Rahav E, Giannetto M J, Bar-Zeev E
Israel Oceanographic and Limnological Research, National Institute of Oceanography, Haifa, 8030, Israel.
Department of Chemical and Environmental Engineering, Yale University, New Haven, CT 06520, USA.
Sci Rep. 2016 Jun 16;6:27858. doi: 10.1038/srep27858.
N2 fixation should be a critical process in the nitrogen-poor surface water of the eastern Mediterranean Sea. Despite favorable conditions, diazotroph abundance and N2 fixation rates remains low for reasons yet explained. The main goal of this study was to investigate the limiting nutrients for diazotrophy in this oligotrophic environment. Hence, we conducted dedicated bottle-microcosms with eastern Mediterranean Sea water that were supplemented with mono and polysaccharides as well as inorganic nitrogen and phosphorous. Our results indicate that the diazotrophic community expressing nifH was primarily represented by heterotrophic Proteobacteria. N2 fixation and heterotrophic bacterial activity increased up-to tenfold following two days of dark incubations, once seawater was supplemented with organic carbon substrate in the form of glucose (monosaccharides) or gum-xanthan (polysaccharide surrogate). Furthermore, our results point that carbon-rich polysaccharides, such as transparent exopolymer particles, enhance heterotrophic N2 fixation, by forming microenvironments of intense metabolic activity, high carbon: nitrogen ratio, and possibly low O2 levels. The conclusions of this study indicate that diazotrophs in the eastern Mediterranean coast are primarily limited by organic carbon substrates, as possibly in many other marine regions.
在东地中海氮含量较低的表层水中，固氮作用应是一个关键过程。尽管条件适宜，但由于尚未明确的原因，固氮微生物的丰度和固氮率仍然很低。本研究的主要目标是调查在这种贫营养环境中固氮作用的限制营养物质。因此，我们用东地中海海水进行了专门的瓶式微宇宙实验，添加了单糖和多糖以及无机氮和磷。我们的结果表明，表达nifH的固氮微生物群落主要由异养变形菌组成。一旦以葡萄糖（单糖）或黄原胶（多糖替代物）形式的有机碳底物添加到海水中，经过两天的黑暗培养后，固氮作用和异养细菌活性增加了高达十倍。此外，我们的结果表明，富含碳的多糖，如透明胞外聚合物颗粒，通过形成强烈代谢活动、高碳氮比以及可能低氧水平的微环境，增强了异养固氮作用。本研究的结论表明，东地中海沿岸的固氮微生物主要受有机碳底物的限制，许多其他海洋区域可能也是如此。
