Determination of Atmospheric Methyl Bromide by Cryotrapping-Gas Chromatography and Application to Soil Kinetic Studies Using a Dynamic Dilution System

Volatile Gas Production by Methyl Halide Transferase: An In Situ Reporter Of Microbial Gene Expression In Soil

Traditional visual reporters of gene expression have only very limited use in soils because their outputs are challenging to detect through the soil matrix. This severely restricts our ability to study time-dependent microbial gene expression in one of the Earth's largest, most complex habitats. Here we describe an approach to report on dynamic gene expression within a microbial population in a soil under natural water levels (at and below water holding capacity) via production of methyl halides using a methyl halide transferase. As a proof-of-concept application, we couple the expression of this gas reporter to the conjugative transfer of a bacterial plasmid in a soil matrix and show that gas released from the matrix displays a strong correlation with the number of transconjugant bacteria that formed. Gas reporting of gene expression will make possible dynamic studies of natural and engineered microbes within many hard-to-image environmental matrices (soils, sediments, sludge, and biomass) at sample scales exceeding those used for traditional visual reporting.

Environmental fate of methyl bromide as a soil fumigant

The great variation among results of recent experiments measuring the total emission of MeBr from fields implies that many factors influence MeBr transport and transformation in the soil-water-air system and its ultimate loss from the soil surface. It has been demonstrated that variables related to application methods (e.g., injection depth, use and type of surface tarp), soil properties (e.g., water content, bulk density, soil organic matter), and climatic conditions (e.g.. air temperature, wind speed, barometric pressure) have pronounced effects on MeBr volatilization following soil injection. The following conclusions can be drawn from this experimental information. Tarping consistently, increased the residence time and concentration of MeBr residing in the soil. Prolonged retention of MeBr in the soil resulted in more extensive degradation and reduced cumulative emissions. Research indicates that the polyethylene film typically used for the surface cover is relatively permeable to MeBr and allows significant emissions compared to virtually impermeable plastic films. This effect is more pronounced during periods of high temperature. Soil type, soil water content, and bulk density are important factors affecting MeBr transport and transformation in soil, which ultimately affect volatilization. The total volatilization from a soil with high organic matter content may be drastically reduced relative to that from a low organic matter soil. Amendment of the surface soil with organic matter or nucleophilic compounds that promote increased degradation may offer another method for reducing volatilization. MeBr volatilization may also be decreased by increasing soil water content and bulk density, mainly because of the reduced gas-phase diffusion resulting from reduced soil air-filled porosity. To minimize volatilization, MeBr should be applied during periods of cool temperature, injected relatively deep in organic-rich, moist soil, and the soil surface packed and tarped immediately after the application. Depending on site-specific conditions, a new high-barrier plastic should be used. Injecting MeBr during periods of warm temperature, at a shallow depth in dry, loose soil without the use of low-permeability plastic barriers, will likely result in maximum volatilization rates and therefore should be discouraged. Before adopting any new emission reduction technology, the pest control characteristics of the new methodology should be assessed under soil and environmental conditions typical of the region to optimize efficacy while minimizing environmental contamination. There is considerable current scientific evidence indicating that eliminating MeBr use for soil fumigation may not have a significant impact on stratospheric ozone depletion. Management practices can and have been developed that essentially eliminate atmospheric emissions of MeBr and other fumigant compounds following soil application. Some scientists have suggested that there are natural buffers and various unknown sources of MeBr that make it impossible to ascertain that eliminating soil fumigation with MeBr will significantly improve stratospheric ozone levels. It is quite certain, however, that the phase-out will make it much more difficult for growers to economically provide an adequate and healthful food supply in the U.S. and elsewhere in the world. As the phase-out date approaches, there remains a great need for information about MeBr and stratospheric ozone depletion. Stratospheric ozone must be protected, but recent experiments suggest that it can be protected while still allowing MeBr to be used for soil fumigation. A new approach may be warranted in which state and federal regulations recognize that every chemical is a potential environmental contaminant, depending on the properties of the chemical and the environmental conditions prevailing following its application. Ideally, regulations should incorporate incentives to develop technology that minimizes the likelihood that a chemical becomes an environmental and/or public health problem. Rather than instituting an irrevocable ban, allowing for a suspension of chemical use until the appropriate technology is developed to control the undesirable characteristic(s) of the chemical use would provide much more flexibility to growers and may enhance environmental protection by adopting a proactive approach in which growers, chemical manufacturers, regulators, and the public can have confidence.

