Chlorine Isotope Effects and Composition of Naturally Produced Organochlorines from Chloroperoxidases, Flavin-Dependent Halogenases, and in Forest Soil

The use of stable chlorine isotopic signatures (delta Cl-37) of organochlorine compounds has been suggested as a tool to determine both their origins and transformations in the environment. Here we investigated the delta Cl-37 fractionation of two important pathways for enzymatic natural halogenation: chlorination by chloroperoxidase (CPO) and flavin-dependent halogenases (FDH). Phenolic products of CPO were highly Cl-37 depleted (delta Cl-37 = -12.6 +/- 0.9 parts per thousand); significantly more depleted than all known industrially produced organochlorine compounds (delta Cl-37 = -7 to +6 parts per thousand). In contrast, four FDH products did not exhibit any observable isotopic shifts (delta Cl-37 = -0.3 +/- 0.6 parts per thousand). We attributed the different isotopic effect to the distinctly different chlorination mechanisms employed by the two enzymes. Furthermore, the delta Cl-37 in bulk organochlorines extracted from boreal forest soils were only slightly depleted in Cl-37 relative to inorganic Cl. In contrast to previous suggestions that CPO plays a key role in production of soil organochlorines, this observation points to the additional involvement of either other chlorination pathways, or that dechlorination of naturally produced organochlorines can neutralize delta Cl-37 shifts caused by CPO chlorination. Overall, this study demonstrates that chlorine isotopic signatures are highly useful to understand sources and cycling of organochlorines in nature. Furthermore, this study presents delta Cl-37 values of FDH products as well of bulk organochlorines extracted from pristine forest soil for the first time.