Department of Crop and Soil Sciences, University of Georgia, Athens, 30602, GA, USA.
Department of Marine Sciences, University of Georgia, Athens, GA, USA.
Microbiome. 2018 Oct 23;6(1):189. doi: 10.1186/s40168-018-0574-5.
Many environments contain redox transition zones, where transient oxygenation events can modulate anaerobic reactions that influence the cycling of iron (Fe) and carbon (C) on a global scale. In predominantly anoxic soils, this biogeochemical cycling depends on Fe mineral composition and the activity of mixed Fe(III)-reducer populations that may be altered by periodic pulses of molecular oxygen (O).
We repeatedly exposed anoxic (4% H:96% N) suspensions of soil from the Luquillo Critical Zone Observatory to 1.05 × 10, 1.05 × 10, and 1.05 × 10 mmol O kg soil h during pulsed oxygenation treatments. Metatranscriptomic analysis and Fe Mössbauer spectroscopy were used to investigate changes in Fe(III)-reducer gene expression and Fe(III) crystallinity, respectively.
Slow oxygenation resulted in soil Fe-(oxyhydr)oxides of higher crystallinity (38.1 ± 1.1% of total Fe) compared to fast oxygenation (30.6 ± 1.5%, P < 0.001). Transcripts binning to the genomes of Fe(III)-reducers Anaeromyxobacter, Geobacter, and Pelosinus indicated significant differences in extracellular electron transport (e.g., multiheme cytochrome c, multicopper oxidase, and type-IV pilin gene expression), adhesion/contact (e.g., S-layer, adhesin, and flagellin gene expression), and selective microbial competition (e.g., bacteriocin gene expression) between the slow and fast oxygenation treatments during microbial Fe(III) reduction. These data also suggest that diverse Fe(III)-reducer functions, including cytochrome-dependent extracellular electron transport, are associated with type-III fibronectin domains. Additionally, the metatranscriptomic data indicate that Methanobacterium was significantly more active in the reduction of CO to CH and in the expression of class(III) signal peptide/type-IV pilin genes following repeated fast oxygenation compared to slow oxygenation.
This study demonstrates that specific Fe(III)-reduction mechanisms in mixed Fe(III)-reducer populations are uniquely sensitive to the rate of O influx, likely mediated by shifts in soil Fe(III)-(oxyhydr)oxide crystallinity. Overall, we provide evidence that transient oxygenation events play an important role in directing anaerobic pathways within soil microbiomes, which is expected to alter Fe and C cycling in redox-dynamic environments.
许多环境都包含氧化还原过渡区，其中瞬态氧合事件可以调节影响全球铁 (Fe) 和碳 (C) 循环的厌氧反应。在主要缺氧的土壤中，这种生物地球化学循环取决于 Fe 矿物组成和混合 Fe(III)还原剂群体的活性，而后者可能会受到周期性分子氧 (O) 脉冲的影响。
我们反复将来自卢奎洛关键带观测站的缺氧 (4% H:96% N) 土壤悬浮液暴露于脉冲供氧处理中的 1.05 × 10、1.05 × 10 和 1.05 × 10 mmol O kg 土壤 h。使用宏转录组分析和 Fe Mössbauer 光谱分别研究了 Fe(III)还原剂基因表达和 Fe(III)结晶度的变化。
与快速供氧相比，缓慢供氧导致土壤 Fe-(oxyhydr)oxides 具有更高的结晶度（总 Fe 的 38.1 ± 1.1%）（P < 0.001）。对 Fe(III)还原剂 Anaeromyxobacter、Geobacter 和 Pelosinus 基因组进行 binning 的转录本表明，在微生物 Fe(III)还原过程中，与慢速和快速供氧处理相比，细胞外电子传输（例如多血红素细胞色素 c、多铜氧化酶和 IV 型菌毛基因表达）、粘附/接触（例如 S-层、粘附素和菌毛基因表达）和选择性微生物竞争（例如细菌素基因表达）存在显着差异。这些数据还表明，与 III 型纤维连接蛋白结构域相关的各种 Fe(III)还原剂功能，包括细胞色素依赖的细胞外电子传输，在微生物 Fe(III)还原过程中具有重要作用。此外，宏转录组数据表明，与慢速供氧相比，重复快速供氧后，Methanobacterium 在 CO 还原为 CH 和表达 III 类信号肽/IV 型菌毛基因方面的活性明显更高。
本研究表明，混合 Fe(III)还原剂群体中的特定 Fe(III)还原机制对 O 流入率具有独特的敏感性，这可能是由土壤 Fe(III)-(oxyhydr)oxide 结晶度的变化介导的。总的来说，我们提供的证据表明，瞬态氧合事件在指导土壤微生物组中的厌氧途径方面发挥着重要作用，这预计会改变氧化还原动态环境中的 Fe 和 C 循环。