Consumption of tropospheric levels of methyl bromide by C(1) compound-utilizing bacteria and comparison to saturation kinetics

Pure cultures of methylotrophs and methanotrophs are known to oxidize methyl bromide (MeBr); however, their ability to oxidize tropospheric concentrations (parts per trillion by volume [pptv]) has not been tested. Methylotrophs and methanotrophs were able to consume MeBr provided at levels that mimicked the tropospheric mixing ratio of MeBr (12 pptv) at equilibrium with surface waters ( approximately 2 pM). Kinetic investigations using picomolar concentrations of MeBr in a continuously stirred tank reactor (CSTR) were performed using strain IMB-1 and Leisingeria methylohalidivorans strain MB2(T) - terrestrial and marine methylotrophs capable of halorespiration. First-order uptake of MeBr with no indication of threshold was observed for both strains. Strain MB2(T) displayed saturation kinetics in batch experiments using micromolar MeBr concentrations, with an apparent K(s) of 2.4 microM MeBr and a V(max) of 1.6 nmol h(-1) (10(6) cells)(-1). Apparent first-order degradation rate constants measured with the CSTR were consistent with kinetic parameters determined in batch experiments, which used 35- to 1 x 10(7)-fold-higher MeBr concentrations. Ruegeria algicola (a phylogenetic relative of strain MB2(T)), the common heterotrophs Escherichia coli and Bacillus pumilus, and a toluene oxidizer, Pseudomonas mendocina KR1, were also tested. These bacteria showed no significant consumption of 12 pptv MeBr; thus, the ability to consume ambient mixing ratios of MeBr was limited to C(1) compound-oxidizing bacteria in this study. Aerobic C(1) bacteria may provide model organisms for the biological oxidation of tropospheric MeBr in soils and waters.

Rapid Consumption of Low Concentrations of Methyl Bromide by Soil Bacteria

A dynamic dilution system for producing low mixing ratios of methyl bromide (MeBr) and a sensitive analytical technique were used to study the uptake of MeBr by various soils. MeBr was removed within minutes from vials incubated with soils and ~10 parts per billion by volume of MeBr. Killed controls did not consume MeBr, and a mixture of the broad-spectrum antibiotics chloramphenicol and tetracycline inhibited MeBr uptake by 98%, indicating that all of the uptake of MeBr was biological and by bacteria. Temperature optima for MeBr uptake suggested a biological sink, yet soil moisture and temperature optima varied for different soils, implying that MeBr consumption activity by soil bacteria is diverse. The eucaryotic antibiotic cycloheximide had no effect on MeBr uptake, indicating that soil fungi were not involved in MeBr removal. MeBr consumption did not occur anaerobically. A dynamic flowthrough vial system was used to incubate soils at MeBr mixing ratios as low as those found in the remote atmosphere (5 to 15 parts per trillion by volume [pptv]). Soils consumed MeBr at all mixing ratios tested. Temperate forest and grassy lawn soils consumed MeBr most rapidly (rate constant [k] = 0.5 min-1), yet sandy temperate, boreal, and tropical forest soils also readily consumed MeBr. Amendments of CH4 up to 5% had no effect on MeBr uptake even at CH4:MeBr ratios of 10(7), and depth profiles of MeBr and CH4 consumption exhibited very different vertical rate optima, suggesting that methanotrophic bacteria, like those presently in culture, do not utilize MeBr when it is at atmospheric mixing ratios. Data acquired with gas flux chambers in the field demonstrated the very rapid in situ consumption of MeBr by soils. Uptake of MeBr at mixing ratios found in the remote atmosphere occurs via aerobic bacterial activity, displays first-order kinetics at mixing ratios from 5 pptv to ~1 part per million per volume, and is rapid enough to account for 25% of the global annual loss of atmospheric MeBr.