Radiocarbon Evidence for a Naturally Produced, Bioaccumulating Halogenated Organic Compound

Abstract

Halogenated organic compounds (HOCs) such as 1,1'-dimethyl-3,3',4,4'-tetrabromo-5,5'-dichloro-2,2'-bipyrrole (DBP-Br4Cl2) and heptachloro-1'-methyl-1,2'-bipyrrole (Q1) have been detected worldwide, sometimes at high levels in Antarctic air, seabird eggs, the blubber of marine mammals, and, most notably, even human milk. To date, it has been difficult to determine whether these compounds are natural products or derived from industrial synthesis. Molecular-level 14C analysis of these compounds is particularly appealing because most industrial compounds are manufactured from petrochemicals (14C-free) and natural compounds should have "modern" or "contemporary" 14C levels. To investigate the source of DBP-Br4Cl2, we isolated 600 microg of this compound (150 microg of carbon) from marine animal extracts by employing gel permeation chromatography, Florisil column chromatography, and two-dimensional preparative capillary gas chromatography. The purified DBP-Br4Cl2 was split into two samples (75 microg of carbon each) and analyzed by accelerator mass spectrometry for 14C content. The delta14C values were -449 percent per thousand and -467 percent per thousand, corresponding to conventional 14C ages of 4740 and 5000 years before present (BP), respectively. The presence of detectable 14C in the DBP-Br4Cl2 strongly points to at least a natural or biogenic source. However, these delta14C values for DBP-Br4Cl2 are more depleted than expected for a recently synthesized natural product. Several explanations are discussed, but additional samples

Identification of highly brominated analogues of Q1 in marine mammals

“…It has been suggested previously that bioaccumulated bipyrroles are produced naturally (Reddy et al, 2004;Wu et al, 2002). The presence of radiocarbon in one bioaccumulated DMBP implies, at least in part, a biogenic origin (Reddy et al, 2004).…”

“…The presence of radiocarbon in one bioaccumulated DMBP implies, at least in part, a biogenic origin (Reddy et al, 2004). Additionally, a structurally similar hexabrominated 2,2'-bipyrrole is a known marine bacterial metabolite (Andersen et al, 1974).…”

“…In support of their natural origin, one bioaccumulated DMBP has been shown to be at least in 3 part biogenic from analysis of its natural abundance radiocarbon content (Reddy et al, 2004). However, very few marine natural products have been identified with such a high degree of halogenation.…”

Natural 14C in Saccoglossus bromophenolosus compared to 14C in surrounding sediments

“…An additional benefit of this approach is that the absolute amount of 14 C in natural products may vary slightly depending on the type of organism that synthesized them, and also when and where the organism grew. To obtain some baseline data, Reddy et al (2004) isolated 2, 4-dibromophenol (2, 4-DBP) and 2, 3, 4-tribromopyrrole (2, 3, 4-TBP) from Saccoglossus bromophenolosus and S. kowalevskii, respectively. These saccoglossoids were selected because they are easily sampled, and these compounds because significant research has been performed on their biosynthesis, chemical ecology and biogeochemistry (Higa et al 1980, Fielman & Targett 1995, King et al 1995, Woodin et al 1997.…”

“…These saccoglossoids were selected because they are easily sampled, and these compounds because significant research has been performed on their biosynthesis, chemical ecology and biogeochemistry (Higa et al 1980, Fielman & Targett 1995, King et al 1995, Woodin et al 1997. Reddy et al (2004) found that both compounds were natural as they contained detectable levels of 14 C (and hence were not from a petrogenic source). The actual 14 C values were more depleted (Δ 14 C = -170 ‰ for 2, 4-DBP and -130 ‰ for 2, 3, 4-TBP) than recently fixed carbon, which for coastal and open ocean surface waters is > 0 ‰ (Fig.…”