Abstract Sedimentary dissolved organic matter (DOM) is an important pool of intermediates produced and consumed during early diagenesis of organic matter in the anoxic subseafloor. Rapid degradation of organic matter in the coastal sediment results in stratification of redox zones. However, to date little is known about the selectivity with respect to organic matter of the initial microbially mediated anaerobic degradation processes under contrasting redox conditions and how these affect the composition of DOM. In order to study the effect of sulfate reducing vs. methanogenic conditions on DOM quality and degradation, sediments (0–18 cm) from the Rhone River Delta were incubated, with redox conditions being controlled by sulfate amendment. The progress of incubation was monitored by H2, CH4, sulfate, DIC, DOC and acetate production. DOM composition was determined by 3D Fluorescence Spectroscopy, i.e., Excitation Emission Matrix Spectroscopy (EEMs), and ultra-high-resolution Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FT-ICR-MS). Parallel factor analysis (PARAFAC) of EEMs was used to distinguish different groups of DOM (humic-like and protein-like compounds) to evaluate composition, conjugation and size of DOM. Prior to incubation, humic-like sedimentary DOM predominated; nearly half of the molecular formulae (>5000) identified by FT-ICR-MS were CHO and one third were CHNO compounds. During incubation, protein-like DOM and CHNO formulae with 3 and 4 N atoms formulae were rare under both sulfate reducing and methanogenic conditions. Incubation under sulfate reducing conditions resulted in rapid release and net accumulation of dissolved organic carbon (DOC). Protein-like DOM was rapidly cycled and humic-like DOM accumulated. Consistently, CHNO formulae with 3 and 4 N atoms decreased faster, whereas formulae with one N and oxygen-rich unsaturated compounds became more concentrated. In contrast, during incubation under methanogenic conditions, there was no net accumulation of DOC; blue-shift of humic-like peaks suggest the transformation possibly associated with loss of oxygen-bearing functional groups in conjugated structures of humic substances. This interpretation is consistent with the relative decrease of oxygenation and carbon number in the pool of aromatic and highly unsaturated compounds observed by FT-ICR-MS analysis. Approximately 90% of molecular formulae that were lost under methanogenic conditions were accumulated under sulfate reducing conditions. Our results suggest that under sulfate reducing conditions degradation of organic matter results in the accumulation of highly oxidized DOM, while protein-like compounds are selectively consumed. When the redox regime changes to methanogenic conditions, microbes apparently utilize the humic-like and oxygen-rich compounds of the oxidized DOM pool that accumulated under sulfate reducing conditions. Consequently, redox regimes and the associated biogeochemical processes influence rate and fractions of DOM released by and consumed in the deep biosphere, which could ultimately shape the composition of the preserved sedimentary organic matter and the DOM released to the ocean.

中文翻译：

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氧化还原条件对西地中海罗纳河附近海洋沉积物中溶解有机物 (DOM) 质量的影响

摘要 沉积溶解有机质 (DOM) 是缺氧海底有机质早期成岩过程中产生和消耗的重要中间体库。沿海沉积物中有机物的快速降解导致氧化还原带分层。然而，迄今为止，关于在对比氧化还原条件下初始微生物介导的厌氧降解过程对有机物的选择性以及它们如何影响 DOM 的组成知之甚少。为了研究硫酸盐还原与产甲烷条件对 DOM 质量和降解的影响，培养了罗纳河三角洲的沉积物（0-18 厘米），氧化还原条件由硫酸盐修正物控制。通过H 2 、CH 4 、硫酸盐、DIC、DOC和乙酸盐的产生监测温育进程。DOM 组成由 3D 荧光光谱法，即激发发射矩阵光谱法 (EEM) 和超高分辨率傅立叶变换离子回旋共振质谱法 (FT-ICR-MS) 确定。EEM 的平行因子分析 (PARAFAC) 用于区分不同组的 DOM（类腐殖质和类蛋白质化合物），以评估 DOM 的组成、共轭和大小。在孵化之前，类腐殖质沉积 DOM 占主导地位；FT-ICR-MS 鉴定的分子式（>5000）中，近一半是 CHO，三分之一是 CHNO 化合物。在孵化过程中，在硫酸盐还原和产甲烷条件下，具有 3 和 4 个 N 原子分子式的类蛋白质 DOM 和 CHNO 分子式很少见。在硫酸盐还原条件下培养导致溶解有机碳 (DOC) 的快速释放和净积累。类蛋白质 DOM 快速循环，积累类腐殖质 DOM。一致地，具有 3 个和 4 个 N 原子的 CHNO 分子式下降得更快，而具有一个 N 和富氧不饱和化合物的分子式变得更加浓缩。相比之下，在产甲烷条件下培养期间，DOC 没有净积累；类腐殖质峰的蓝移表明这种转变可能与腐殖质共轭结构中含氧官能团的损失有关。这种解释与通过 FT-ICR-MS 分析观察到的芳香族和高度不饱和化合物池中氧合和碳数的相对减少是一致的。在产甲烷条件下丢失的大约 90% 的分子式在硫酸盐还原条件下积累。我们的结果表明，在硫酸盐还原条件下，有机物的降解导致高度氧化的 DOM 的积累，而蛋白质类化合物则被选择性消耗。当氧化还原状态变为产甲烷条件时，微生物显然会利用在硫酸盐还原条件下积累的氧化 DOM 池中的类腐殖质和富氧化合物。因此，氧化还原机制和相关的生物地球化学过程会影响深部生物圈释放和消耗的 DOM 的速率和比例，这可能最终影响保存的沉积有机物的组成和释放到海洋中的 DOM。当氧化还原状态变为产甲烷条件时，微生物显然会利用在硫酸盐还原条件下积累的氧化 DOM 池中的类腐殖质和富氧化合物。因此，氧化还原机制和相关的生物地球化学过程会影响深部生物圈释放和消耗的 DOM 的速率和比例，这可能最终影响保存的沉积有机物的组成和释放到海洋中的 DOM。当氧化还原状态变为产甲烷条件时，微生物显然会利用在硫酸盐还原条件下积累的氧化 DOM 池中的类腐殖质和富氧化合物。因此，氧化还原机制和相关的生物地球化学过程会影响深部生物圈释放和消耗的 DOM 的速率和比例，这可能最终影响保存的沉积有机物的组成和释放到海洋中的 DOM。