o-xylene, and p-dichlorobenzene) were selected to examine their spatial/temporal variations. All of the 10 VOCs in Paterson were significantly higher than the background site (p<0.01). Ethylbenzene, m,p-xylene, o-xylene, and p-dichlorobenzene measured at the commercial site were significantly higher than the industrial/mobile sites (p<0.01). Seven VOCs (acetylene, benzene, dichloromethane, methyl ethyl ketone, styrene, toluene, and p-dichlorobenzene) were significantly different by season (p<0.05), that is, higher in cold seasons than in warm seasons. In addition, dichloromethane, methyl ethyl ketone, and toluene were significantly higher on weekdays than weekend days (p<0.05). These results are consistent with literature data, indicating the impact of anthropogenic VOC sources on air pollution in Paterson. Positive Matrix Factorization (PMF) analysis was applied for 24-hour integrated VOC measurements in Paterson over one year and identified six contributing factors, including motor vehicle exhausts (20%), solvents uses (19%), industrial emissions (16%), mobile+stationery sources (12%), small shop emissions (11%), and others (22%). Additional locational analysis confirmed the identified sources were well matched with point sources located upwind in Paterson. The study demonstrated the community-scale monitoring approach can capture spatial variation of VOCs in an urban community with mixed VOC sources. It also provided robust data to identify major sources of concern in the community.     

A systematic selection of the non-aqueous phase in a bacterial two liquid phase bioreactor treating α-pinene

A systematic evaluation of the selection criteria of non-aqueous phases in two liquid phase bioreactors (TLPBs), also named two-phase partitioning bioreactors (TPPBs), was carried out using the biodegradation of α-pinene by Pseudomonas fluorescens NCIMB 11671 as a model process. A preliminary solvent screening was thus carried out among the most common non-aqueous phases reported in literature for volatile organic contaminants biodegradation in TLPBs: silicon oil, paraffin oil, hexadecane, diethyl sebacate, dibutyl-phtalate, FC 40, 1,1,1,3,5,5,5-heptamethyltrisiloxane (HMS), and 2,2,4,4,6,8,8-heptamethylnonane (HMN). FC 40, silicone oil, HMS, and HMN were first selected based on its biocompatibility, resistance to microbial attack, and α-pinene mass transport characteristics. FC 40, HMS, HMN, and silicone oil at 10% (v/v) enhanced α-pinene mass transport from the gas to the liquid phase by a factor of 3.8, 14.8, 11.4, and 8.6, respectively, compared to a single-phase aqueous system. FC 40 and HMN were finally compared for their ability to enhance α-pinene biodegradation in a mechanically agitated bioreactor. The use of FC 40 or HMN (both at 10% v/v) sustained non-steady state removal efficiencies (RE) and elimination capacities (EC) approximately 7 and 12 times higher than those achieved in the system without an organic phase, respectively. In addition, preliminary results showed that P fluorescens could uptake and mineralize α-pinene directly from the non aqueous phase.     

Long-term legacies and partial recovery of mycorrhizal communities after invasive plant removal

Invasive plants can have strong impacts on native communities, which have prompted intense efforts at invasive removal. However, relatively little is known about how native communities will reassemble after a dominant invader has been removed from the system. Legacy effects of invasive plants on soil microbial communities may alter native plant community reassembly long after the invader is gone. Here we found that arbuscular mycorrhizal fungal (AMF) communities have shown some recovery in experimental plots following 6 years of removal of the invasive Alliaria petiolata (garlic mustard, a species known to degrade AMF communities) in terms of taxonomic richness and community composition. However, despite this recovery, the density of A. petiolata at the beginning of the experiment (in 2004) still correlated with lower AMF richness and altered community composition after 6 years of annual weeding, suggesting long-term legacies of dense A. petiolata infestations. Because native plant and mycorrhizal fungal communities may show interdependence, reassembly of one community may be limited by the reassembly of the other. Restoration may be more effective if practices address both communities simultaneously.     

