In lakes, large amounts of methane are produced in anoxic sediments. Methane-oxidizing bacteria effectively convert this potent greenhouse gas into biomass and carbon dioxide. These bacteria are present throughout the water column, where methane concentrations can range from nanomolar to millimolar. In this study, we tested the hypothesis that methanotroph assemblages in a seasonally stratified freshwater lake are adapted to the contrasting methane concentrations in the epi- and hypolimnion. We further hypothesized that lake overturn would change the apparent methane oxidation kinetics as more methane becomes available in the epilimnion. In addition to the change in the methane oxidation kinetics, we investigated changes in the transcription of genes encoding methane monooxygenase, the enzyme responsible for the first step of methane oxidation, with metatranscriptomics. Using laboratory incubations of the natural microbial communities, we show that the half-saturation constant (K_m) for methane – the methane concentration at which half the maximum methane oxidation rate is reached – was 20 times higher in the hypolimnion than in the epilimnion during stable stratification. During lake overturn, however, the kinetic constants in the epi- and hypolimnion converged along with a change in the transcriptionally active methanotroph assemblage. Conventional particulate methane monooxygenase appeared to be responsible for methane oxidation under different methane concentrations. Our results suggest that methane availability is one important factor for creating niches for methanotroph assemblages with well-adapted methane oxidation kinetics. This rapid selection and succession of adapted lacustrine methanotroph assemblages allowed the previously reported high removal efficiency of methane transported to the epilimnion to be maintained – even under rapidly changing conditions during lake overturn. Consequently, only a small fraction of methane stored in the anoxic hypolimnion is emitted to the atmosphere.

中文翻译：

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湖泊混合方案选择了甲烷营养菌组合物的表观甲烷氧化动力学

在湖泊中，缺氧沉积物中会产生大量甲烷。甲烷氧化细菌可有效地将这种强大的温室气体转化为生物质和二氧化碳。这些细菌遍布水柱，甲烷的浓度范围可以从纳摩尔到毫摩尔。在这项研究中，我们检验了以下假设：季节性分层的淡水湖中的甲烷营养菌组合适应于上层和下层的甲烷浓度对比。我们进一步假设湖泊的倾覆将改变表观甲烷氧化动力学，因为更多的甲烷可从上层中获得。除了甲烷氧化动力学的变化之外，我们还研究了编码甲烷单加氧酶（负责甲烷氧化第一步的酶）的基因的转录变化，与元转录组学。利用自然微生物群落的实验室培养，我们显示出半饱和常数（ķ_米）中的甲烷（达到最大甲烷氧化速率的一半时的甲烷浓度）在稳定分层过程中，次上苯丙氨酸的浓度比上次上苯丙酸的浓度高20倍。然而，在湖面倾覆过程中，上层和下层的动力学常数随着转录活性甲烷营养菌组合的变化而收敛。在不同的甲烷浓度下，常规的颗粒甲烷单加氧酶似乎是造成甲烷氧化的原因。我们的研究结果表明，甲烷的利用度是为甲烷营养生物创造合适的甲烷氧化动力学的重要因素之一。这种快速选择和接班的适应性湖甲烷甲烷营养组合使得即使在湖泊倾覆过程中迅速变化的条件下，也可以保持先前报道的甲烷向the上甲烷的高去除效率。因此，仅将一小部分存储在缺氧低渗水中的甲烷排放到大气中。