Direct and indirect formation of organohalogens by microorganisms

有机卤素影响大气化学，从而影响地球气候。因此，有必要确定有机卤素的来源，并获得导致其形成的不同机制的详细了解。从最近的研究来看，生物（酶催化）以及涉及腐殖质和铁矿物的非生物氧化还原反应都可以导致有机卤素的形成。然而，这些过程的机制、活性有机和铁的种类、影响酶和非生物反应的全球分布和地球化学参数，以及非生物和生物过程对全球有机卤素收支的相对贡献，目前尚不清楚。本研究项目的主要目标是：1)确定铁矿物和腐殖质的微生物氧化还原反应是否能产生活性铁和腐殖质化合物，从而导致挥发性有机卤素（VOX）的非生物形成；2)量化环境中卤化酶的多样性、丰度和活性。为此，将收集有机卤素释放场的土壤样本。将用这些样品建立实验室微观环境，以确定腐殖质物质、铁矿物和铁/腐殖质代谢细菌对VOX形成的作用，并评估地球化学条件变化的影响，包括氧化还原状态、水合作用（水）状态和温度对VOX形成的影响。在此基础上，我们将进一步分析和量化微生物群落、南部非洲和俄罗斯野外生境中卤化酶的遗传多样性、原位基因表达以及控制酶活性的地球化学参数。全面了解微生物直接和间接形成有机卤素的过程，不仅有助于我们进一步了解自然生物卤化过程，而且有助于科学量化有机卤素化合物的来源、汇和全球负担。

Organohalogens influence atmospheric chemistry and thereby earth climate. Thus, it is necessary to identify the sources of organohalogens and to obtain a detailed understanding of the different mechanisms that lead to their formation. From recent studies it became evident that both biotic (enzymatically catalysed), but also abiotic redox reactions involving humic substances and iron minerals can lead to the formation of organohalogens. However, the mechanisms of these processes, the identity of the reactive organic and iron species, the global distribution and geochemical parameters influencing the enzymatic and abiotic reactions, and the relative contribution of abiotic vs. biotic processes to the global organohalogen budget are currently unclear. The main goals of this research project are: i) to determine whether microbial redox reactions of iron minerals and humic substances can produce reactive iron species and humic compounds that lead to abiotic formation of volatile organohalogens (VOX) and ii) to quantify the diversity, abundance, and activity of halogenating enzymes in the environment. For this purpose soil samples from organohalogenreleasing field sites will be collected. Laboratory microcosms will be setup with these samples to determine the role of humic substances, iron minerals, and iron-/humics-metabolizing bacteria for VOX formation and to evaluate the effect of changing geochemical conditions, including redox status, hydration (water) status, and temperature on VOX formation. Further we propose to analyze and quantify the genetic diversity and in situ gene expression of halogenating enzyme and the geochemical parameters controlling enyzme activity in the microcosms and at the Southern African and Russian field sites. Comprehensive knowledge on the direct and indirect formation of organohalogens by microorganisms will not only further our understanding of natural biological halogenations processes, but also facilitate scientific efforts to quantify the sources, sinks, and global burdens of organic halogen compounds.