Methane is a potent greenhouse gas; methane production and consumption within seafloor sediments has generated intense interest. Anaerobic oxidation of methane (AOM) and methanogenesis (MOG) primarily occur at the depth of the sulfate–methane transition zone or underlying sediment respectively. Methanogenesis can also occur in the sulfate‐reducing sediments through the utilization of non‐competitive methylated compounds; however, the occurrence and importance of this process are not fully understood. Here, we combined a variety of data, including geochemical measurements, rate measurements and molecular analyses to demonstrate the presence of a cryptic methane cycle in sulfate‐reducing sediments from the continental shelf of the northern South China Sea. The abundance of methanogenic substrates as well as the high MOG rates from methylated compounds indicated that methylotrophic methanogenesis was the dominant methanogenic pathway; this conclusion was further supported by the presence of the methylotrophic genus Methanococcoides. High potential rates of AOM were observed in the sediments, indicating that methane produced in situ could be oxidized simultaneously by AOM, presumably by ANME‐2a/b as indicated by 16S rRNA gene analysis. A significant correlation between the relative abundance of methanogens and methanotrophs was observed over sediment depth, indicating that methylotrophic methanogenesis could potentially fuel AOM in this environment. In addition, higher potential rates of AOM than sulfate reduction rates at in situ methane conditions were observed, making alternative electron acceptors important to support AOM in sulfate‐reducing sediment. AOM rates were stimulated by the addition of Fe/Mn oxides, suggesting AOM could be partially coupled to metal oxide reduction. These results suggest that methyl‐compounds driven methane production drives a cryptic methane cycling and fuels AOM coupled to the reduction of sulfate and other electron acceptors.

中文翻译：

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南海减少硫酸盐沉积物中甲基化合物驱动的底栖碳循环

甲烷是一种有力的温室气体。海底沉积物中甲烷的生产和消费引起了人们的极大兴趣。甲烷的厌氧氧化（AOM）和甲烷生成（MOG）主要发生在硫酸盐-甲烷过渡带或下层沉积物的深度。通过利用非竞争性甲基化化合物，在硫酸盐还原沉积物中也可能发生甲烷生成。但是，此过程的发生和重要性尚不完全清楚。在这里，我们结合了多种数据，包括地球化学测量，速率测量和分子分析，以证明南海北部陆架的硫酸盐还原沉积物中存在隐秘的甲烷循环。产甲烷的底物丰富以及甲基化化合物的高MOG速率表明，甲基营养型产甲烷是主要的产甲烷途径。甲基营养族的存在进一步支持了这一结论。甲烷菌。在沉积物中观察到高潜在的AOM速率，这表明原位产生的甲烷可以同时被AOM氧化，如16S rRNA基因分析所示，可能被ANME-2a / b氧化。在沉积物深度上观察到产甲烷菌和甲烷营养菌的相对丰度之间存在显着相关性，这表明在这种环境下，甲基营养菌的甲烷化作用可能会助长AOM。此外，AOM的潜在速率比原位硫酸盐还原速率更高观察到了甲烷的条件，这使得替代电子受体对于支持AOM减少硫酸盐沉积物中的作用很重要。Fe / Mn氧化物的加入会刺激AOM速率，表明AOM可能与金属氧化物的还原部分相关。这些结果表明，由甲基化合物驱动的甲烷生产推动了隐秘的甲烷循环，并为AOM燃料加了硫酸盐和其他电子受体的还原。