Arshad Arslan, Speth Daan R, de Graaf Rob M, Op den Camp Huub J M, Jetten Mike S M, Welte Cornelia U
Department of Microbiology, Institute for Water and Wetland Research, Radboud University Nijmegen, Netherlands.
Front Microbiol. 2015 Dec 18;6:1423. doi: 10.3389/fmicb.2015.01423. eCollection 2015.
Methane oxidation is an important process to mitigate the emission of the greenhouse gas methane and further exacerbating of climate forcing. Both aerobic and anaerobic microorganisms have been reported to catalyze methane oxidation with only a few possible electron acceptors. Recently, new microorganisms were identified that could couple the oxidation of methane to nitrate or nitrite reduction. Here we investigated such an enrichment culture at the (meta) genomic level to establish a metabolic model of nitrate-driven anaerobic oxidation of methane (nitrate-AOM). Nitrate-AOM is catalyzed by an archaeon closely related to (reverse) methanogens that belongs to the ANME-2d clade, tentatively named Methanoperedens nitroreducens. Methane may be activated by methyl-CoM reductase and subsequently undergo full oxidation to carbon dioxide via reverse methanogenesis. All enzymes of this pathway were present and expressed in the investigated culture. The genome of the archaeal enrichment culture encoded a variety of enzymes involved in an electron transport chain similar to those found in Methanosarcina species with additional features not previously found in methane-converting archaea. Nitrate reduction to nitrite seems to be located in the pseudoperiplasm and may be catalyzed by an unusual Nar-like protein complex. A small part of the resulting nitrite is reduced to ammonium which may be catalyzed by a Nrf-type nitrite reductase. One of the key questions is how electrons from cytoplasmically located reverse methanogenesis reach the nitrate reductase in the pseudoperiplasm. Electron transport in M. nitroreducens probably involves cofactor F420 in the cytoplasm, quinones in the cytoplasmic membrane and cytochrome c in the pseudoperiplasm. The membrane-bound electron transport chain includes F420H2 dehydrogenase and an unusual Rieske/cytochrome b complex. Based on genome and transcriptome studies a tentative model of how central energy metabolism of nitrate-AOM could work is presented and discussed.
甲烷氧化是减少温室气体甲烷排放及进一步缓解气候变暖的重要过程。据报道，需氧和厌氧微生物均可催化甲烷氧化，且仅有少数几种可能的电子受体。最近，人们发现了一些新的微生物，它们能够将甲烷氧化与硝酸盐或亚硝酸盐还原耦合起来。在此，我们在（宏）基因组水平上研究了这样一种富集培养物，以建立硝酸盐驱动的甲烷厌氧氧化（硝酸盐-厌氧甲烷氧化）的代谢模型。硝酸盐-厌氧甲烷氧化由一种与（逆向）产甲烷菌密切相关的古菌催化，该古菌属于ANME-2d进化枝，暂命名为嗜硝还原甲烷菌。甲烷可能由甲基辅酶M还原酶激活，随后通过逆向产甲烷作用完全氧化为二氧化碳。该途径的所有酶均存在于所研究的培养物中并表达。古菌富集培养物的基因组编码了多种参与电子传递链的酶，这些酶类似于在甲烷八叠球菌属物种中发现的酶，且具有甲烷转化古菌中以前未发现的其他特征。硝酸盐还原为亚硝酸盐似乎发生在假周质中，可能由一种不寻常的类Nar蛋白复合物催化。产生的亚硝酸盐的一小部分被还原为铵，这可能由一种Nrf型亚硝酸盐还原酶催化。关键问题之一是位于细胞质中的逆向产甲烷作用产生的电子如何到达假周质中的硝酸盐还原酶。嗜硝还原甲烷菌中的电子传递可能涉及细胞质中的辅因子F420、细胞质膜中的醌和假周质中的细胞色素c。膜结合电子传递链包括F420H2脱氢酶和一种不寻常的铁硫蛋白/细胞色素b复合物。基于基因组和转录组研究，我们提出并讨论了一个关于硝酸盐-厌氧甲烷氧化核心能量代谢可能如何运作的初步模型。
