Department of Biogeochemistry, Max Planck Institute for Terrestrial Microbiology, Marburg, Germany.
ISME J. 2012 Jul;6(7):1302-13. doi: 10.1038/ismej.2011.194. Epub 2011 Dec 22.
Although it is well documented that the lack of nitrogen in the diet of wood-feeding termites is compensated by the nitrogen-fixing capacity of their gut microbiota, the bacteria responsible for this activity are largely unknown. Here, we analyzed the diversity and expression of nitrogenase genes (homologs of nifH) in four species of dry-wood termites (Kalotermitidae), which thrive on a particularly nitrogen-poor resource. Although each species harbored a highly diverse suite of termite-specific homologs in their microliter-sized hindgut, only a core set related to nifH genes of Treponema and Azoarcus spp., 'Azobacteroides pseudotrichonymphae', the first member of the Bacteroidales identified as a diazotroph, and termite-gut-specific anfH genes of hitherto unknown origin were preferentially expressed. Transcription patterns corroborated that the populations of active diazotrophs differ fundamentally between termite genera. Capillary-picked suspensions of the flagellates Devescovina arta and Snyderella tabogae revealed that their bacterial ectosymbionts each possess two paralogs of nifH, which apparently have been acquired consecutively during evolution of Bacteroidales, but only one of them (anfH) is actively expressed. Transcription patterns correlated neither with the molybdenum content of the diet nor with intestinal hydrogen concentrations, measured with microsensors. We propose that the nitrogen-fixing community in different dry-wood termites is shaped by the symbionts of their specific flagellate populations. Our findings suggest that the diazotrophic nature of 'Armantifilum devescovinae' has an important role in the nitrogen metabolism of dry-wood termites and is the driving force of co-evolution with its flagellate host.
尽管有大量文献记载，表明取食木质的白蚁的饮食中缺乏氮元素，但可通过其肠道微生物群的固氮能力得到补偿，但负责这种活动的细菌在很大程度上仍是未知的。在这里，我们分析了 4 种干木白蚁（白蚁科）的多样性和固氮酶基因（nifH 同源物）的表达，这些白蚁取食的是一种特别缺乏氮的资源。尽管每个物种的微型后肠中都存在高度多样化的白蚁特异性同源物，但只有一组与 Treponema 和 Azoarcus spp. 的 nifH 基因、“拟食木质真杆菌”（第一个被鉴定为固氮菌的拟杆菌门成员）以及与未知来源的白蚁肠道特异性 anfH 基因相关的核心基因集得到了优先表达。转录模式证实，活跃固氮菌的种群在白蚁属之间存在根本差异。用毛细管挑选的鞭毛虫 Devescovina arta 和 Snyderella tabogae 悬浮液表明，它们的细菌外共生体各自拥有 2 个 nifH 基因的旁系同源物，这些基因显然是在拟杆菌门的进化过程中连续获得的，但只有一个（anfH）是活跃表达的。转录模式既与饮食中的钼含量无关，也与用微传感器测量的肠道氢浓度无关。我们提出，不同干木白蚁中的固氮群落是由其特定鞭毛虫种群的共生体塑造的。我们的研究结果表明，“Armantifilum devescovinae”的固氮性质在干木白蚁的氮代谢中具有重要作用，是与它们的鞭毛虫宿主共同进化的驱动力。
