Steunou Anne-Soisig, Jensen Sheila I, Brecht Eric, Becraft Eric D, Bateson Mary M, Kilian Oliver, Bhaya Devaki, Ward David M, Peters John W, Grossman Arthur R, Kühl Michael
Department of Plant Biology, The Carnegie Institution, Stanford University, Stanford, CA, USA.
ISME J. 2008 Apr;2(4):364-78. doi: 10.1038/ismej.2007.117. Epub 2008 Mar 6.
Nitrogen fixation, a prokaryotic, O2-inhibited process that reduces N2 gas to biomass, is of paramount importance in biogeochemical cycling of nitrogen. We analyzed the levels of nif transcripts of Synechococcus ecotypes, NifH subunit and nitrogenase activity over the diel cycle in the microbial mat of an alkaline hot spring in Yellowstone National Park. The results showed a rise in nif transcripts in the evening, with a subsequent decline over the course of the night. In contrast, immunological data demonstrated that the level of the NifH polypeptide remained stable during the night, and only declined when the mat became oxic in the morning. Nitrogenase activity was low throughout the night; however, it exhibited two peaks, a small one in the evening and a large one in the early morning, when light began to stimulate cyanobacterial photosynthetic activity, but O2 consumption by respiration still exceeded the rate of O2 evolution. Once the irradiance increased to the point at which the mat became oxic, the nitrogenase activity was strongly inhibited. Transcripts for proteins associated with energy-producing metabolisms in the cell also followed diel patterns, with fermentation-related transcripts accumulating at night, photosynthesis- and respiration-related transcripts accumulating during the day and late afternoon, respectively. These results are discussed with respect to the energetics and regulation of N2 fixation in hot spring mats and factors that can markedly influence the extent of N2 fixation over the diel cycle.
固氮作用是一个原核的、受氧气抑制的过程，它将氮气还原为生物量，在氮的生物地球化学循环中至关重要。我们分析了黄石国家公园一个碱性温泉微生物垫中不同生态型的聚球藻的固氮基因转录本水平、固氮酶铁蛋白亚基（NifH）和固氮酶活性在昼夜周期中的变化。结果显示，固氮基因转录本在傍晚时增加，随后在夜间逐渐下降。相比之下，免疫学数据表明，NifH多肽的水平在夜间保持稳定，仅在早晨微生物垫变为有氧状态时才下降。固氮酶活性在整个夜间都很低；然而，它出现了两个峰值，一个小峰值在傍晚，一个大峰值在清晨，此时光照开始刺激蓝藻的光合活性，但呼吸作用消耗氧气的速率仍超过氧气释放的速率。一旦光照强度增加到微生物垫变为有氧状态的程度，固氮酶活性就会受到强烈抑制。细胞中与能量产生代谢相关的蛋白质转录本也呈现昼夜模式，与发酵相关的转录本在夜间积累，与光合作用和呼吸作用相关的转录本分别在白天和傍晚积累。我们结合温泉微生物垫中固氮作用的能量学和调控以及在昼夜周期中可显著影响固氮程度的因素对这些结果进行了讨论。
