基于单体稳定同位素探究滨海湿地沉积物中多溴联苯醚的厌氧微生物降解路径和生物可利用性

Coastal wetland is the important sink of terrestrially organic pollutants. The biodegradation and bioavailability of organic contaminants by anaerobic microbes in the sediments pose the significant effect on the ecosystem of coastal wetland. However, the knowledge on these issues is still limited now. One of the reasons is the lack of key indicators to accurately trace the microbial biodegradation pathways and bioavailability of organic contaminants. The light isotopes of contaminants were preferentially utilized in the microbial degradation, leading to the enrichment of heavy isotopes in the residues. Therefore, the changes in composition of stable isotopes can be used to accurately indicate the degradation processes of contaminants. The present project proposes to use the compound-specific stable isotope analysis to trace the biodegradation and bioavailability of polybrominated diphenyl ethers (PBDEs) by the anaerobic microbes in sediments from Liaohe coastal wetland. The objectives of this project are that: 1) accurately identify the degradation pathways of PBDEs by combining the degradation products and features of stable carbon isotope fractionation; 2) providing a new approach to dynamically and quantitatively estimate the bioavailability of PBDEs by using the stable carbon isotopes. This work possesses distinct characteristics of region and analytical method. It not only improves the knowledge on the environmental behaviors and fate of terrestrial contaminants, but also provides the importantly scientific foundation for the monitoring and management of terrestrial pollutant in the coastal wetland.

滨海湿地作为陆源有机污染物的重要聚集地，沉积物中有机污染物的厌氧微生物降解和生物可利用性对滨海湿地生态系统产生重要影响。然而目前对这些问题的认识仍不足，原因之一在于缺少能准确示踪有机污染物微生物降解路径和生物可利用性的关键指标。污染物微生物降解过程中优先利用轻同位素，从而导致残留物中重同位素的富集，因此稳定同位素组成变化可准确指示污染物的降解过程。本项目以溴系阻燃剂多溴联苯醚（PBDEs）为目标陆源污染物，辽河口滨海湿地为研究区域，研究沉积物中PBDEs在厌氧微生物降解过程中稳定碳同位素的分馏特征，结合降解产物的鉴定，准确识别PBDEs的降解路径；利用单体稳定碳同位素建立动态和定量评估沉积物中PBDEs厌氧微生物可利用性的新方法。本项目研究具有鲜明的区域和分析方法特色，可促进对陆源有机污染物在滨海湿地生态系统中环境行为和归宿的认识，同时也为滨海湿地陆源污染的监控和治理提供重要的科学基础。

以辽河口滨海湿地为研究对象，基于单体稳定同位素分析，探究了湿地沉积物中常见溴代阻燃剂BDE-47、BDE-153和BDE-209的厌氧微生物降解路径和生物可利用性，克服了传统以浓度变化和产物鉴定为研究手段的局限性。研究表明，PBDEs在厌氧微生物降解过程中，随着浓度的减少，残留物中碳稳定同位素δ13C值不断增大，而灭菌沉积物中δ13C值无明显变化，首次揭示了厌氧微生物降解可导致PBDEs碳同位素稳定发生分馏。基于Rayleigh同位素动力学分馏方程，以碳稳定同位素组成δ13C和富集因子εC为指标，建立了动态和定量评估湿地沉积物中PBDEs厌氧微生物降解程度和生物可利用性的新方法。此外，基于碳稳定同位素和微生物群落分析，证实了PBDEs不同单体厌氧微生物降解之间存在竞争作用，而这种竞争作用主要归因于对共有脱卤菌的竞争。通过稳定同位素分析和产物鉴定，发现还原脱溴生成低溴单体是PBDEs厌氧微生物降解的主要路径。首次采用二维碳-溴稳定同位素分析，结合降解产物的鉴定，揭示了SN2亲核芳基取代为PBDEs厌氧微生物还原脱溴机理，具体为微生物还原脱卤酶的辅酶因子与PBDEs苯环发生加成反应，同时伴随C-Br键的断裂，该过程中在电子供体和受体之间存在内层电子的转移。本项目研究成果对滨海湿地乃至其他地区沉积物中污染物的监控和治理提供了重要理论基础。