Nitrogen fixation in a chemoautotrophic lucinid symbiosis
Nitrogen fixation in a chemoautotrophic lucinid symbiosis
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DOI：
10.1038/nmicrobiol.2016.193
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发表时间：
2017-01-01
影响因子：
28.3
通讯作者：
Markert, Stephanie
中科院分区：
文献类型：
--
作者：
Koenig, Sten;Gros, Olivier;Markert, Stephanie
浅水双壳类Codania orbularis与其鳃中的硫氧化细菌共生。内共生菌固定二氧化碳，从而产生有机碳化合物，支持宿主的生长。为了详细研究未培养的共生菌的代谢和共生菌与宿主的相互作用，我们对纯化的细菌进行了蛋白质组学分析。出乎意料的是，我们的结果揭示了一个迄今为止完全不被认识的共生体的生理特征：共生体的基因组编码了生物固氮(重氮营养)所需的所有蛋白质。用蛋白质组学方法证实了这些基因在标准环境条件下的表达。通过酶活性测定，验证了共生体中固氮酶的活性。细菌固氮酶还原酶nifH的系统发育分析表明，该共生体与海草底泥中的自由生活固氮变形杆菌关系密切。轮状念珠藻共生体，在这里暂时命名为‘Candidatus Thiodiazotroa endolucinida’，因此可能不仅维持双壳类对碳的需求。轮藻还可能受益于其共生体稳定的固定氮供应--这种情况在可比的化学自养共生体中是前所未有的。
The shallow water bivalve Codakia orbicularis lives in symbiotic association with a sulfur-oxidizing bacterium in its gills. The endosymbiont fixes CO2 and thus generates organic carbon compounds, which support the host's growth. To investigate the uncultured symbiont's metabolism and symbiont-host interactions in detail we conducted a proteogenomic analysis of purified bacteria. Unexpectedly, our results reveal a hitherto completely unrecognized feature of the C. orbicularis symbiont's physiology: the symbiont's genome encodes all proteins necessary for biological nitrogen fixation (diazotrophy). Expression of the respective genes under standard ambient conditions was confirmed by proteomics. Nitrogenase activity in the symbiont was also verified by enzyme activity assays. Phylogenetic analysis of the bacterial nitrogenase reductase NifH revealed the symbiont's close relationship to free-living nitrogen-fixing Proteobacteria from the seagrass sediment. The C. orbicularis symbiont, here tentatively named 'Candidatus Thiodiazotropha endolucinida', may thus not only sustain the bivalve's carbon demands. C. orbicularis may also benefit from a steady supply of fixed nitrogen from its symbiont-a scenario that is unprecedented in comparable chemoautotrophic symbioses.