The phylum Chloroflexi contains several isolated bacteria that have been found to respire a diverse array of halogenated anthropogenic chemicals. The distribution and role of these Chloroflexi in uncontaminated terrestrial environments, where abundant natural organohalogens could function as potential electron acceptors, have not been studied. Soil samples (116 total, including 6 sectioned cores) from a range of uncontaminated sites were analyzed for the number of Dehalococcoides-like Chloroflexi 16S rRNA genes present. Dehalococcoides-like Chloroflexi populations were detected in all but 13 samples. The concentrations of organochlorine ([organochlorine]), inorganic chloride, and total organic carbon (TOC) were obtained for 67 soil core sections. The number of Dehalococcoides-like Chloroflexi 16S rRNA genes positively correlated with [organochlorine]/TOC while the number of Bacteria 16S rRNA genes did not. Dehalococcoides-like Chloroflexi were also observed to increase in number with a concomitant accumulation of chloride when cultured with an enzymatically produced mixture of organochlorines. This research provides evidence that organohalide-respiring Chloroflexi are widely distributed as part of uncontaminated terrestrial ecosystems, they are correlated with the fraction of TOC present as organochlorines, and they increase in abundance while dechlorinating organochlorines. These findings suggest that organohalide-respiring Chloroflexi may play an integral role in the biogeochemical chlorine cycle.T he phylum Chloroflexi is a deeply branching and diverse phylum containing isolates that are aerobic and anaerobic thermophiles, filamentous anoxygenic phototrophs, and anaerobic organohalide respirers (17,20,32,39). Chloroflexi have been estimated to dominate the microbial community of some seafloor sediments and also can make up 12% and 16% of the community in the B horizon of temperate grasslands and alpine meadows, respectively (9, 21, 47). Many of the Chloroflexi present in these environments have been found to form deeply branching lineages unrelated to any isolated strains of Chloroflexi. In addition, there is a lack of physiological data regarding the niche of these highabundance Chloroflexi.The Chloroflexi phylum contains several isolates that have been shown to be obligate organohalide respirers. These isolates include the genus Dehalococcoides and, more recently, Dehalobium chlorocoercia DF-1, strain o-17, and Dehalogenimonas lykanthroporepellens strains BL-DC-8 and BL-DC-9 (10, 31, 32, 50). Although the Dehalococcoides isolates have nearly identical 16S rRNA sequence similarities, Dehalobium, strain o-17, and Dehalogenimonas are more distantly related, with 89 to 91% 16S rRNA gene sequence identity to each other and approximately 87 to 90% 16S rRNA gene sequence identity to the cultured Dehalococcoides species (5, 31, 50). Members of the genus Dehalococcoides have been found to dechlorinate a wide range of persistent organic contaminants, and as a part of mixed consortia, Dehalococcoideslike species are t...

Interactions between organic matter and mineral matrices are critical to the preservation of soil and sediment organic matter. In addition to clay minerals, Fe(III) oxides particles have recently been shown to be responsible for the protection and burial of a large fraction of sedimentary organic carbon (OC). Through a combination of synchrotron X-ray techniques and high-resolution images of intact sediment particles, we assessed the mechanism of interaction between OC and iron, as well as the composition of organic matter co-localized with ferric iron. We present scanning transmission x-ray microscopy images at the Fe L3 and C K1 edges showing that the organic matter co-localized with Fe(III) consists primarily of C=C, C=O and C-OH functional groups. Coupling the co-localization results to iron K-edge X-ray absorption spectroscopy fitting results allowed to quantify the relative contribution of OC-complexed Fe to the total sediment iron and reactive iron pools, showing that 25–62% of total reactive iron is directly associated to OC through inner-sphere complexation in coastal sediments, as much as four times more than in low OC deep sea sediments. Direct inner-sphere complexation between OC and iron oxides (Fe-O-C) is responsible for transferring a large quantity of reduced OC to the sedimentary sink, which could otherwise be oxidized back to CO2.

