PDF(Pubmed)
Abstract：
Biological nitrogen fixation (BNF) by methanotrophic bacteria has been shown to play an important role in maintaining fertility. However, this process is still limited to aerobic methane oxidation with sufficient oxygen. It has remained unknown whether and how methanotrophic BNF proceeds in hypoxic environments. Herein, we incubated paddy soils with a ferrihydrite-containing mineral salt medium to enrich methanotrophic bacteria in the presence of methane (20%, v/v) under oxygen constraints (0.27%, v/v). The resulting microcosms showed that ferrihydrite-dependent aerobic methane oxidation significantly contributed (81%) to total BNF, increasing the 15N fixation rate by 13-fold from 0.02 to 0.28 μmol 15N2 (g dry weight soil) -1 d-1. BNF was reduced by 97% when ferrihydrite was omitted, demonstrating the involvement of ferrihydrite in methanotrophic BNF. DNA stable-isotope probing indicated that Methylocystis, Methylophilaceae, and Methylomicrobium were the dominant methanotrophs/methylotrophs that assimilated labeled isotopes (13C or 15N) into biomass. Metagenomic binning combined with electrochemical analysis suggested that Methylocystis and Methylophilaceae had the potential to perform methane-induced BNF and likely utilized riboflavin and c-type cytochromes as electron carriers for ferrihydrite reduction. It was concluded that ferrihydrite mediated methanotrophic BNF by methanotrophs/methylotrophs solely or in conjunction with iron-reducing bacteria. Overall, this study revealed a previously overlooked yet pronounced coupling of iron-dependent aerobic methane oxidation to BNF and improves our understanding of methanotrophic BNF in hypoxic zones.
摘要：
甲烷营养细菌的生物固氮（BNF）已被证明在维持肥力中起重要作用。然而,该过程仍然限于具有足够氧气的好氧甲烷氧化。尚不清楚甲烷营养BNF是否以及如何在低氧环境中进行。在这里，我们用含有水铁矿的矿物盐培养基培养水稻土以在甲烷存在下富集甲烷营养细菌（20％，v/v)在氧气约束下(0.27%，v/v）。由此产生的微观世界表明，水铁矿依赖的好氧甲烷氧化对总BNF有显著贡献(81%)，将15N固定率从0.02提高到0.28μmol15N2（g干重土壤）-1d-1，提高了13倍。当忽略水铁矿时，BNF降低了97%，证明水铁矿参与甲烷营养BNF。DNA稳定同位素探测表明，嗜甲基科，和甲基微生物是主要的甲烷氧化菌/甲基氧化菌，将标记的同位素（13C或15N）同化为生物量。宏基因组分级与电化学分析相结合表明，甲基囊虫和嗜甲基科具有进行甲烷诱导的BNF的潜力，并且可能利用核黄素和c型细胞色素作为减少水铁矿的电子载体。结论是，水铁矿仅通过甲烷氧化菌/甲基营养菌或与铁还原细菌一起介导甲烷氧化BNF。总的来说,这项研究揭示了以前被忽视但明显的铁依赖好氧甲烷氧化与BNF的耦合，并提高了我们对缺氧区甲烷营养BNF的理解。
