Variations in α‐Hexachlorocyclohexane enantiomer ratios in relation to microbial activity in a temperate estuary

Abstract

Changes in the enantiomer ratios (ERs) of chiral pollutants in the environment are often considered evidence of biological alteration despite the lack of data on causal or mechanistic relationships between microbial parameters and ER values. Enantiomer ratios that deviate from 1:1 in the environment provide evidence for the preferential microbial degradation of one enantiomer, whereas ER values equal to 1 provide no evidence for microbial degradation and may mistakenly be interpreted as evidence that biodegradation is not important. In an attempt to link biological and geochemical information related to enantioselective processes, we measured the ERs of the chiral pesticide alpha-hexachlorocyclohexane (alpha-HCH) and bacterial activity (normalized to abundance) in surface waters of the York River (VA, USA) bimonthly throughout one year. Despite lower overall alpha-HCH concentrations, alpha-HCH ER values were unexpectedly close to 1:1 in the freshwater region of the estuary with the highest bacterial activity. In contrast, ER values were nonracemic (ER double dagger 1) and alpha-HCH concentrations were significantly higher in the higher salinity region of the estuary, where bacterial activity was lower. Examination of these data may indicate that racemic environmental ER values are not necessarily reflective of a lack of biodegradation or recent input into the environment, and that nonenantioselective biodegradation may be important in certain areas.

Probing the Enantioselectivity of Chiral Pesticides

“…Other research has shown that microbial transformation is not necessarily enantioselective (27). Significant nonstereoselective transformation of -HCH was observed in parts of a Virginia estuary where relevant bacterial activity was the highest, whereas highly stereoselective degradation occurred in other regions of the estuary.…”

Assessing ongoing sources of dissolved‐phase polychlorinated biphenyls in a contaminated stream

“…The EF values, which are near racemic in the vicinity of fresh sources and then depart from racemic with distance from the source, can provide evidence for recent fresh sources to the systems [17]. The only two chiral congeners detected in the passive samplers were PCBs 91 (2,2 0 ,3,4 0 ,6-pentaCB) and 95 (2,2 0 ,3,5 0 ,6-pentaCB).…”

“…Enantioselective analysis of chiral compounds is a proven tool for source apportionment and chemical characterization [14,15]. Chiral signatures can provide insight into the biotransformation of contaminants or the source of contaminants in aquatic food webs because biological processes or a weathered contribution will change enantiomer compositions [16,17].…”

Occurrence of Chiral Bioactive Compounds in the Aquatic Environment: A Review

“…Although much less used, LC-DAD, LC-UV and LC-FD detection have been used in some of the studies [11,26,27,54,63,78,79,86]. On the other hand, most pesticides are transparent towards UV radiation and therefore, ECD and MS have been the most used detection techniques [12,97,[112][113][114][115].…”

“…The authors showed that different microbiological process influenced the degradation of α-HCH isomers and suggested a correlation between the ER of (+/-) α-HCH and the concentrations of isomers α-HCH and γ-HCH. Another study developed by Padma et al 2003 investigated the variation in ER of α-HCH enantiomers in the York river estuary due to the microbiological activity (Table 2) [114]. Surprising, the α-HCH ER values were close to 1 in the freshwater region of the estuary, i.e., in the head of the river, where the bacterial activity was high.…”