Halogenated hydrocarbon formation in a moderately acidic salt lake in Western Australia - Role of abiotic and biotic processes

Environmental context Volatile halogenated organic compounds (VOX) contribute to ozone depletion and global warming. Here we demonstrate that acidic salt lake sediments in Western Australia contribute to the global natural emission of these compounds and that the emissions are primarily of biotic origin. Elucidating major sources and sinks of VOX is a key task in environmental chemistry because their formation and degradation have major effects on atmospheric chemistry and thus earth climate. Abstract Volatile organohalogen compounds (VOX) are known environmental pollutants and contribute to stratospheric ozone depletion. Natural formation of VOX has been shown for many environments from the deep sea to forest soils and Antarctica. Recently, we showed that VOX are emitted from pH-neutral salt lakes in Western Australia and that they are mainly of biotic origin. To which extent this biotic organohalogen formation in salt lakes is pH-dependent and whether VOX are also formed under acidic conditions are unknown. Therefore, we quantified VOX emissions from an acidic salt lake in Western Australia (Lake Orr) in biotic and abiotic (γ ray-irradiated) microcosm experiments under controlled laboratory conditions. The experiments revealed that biotic halogenation processes also occurred under acidic conditions (pH range 3.8-4.8), though the emissions were approximately one order of magnitude lower (nanogram per kilogram dry sediment range) than from pH-neutral lake sediments. Among the detected substances were brominated, e.g. tribromomethane, as well as chlorinated compounds (e.g. trichloromethane). The addition of lactate and acetate, and ferrihydrite showed no stimulation of VOX formation in our microcosms. Hence, the stimulation of Fe-metabolising microorganisms and their potential effect on the formation of reactive Fe species did not promote VOX emissions, suggesting a direct enzymatic formation of the emitted compounds.