Natural organobromine in terrestrial ecosystems

Abstract

Recent studies have shown that bromine undergoes biogeochemical cycling involving natural formation and degradation of organobromine compounds in marine systems. In the terrestrial environment, where background bromine levels tend to be low, the biogeochemistry of this element remains largely unexamined. We traced the path of bromine through plant growth, senescence, and decay of leaf litter on the forest floor. Using sensitive X-ray spectroscopic techniques, we show that all bromine in humified plant material, organic-rich surface soils, and isolated humic substances is bonded to carbon. Analysis of bromide-enriched plants suggests that bromide absorbed by the growing plants ultimately converts to organobromine when the plant litter decays. Application of isolated chloroperoxidase, a halogenating enzyme, to healthy plant material results in extensive bromination, with organobromine formed preferentially over organochlorine. The relative ease of bromide oxidation appears to promote biogeochemical transformations of Br from inorganic to organic forms, leading to its incorporation into soil organic matter through enzymatic processes related to plant litter decomposition. In combination with low concentration and susceptibility to leaching and plant uptake, natural bromination processes lead to the exhaustion of inorganic bromide in surface soils, making organic matter a reservoir of bromine in the terrestrial environment. This study provides the first detailed look into the terrestrial bromine cycle and lays the foundation for future studies of natural organobromine degradation, which may shed light on the fate of anthropogenic organobromine pollutants in the soil environment.