Biological chlorine cycling in the Arctic Coastal Plain

This study explores biological chlorine cycling in coastal Arctic wet tundra soils. While many previous chlorine-cycling studies have focused on contaminated environments, it is now recognized that chlorine can cycle naturally between inorganic and organic forms in soils. However, these pathways have not previously been described for an Arctic ecosystem. We measured soil organic and inorganic Cl pools, characterized soils and plant tissues with chlorine K-edge X-ray absorption near-edge spectroscopy (Cl-XANES), measured dechlorination rates in laboratory incubations, and analyzed metagenomes and 16S rRNA genes along a chronosequence of revegetated drained lake basins. Concentrations of soil organic chlorinated compounds (Clorg) were correlated with organic matter content, with a steeper slope in older soils. The concentration and chemical diversity of Clorg increased with soil development, with Clorg in younger soils more closely resembling that of vegetation, and older soils having more complex and variable Cl-XANES signatures. Plant Clorg concentrations were higher than previously published values, and can account for the rapid accumulation of Clorg in soils. The high rates of Clorg input from plants also implies that soil Clorg pools turn over many times during soil development. Metagenomic analyses revealed putative genes for synthesis (haloperoxidases, halogenases) and breakdown (reductive dehalogenases, halo-acid dehalogenases) of Clorg, originating from diverse microbial genomes. Many genome sequences with close similarity to known organohalide respirers (e.g. Dehalococcoides) were identified, and laboratory incubations demonstrated microbial organohalide respiration in vitro. This study provides multiple lines of evidence for a complex and dynamic chlorine cycle in an Arctic tundra ecosystem.