Methyl halide production in fungi

Methyl halide gases are a source of halogen radicals that can react with and destroy stratospheric ozone. The sources of methyl halide gases are both anthropogenic and biogenic, that is, they are human induced and they occur naturally. This research focused on the emission of methyl betides from fungi in the phylum Basidiomycota, which are one of the known biogenic sources. Previous studies have measured methyl halide production and consumption in soils using field chambers. The objective of this study was to compare production from individual fungi in laboratory cultures to the field measured fluxes to examine whether fungi are a significant source of methyl halide emissions. This study included fungi from four different ecosystems: an agricultural field, a temperate forest, a fresh water wetland, and coastal salt marshes in southern New Hampshire, USA. Fungal samples were collected from each site and cultured in the laboratory using tissue culture, wood bait, and direct soil plating methods. Once pure isolates were obtained, the fungi were assayed for methyl halide production using cryotrapping-gas chromatography. Samples of headspace gas were extracted from flasks containing fungi grown in liquid media or soil substrate and measured on a gas chromatograph/electron capture detector (GC/ECD). By sampling individual fungi from different ecosystems and assaying them in media and soil substrate the halide ratio from fungi could be examined. Two types of statistical analyses were used to determine the methyl halide fluxes: linear least squares fit of the methyl halide concentration vs. time, and a Bayesian model with Markov Chain Monte Carlo (MCMC) sampling. Both statistical methods calculated a slope of the flux for each flask. The fluxes were averaged by isolate, and then normalized to mass by dividing by fungal biomass. The Bayesian model provided a rigorous analysis of the data that could be compared to data derived from the traditional linear fit method in order to determine whether a linear regression fit causes an over-estimation of the production rate. Methyl halide production was observed in fungi collected from all of the sites. Specifically, the highest levels of methyl halides were produced in all species of fungi isolated from salt marshes; the highest levels of methyl bromide were produced by basidiomycetes from all sites and zygomycete isolates from the freshwater fen; the non-basidiomycete fungi isolated from the freshwater fen and from the salt marshes produced the highest levels of methyl iodide. Although the primary focus was methyl halide production in basidiomycetes, the results clearly show that the non-basidiomycete fungi are a potential source of methyl halide emissions that may represent a greater environmental significance than expected from basidiomycetes