Lehnen Nadine, Marchant Hannah K, Schwedt Anne, Milucka Jana, Lott Christian, Weber Miriam, Dekaezemacker Julien, Seah Brandon K B, Hach Philipp F, Mohr Wiebke, Kuypers Marcel M M
Max Planck Institute for Marine Microbiology, Celsiusstraße 1, 28359 Bremen, Germany.
Max Planck Institute for Marine Microbiology, Celsiusstraße 1, 28359 Bremen, Germany.
Syst Appl Microbiol. 2016 Oct;39(7):476-483. doi: 10.1016/j.syapm.2016.08.004. Epub 2016 Aug 31.
Seagrass meadows of Posidonia oceanica represent hotspots of productivity in the oligotrophic Mediterranean Sea. The lack of dissolved inorganic nitrogen (DIN) in the seawater suggests that the N-demand of these meadows might be in part supported by microbial dinitrogen (N) fixation. However, currently there are no direct N fixation measurements available for this productive marine macrophyte. Here we investigated N fixation activity associated with P. oceanica leaf, rhizome and root pieces. In 15N incubations, the roots exhibited highest rates of N fixation. The rates varied considerably between replicates, presumably due to a patchy microbial colonization of the roots. Additions of organic carbon compounds (acetate, glucose, sucrose or algal lysate) did not enhance the N fixation rates. Sulfate reduction rates measured alongside were also highest in root incubations. Correspondingly, sequences of the nifH gene (a marker gene for the iron protein of the N-fixing enzyme nitrogenase) related to known sulfate-reducing bacteria were retrieved from P. oceanica roots. Other nifH sequences clustered with known heterotrophic diazotrophs previously identified in other marine macrophytes. In particular, many sequences obtained from P. oceanica roots were similar (>94%) to a saltmarsh rhizosphere-associated heterotrophic diazotroph, indicating that heterotrophic lifestyle might be common among marine macrophyte-associated diazotrophs.
地中海贫营养海域中，大叶藻海草草甸是生产力热点区域。海水中溶解无机氮（DIN）的缺乏表明，这些草甸对氮的需求可能部分由微生物固氮作用来满足。然而，目前尚无针对这种高产海洋大型植物的直接固氮量测定。在此，我们研究了与大叶藻叶片、根茎和根段相关的固氮活性。在15N培养实验中，根的固氮速率最高。各重复实验间的速率差异很大，推测是由于根上微生物定殖呈斑块状分布。添加有机碳化合物（乙酸盐、葡萄糖、蔗糖或藻类裂解物）并未提高固氮速率。同时测定的硫酸盐还原速率在根培养实验中也最高。相应地，从大叶藻根中检索到了与已知硫酸盐还原细菌相关的nifH基因（固氮酶铁蛋白的标记基因）序列。其他nifH序列与先前在其他海洋大型植物中鉴定出的已知异养固氮菌聚类。特别是，从大叶藻根中获得的许多序列与一种盐沼根际相关异养固氮菌相似性超过94%，这表明异养生活方式在与海洋大型植物相关的固氮菌中可能很常见。