Intrinsic potential for immediate biodegradation of toluene in a pristine,energy-limited aquifer

Pristine and energy-limited aquifers are considered to have a low resistance and resilience towards organic pollution. An experiment in an indoor aquifer system revealed an unexpected high intrinsic potential for the attenuation of a short-term toluene contamination. A 30 h pulse of 486 mg of toluene, used as a model contaminant, and deuterated water (D₂O) through an initially pristine, oxic, and organic carbon poor sandy aquifer revealed an immediate aerobic toluene degradation potential. Based on contaminant and tracer break-through curves, as well as mass balance analyses and reactive transport modelling, a contaminant removal of 40 % over a transport distance of only 4.2 m in less than one week of travel time was obtained. The mean first-order degradation rate constant was λ = 0.178 day^?1, corresponding to a half-life time constant T_1/2 of 3.87 days. Toluene-specific stable carbon isotope analysis independently proved that the contaminant mass removal can be attributed to microbial biodegradation. Since average doubling times of indigenous bacterial communities were in the range of months to years, the aerobic biodegradation potential observed is assumed to be present and active in the pristine, energy-limited groundwater ecosystems at any time. Follow-up experiments and field studies will help to quantify the immediate natural attenuation potential of aquifers for selected priority contaminants and will try to identify the key-degraders within the autochthonous microbial communities.     

Arabidopsis thaliana for testing the phytotoxicity of volatile organic compounds

Geosmin and C-8 hydrocarbons are among the major volatile organic compounds (VOC) responsible for the distinctive, musty odor of filamentous fungi. In this study, we developed a plant bioassay for testing the possible toxicity of these compounds, as well as four air freshener products sometimes used to mask their odor. Seeds and vegetative plants of Arabidopsis thaliana were exposed to 1 ppm of 14 different volatile treatments (both single compounds and mixtures) for 72 h and monitored for germination rate, seedling formation, vegetative plant vigor and chlorophyll concentration. All VOCs tested had some inhibitory effect on seed germination or seedling formation; 1-octen-3-one was the most active, giving almost complete inhibition of germination. Geosmin did not prevent germination (radicle protrusion) but seedling formation was arrested 90 %. Of solvents and fragrance products tested, only the scented oil product was as active as the C-8 biogenic compounds in inhibiting seed germination and seedling formation. Two-week-old plants exposed to 1 ppm of individual fungal VOCs for 72 h all exhibited some degree of stress symptoms including smaller leaf size and weight, discoloration, leaf curling, small necrotic lesions, and reduced chlorophyll concentration. Two-week-old vegetative plants exposed to solvents and air freshener products were generally smaller in size. The single most phytotoxic compound tested was 1-octen-3-one which almost completely inhibited seed germination and was lethal to vegetative phase plants. Formaldehyde at 1 ppm killed 2-week-old seedlings but had little effect on seed germination. In conclusion, the A. thaliana bioassay provides an inexpensive approach for testing the toxicity of gas phase molecules.     

Microbial consumption and production of volatile organic compounds at the soil-litter interface

Substantial amounts of volatile organic compounds (VOCs) can be released during decomposition and these compounds can affect atmospheric chemistry, belowground processes, and the structure of microbial communities in litter and soil. However, we have a limited understanding of the types, quantities and ecological impacts of VOCs emitted from litter. Here we used a closed flow-through system and proton transfer reaction mass spectrometry (PTR-MS) to characterize VOC emissions from soil and two litter types (Pinus taeda and Acer rubrum) over a 72-day incubation period. Microbial respiration rates were measured throughout the incubation, and the soils were harvested at the end of the incubation to determine how litter VOCs influenced soil C dynamics, N mineralization rates, and bacterial communities. Using the PTR-MS we identified over 100 VOCs, with 10 VOCs making up the majority of emissions. VOCs accounted for up to 2.5% of the C flux from litter. Soil was a net sink of litter VOCs, absorbing up to 80% of VOCs released by litter, and exposure of soil to litter VOCs increased microbial respiration rates in soil by up to 15%. However, we observed negligible impacts of litter VOCs on soil nutrient levels and bacterial community structure, suggesting that soils must be exposed to higher concentrations of VOCs than observed in our study, to cause effects on these soil characteristics. Overall, VOCs appear to have an important influence on C dynamics at the soil-litter interface and VOC emissions from decomposing litter may represent an understudied component of biosphere–atmosphere interactions.