[1] Organobromine (Br org ) compounds, commonly recognized as persistent, toxic anthropogenic pollutants, are also produced naturally in terrestrial and marine systems. Several enzymatic and abiotic bromination mechanisms have been identified, as well as an array of natural Br org molecules associated with various marine organisms. The fate of the carbon-bromine functionality in the marine environment, however, remains largely unexplored. Oceanographic studies have noted an association between bromine (Br) and organic carbon (C org ) in marine sediments. Even so, there has been no direct chemical evidence that Br in the sediments exists in a stable form apart from inorganic bromide (Br inorg ), which is widely presumed conservative in marine systems. To investigate the scope of natural Br org production and its fate in the environment, we probed Br distribution and speciation in estuarine and marine sediments using in situ X-ray spectroscopy and spectromicroscopy. We show that Br org is ubiquitous throughout diverse sedimentary environments, occurring in correlation with C org and metals such as Fe, Ca, and Zn. Analysis of sinking particulate carbon from the seawater column links the Br org observed in sediments to biologically produced Br org compounds that persist through humification of natural organic matter (NOM). Br speciation varies with sediment depth, revealing biogeochemical cycling of Br between organic and inorganic forms as part of the burial and degradation of NOM. These findings illuminate the chemistry behind the association of Br with C org in marine sediments and cast doubt on the paradigmatic classification of Br as a conservative element in seawater systems.

Chloride-the most abundant ion in sea water 1 -a ects ocean salinity, and thereby seawater density and ocean circulation. Its lack of reactivity gives it an extremely long residence time 2 . Other halogens are known to be incorporated into marine organic matter 3-5 . However, evidence of similar transformations of seawater chloride is lacking, aside from emissions of volatile organochlorine by marine algae 6-8 . Here we report high organochlorine concentrations from 180 to 700 mg kg −1 in natural particulate organic matter that settled into sediment traps at depths between 800 and 3,200 m in the Arabian Sea, taken between 1994 and 1995. X-ray spectromicroscopic imaging of chlorine bonding reveals that this organochlorine exists primarily in concentrated aliphatic forms consistent with lipid chlorination, along with a more di use aromatic fraction. High aliphatic organochlorine in particulate material from cultured phytoplankton suggests that primary production is a source of chlorinated organic matter. We also found that particulate algal detritus can act as an organic substrate for abiotic reactions involving Fe 2+ , H 2 O 2 or light that incorporate chlorine into organic matter at levels up to several grams per kilogram. We conclude that transformations of marine chloride to non-volatile organochlorine through biological and abiotic pathways represent an oceanic sink for this relatively unreactive element.Settling biogenic particulates remove much material from the surface ocean, and in addition to carbon this 'biological pump' entrains many other elements 9,10 . This route is established for the halogens Br and I (refs 3-5), whose low oxidation potential makes them amenable to incorporation into organic molecules. Cl, with the highest electron affinity of any element, has never been shown to participate in this process. Organochlorines are known in the ocean, but primarily as volatile compounds emitted to the atmosphere 6-8 , anthropogenic contaminants with various fates 11-13 , and trace compounds in microbes or dissolved organic matter 14,15 with no significant recognized fluxes.Both enzymatic and abiotic pathways have recently been shown to incorporate Br into particulate organic matter (POM) in sea water 16 . We reasoned that similar reactions can incorporate Cl into marine organic particulates. We tested this hypothesis by first investigating whether settling organic particulates show significant organochlorine content. Second, we looked for biological production of particulate organochlorines by phytoplankton, the main progenitors of POM in the oceans. Third, we examined the potential of abiotic processes, including photochemical, peroxidative and Fenton-like mechanisms, to chlorinate algal detritus.In each of these tests, we used the analytical capability of X-ray absorption near-edge structure (XANES) spectroscopy to distinguish organic from inorganic forms of Cl ( Fig. 1 and Supplementary Fig. 1). Spectral features near 2,822 eV (the Cl 'K-edge') denote electronic transitions from the Cl-1s shell to e...