Ohkuma Moriya, Noda Satoko, Hattori Satoshi, Iida Toshiya, Yuki Masahiro, Starns David, Inoue Jun-ichi, Darby Alistair C, Hongoh Yuichi
Japan Collection of Microorganisms/Microbe Division, RIKEN BioResource Center, and Biomass Research Platform Team, RIKEN Biomass Engineering Program Cooperation Division, RIKEN Center for Sustainable Resource Science, Ibaraki 305-0074, Japan;
Japan Collection of Microorganisms/Microbe Division, RIKEN BioResource Center, and Interdisciplinary Graduate School of Medicine and Engineering, University of Yamanashi, Yamanashi 400-8511, Japan;
Proc Natl Acad Sci U S A. 2015 Aug 18;112(33):10224-30. doi: 10.1073/pnas.1423979112. Epub 2015 May 15.
Symbiotic associations of cellulolytic eukaryotic protists and diverse bacteria are common in the gut microbial communities of termites. Besides cellulose degradation by the gut protists, reductive acetogenesis from H2 plus CO2 and nitrogen fixation by gut bacteria play crucial roles in the host termites' nutrition by contributing to the energy demand of termites and supplying nitrogen poor in their diet, respectively. Fractionation of these activities and the identification of key genes from the gut community of the wood-feeding termite Hodotermopsis sjoestedti revealed that substantial activities in the gut--nearly 60% of reductive acetogenesis and almost exclusively for nitrogen fixation--were uniquely attributed to the endosymbiotic bacteria of the cellulolytic protist in the genus Eucomonympha. The rod-shaped endosymbionts were surprisingly identified as a spirochete species in the genus Treponema, which usually exhibits a characteristic spiral morphology. The endosymbionts likely use H2 produced by the protist for these dual functions. Although H2 is known to inhibit nitrogen fixation in some bacteria, it seemed to rather stimulate this important mutualistic process. In addition, the single-cell genome analyses revealed the endosymbiont's potentials of the utilization of sugars for its energy requirement, and of the biosynthesis of valuable nutrients such as amino acids from the fixed nitrogen. These metabolic interactions are suitable for the dual functions of the endosymbiont and reconcile its substantial contributions in the gut.
纤维素分解真核原生生物与多种细菌的共生关系在白蚁肠道微生物群落中很常见。除了肠道原生生物降解纤维素外，肠道细菌利用H₂和CO₂进行的还原型乙酸生成以及固氮作用，分别通过满足白蚁的能量需求和补充其食物中缺乏的氮，在宿主白蚁的营养中发挥着关键作用。对以木材为食的白蚁斯氏澳白蚁（Hodotermopsis sjoestedti）肠道群落中这些活动的分离以及关键基因的鉴定表明，肠道中的大量活动——近60%的还原型乙酸生成以及几乎完全是固氮作用——唯一地归因于真核共生菌属（Eucomonympha）中纤维素分解原生生物的内共生细菌。令人惊讶的是，这些杆状内共生体被鉴定为密螺旋体属（Treponema）中的一种螺旋体物种，该属通常呈现出特征性的螺旋形态。这些内共生体可能利用原生生物产生的H₂来实现这两种功能。尽管已知H₂会抑制某些细菌的固氮作用，但它似乎反而刺激了这一重要的共生过程。此外，单细胞基因组分析揭示了内共生体利用糖类满足其能量需求的潜力，以及利用固定氮生物合成诸如氨基酸等有价值营养物质的潜力。这些代谢相互作用适合内共生体的双重功能，并协调了其在肠道中的重大贡献。
