Halomethane from halide ion—a highly efficient fungal conversion of environmental significance

Abstract

Globally, at least 5 Mt yr⁻¹ of chloromethane (CH₃Cl) must originate from natural sources, according to estimates based on environmental concentrations, man-made emissions of 26 kt yr⁻¹ being insignificant in comparison^1–3. Although CH₃C1 is a natural product of several fungal species^4,5, no quantitative investigations of its biological production have been reported. We have measured CH₃C1 production by a common wood-rotting fungus, and report that on glucose-based media, the chloride ion (Cl⁻) at concentrations <4 mM was methylated with >90% efficiency. The pattern of CH₃C1 biogenesis was typical of a secondary metabolite and paralleled loss of Cl⁻ from the medium. With cellulosic substrates, CH₃C1 yields ranged between 80 and 95% at Cl⁻ concentrations as high as 25 mM, although production extended over a longer period. Bromo- and iodomethane were formed in high yield from the corresponding halide ions. Such high-efficiency methylation of the halide ion indicates that fungi could make a substantial contribution to the atmospheric CH₃C1 burden.