Houghton Karen M, Carere Carlo R, Stott Matthew B, McDonald Ian R
Te Pū Ao | GNS Science, Wairakei Research Centre, Taupō, New Zealand.
Te Aka Mātuatua | School of Science, Te Whare Wānanga o Waikato | University of Waikato, Hamilton, New Zealand.
Front Microbiol. 2023 Aug 31;14:1253773. doi: 10.3389/fmicb.2023.1253773. eCollection 2023.
Geothermal areas represent substantial point sources for greenhouse gas emissions such as methane. While it is known that methanotrophic microorganisms act as a biofilter, decreasing the efflux of methane in most soils to the atmosphere, the diversity and the extent to which methane is consumed by thermophilic microorganisms in geothermal ecosystems has not been widely explored. To determine the extent of biologically mediated methane oxidation at elevated temperatures, we set up 57 microcosms using soils from 14 Aotearoa-New Zealand geothermal fields and show that moderately thermophilic (>40°C) and thermophilic (>60°C) methane oxidation is common across the region. Methane oxidation was detected in 54% ( = 31) of the geothermal soil microcosms tested at temperatures up to 75°C (pH 1.5-8.1), with oxidation rates ranging from 0.5 to 17.4 μmol g d wet weight. The abundance of known aerobic methanotrophs (up to 60.7% and 11.2% ) and putative anaerobic methanotrophs (up to 76.7% Bathyarchaeota) provides some explanation for the rapid rates of methane oxidation observed in microcosms. However, not all methane oxidation was attributable to known taxa; in some methane-consuming microcosms we detected methanotroph taxa in conditions outside of their known temperature range for growth, and in other examples, we observed methane oxidation in the absence of known methanotrophs through 16S rRNA gene sequencing. Both of these observations suggest unidentified methane oxidizing microorganisms or undescribed methanotrophic syntrophic associations may also be present. Subsequent enrichment cultures from microcosms yielded communities not predicted by the original diversity studies and showed rates inconsistent with microcosms (≤24.5 μmol d), highlighting difficulties in culturing representative thermophilic methanotrophs. Finally, to determine the active methane oxidation processes, we attempted to elucidate metabolic pathways from two enrichment cultures actively oxidizing methane using metatranscriptomics. The most highly expressed genes in both enrichments (methane monooxygenases, methanol dehydrogenases and PqqA precursor peptides) were related to methanotrophs from Methylococcaceae, Methylocystaceae and Methylothermaceae. This is the first example of using metatranscriptomics to investigate methanotrophs from geothermal environments and gives insight into the metabolic pathways involved in thermophilic methanotrophy.
地热区是甲烷等温室气体排放的重要点源。虽然已知甲烷营养微生物起到生物过滤器的作用，可减少大多数土壤中甲烷向大气的排放，但地热生态系统中嗜热微生物消耗甲烷的多样性和程度尚未得到广泛研究。为了确定高温下生物介导的甲烷氧化程度，我们使用来自新西兰14个地热田的土壤建立了57个微观模型，并表明中度嗜热（>40°C）和嗜热（>60°C）的甲烷氧化在该地区很常见。在高达75°C（pH 1.5 - 8.1）的温度下测试的54%（n = 31）的地热土壤微观模型中检测到甲烷氧化，氧化速率范围为0.5至17.4 μmol g-1 d-1湿重。已知的需氧甲烷营养菌（高达60.7% 和11.2% ）和假定的厌氧甲烷营养菌（高达76.7% 的深海古菌）的丰度为微观模型中观察到的快速甲烷氧化速率提供了一些解释。然而，并非所有的甲烷氧化都可归因于已知的分类群；在一些消耗甲烷的微观模型中，我们在已知生长温度范围之外的条件下检测到甲烷营养菌分类群，在其他例子中，通过16S rRNA基因测序，我们在没有已知甲烷营养菌的情况下观察到甲烷氧化。这两个观察结果都表明可能还存在未鉴定的甲烷氧化微生物或未描述的甲烷营养共生关系。随后从微观模型中进行的富集培养产生了原始多样性研究未预测到的群落，并且显示出与微观模型不一致的速率（≤24.5 μmol d-1），突出了培养代表性嗜热甲烷营养菌的困难。最后，为了确定活跃的甲烷氧化过程，我们试图使用宏转录组学从两种积极氧化甲烷的富集培养物中阐明代谢途径。两种富集培养物中表达最高的基因（甲烷单加氧酶、甲醇脱氢酶和PqqA前体肽）与甲基球菌科、甲基孢囊菌科和嗜甲基热菌科的甲烷营养菌有关。这是首次使用宏转录组学研究地热环境中的甲烷营养菌，并深入了解嗜热甲烷营养所涉及的代谢途径。
