Purpose
Humins, as solid-phase redox mediators, are important for enhancing anaerobic biotransformation processes. Although humins are broadly distributed in estuarine and marine sediments, to date, their electron transfer capacity (ETC) from marine sediments has been poorly characterized. This study determined the potential of humins as redox mediators for microbial redox metabolic reactions in sediments along a transect from the Yangtze Estuary to the adjacent East China Sea and analyzed their electron-mediating characteristics.

Materials and methods
Comparative analyses of humins in sediments along this transect were conducted to investigate their ETC as redox mediators for microbial redox metabolic reactions, namely dissimilatory iron reduction (DIR) and dissimilatory nitrate reduction to ammonium (DNRA), using Shewanella oneidensis MR-1. Elemental composition, electrochemical, and Fourier transform infrared analyses were performed to better understand the functional redox groups in humins.

Results and discussion
All the humins functioned as solid-phase redox mediators and enhanced DIR and DNRA by Shewanella oneidensis , although with varying efficacy. The humins extracted from sampling site P1 (P1-HM), located close to the Yangtze Estuary, most effectively enhanced iron and nitrate reduction without nitrite accumulation. Electrochemical analysis confirmed the highest redox activity and ETC in P1-HM and a weakening trend of ETC with increasing distance from the estuary. Fourier transform infrared analysis suggested that quinone moieties might serve as key redox functional groups in humins.

Conclusions
Considering their high yield and ETC, P1-HM could be important redox mediators that influence microbial redox metabolic reactions in sedimentary ecosystems of the Yangtze Estuary. Thus, this study provides insights into the distribution of humins, their ECT, and their potential role as redox mediators for microbial redox reactions, which could improve understanding of elemental biogeochemical cycling processes involving microorganisms in the Yangtze Estuary and East China Sea sedimentary ecosystems and provide important reference information for researches on other estuary areas.



中文翻译：

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长江口至东海沉积物中腐殖质的氧化还原特性及其对微生物氧化还原反应的影响

目的
腐殖质作为固相氧化还原介质，对于增强厌氧生物转化过程非常重要。尽管腐殖质广泛分布在河口和海洋沉积物中，但迄今为止，它们对海洋沉积物的电子转移能力（ETC）的表征还很差。这项研究确定了腐殖质作为微生物氧化还原代谢物的潜力，该物质在沿长江口至邻近的东海样带的沉积物中发生了微生物氧化还原代谢反应，并分析了它们的电子介导特性。

材料和方法
进行了该样带沉积物中的腐殖质的比较分析，以研究其ETC作为微生物氧化还原代谢反应的氧化还原介体，即使用 Shewanella oneidensis MR-1进行的异化铁还原（DIR）和异化硝酸盐还原为铵（DNRA） 。进行了元素组成，电化学和傅立叶变换红外分析，以更好地了解腐殖质中的功能性氧化还原基团。

结果与讨论
所有腐殖质均起着固相氧化还原介质的作用，并被沙瓦 氏菌 增强了DIR和DNRA。 ，尽管疗效不一。从靠近长江口的采样点P1（P1-HM）提取的腐殖质最有效地增强了铁和硝酸盐的还原，而没有亚硝酸盐的积累。电化学分析证实，P1-HM的氧化还原活性和ETC最高，并且随着离河口距离的增加，ETC的趋势减弱。傅里叶变换红外分析表明，醌部分可能是腐殖质中关键的氧化还原官能团。

结论
考虑到它们的高产量和ETC，P1-HM可能是重要的氧化还原介质，影响长江口沉积生态系统中微生物的氧化还原代谢反应。因此，本研究提供了关于腐殖质的分布，其ECT以及它们作为微生物氧化还原反应的氧化还原介体的潜在作用的见解，这可以增进对涉及长江口和东海沉积生态系统中微生物的元素生物地球化学循环过程的了解，并提供对于其他河口领域的研究的重要参考信息。