Krupke Andreas, Mohr Wiebke, LaRoche Julie, Fuchs Bernhard M, Amann Rudolf I, Kuypers Marcel M M
Max Planck Institute for Marine Microbiology, Bremen, Germany.
Helmholtz-Zentrum für Ozeanforschung, Kiel, Germany.
ISME J. 2015 Jul;9(7):1635-47. doi: 10.1038/ismej.2014.253. Epub 2014 Dec 23.
Symbiotic relationships between phytoplankton and N2-fixing microorganisms play a crucial role in marine ecosystems. The abundant and widespread unicellular cyanobacteria group A (UCYN-A) has recently been found to live symbiotically with a haptophyte. Here, we investigated the effect of nitrogen (N), phosphorus (P), iron (Fe) and Saharan dust additions on nitrogen (N2) fixation and primary production by the UCYN-A-haptophyte association in the subtropical eastern North Atlantic Ocean using nifH expression analysis and stable isotope incubations combined with single-cell measurements. N2 fixation by UCYN-A was stimulated by the addition of Fe and Saharan dust, although this was not reflected in the nifH expression. CO2 fixation by the haptophyte was stimulated by the addition of ammonium nitrate as well as Fe and Saharan dust. Intriguingly, the single-cell analysis using nanometer scale secondary ion mass spectrometry indicates that the increased CO2 fixation by the haptophyte in treatments without added fixed N is likely an indirect result of the positive effect of Fe and/or P on UCYN-A N2 fixation and the transfer of N2-derived N to the haptophyte. Our results reveal a direct linkage between the marine carbon and nitrogen cycles that is fuelled by the atmospheric deposition of dust. The comparison of single-cell rates suggests a tight coupling of nitrogen and carbon transfer that stays balanced even under changing nutrient regimes. However, it appears that the transfer of carbon from the haptophyte to UCYN-A requires a transfer of nitrogen from UCYN-A. This tight coupling indicates an obligate symbiosis of this globally important diazotrophic association.
浮游植物与固氮微生物之间的共生关系在海洋生态系统中起着至关重要的作用。最近发现，数量丰富且分布广泛的单细胞蓝细菌A组（UCYN - A）与一种定鞭藻存在共生关系。在此，我们通过nifH表达分析、稳定同位素培养结合单细胞测量，研究了添加氮（N）、磷（P）、铁（Fe）和撒哈拉沙尘对北大西洋亚热带东部海域UCYN - A - 定鞭藻联合体的固氮作用和初级生产的影响。添加Fe和撒哈拉沙尘刺激了UCYN - A的固氮作用，尽管这在nifH表达中并未体现出来。添加硝酸铵以及Fe和撒哈拉沙尘刺激了定鞭藻的二氧化碳固定。有趣的是，使用纳米尺度二次离子质谱的单细胞分析表明，在未添加固定氮的处理中，定鞭藻二氧化碳固定的增加可能是Fe和/或P对UCYN - A固氮的积极作用以及固氮产生的氮向定鞭藻转移的间接结果。我们的研究结果揭示了由沙尘的大气沉降驱动的海洋碳循环与氮循环之间的直接联系。单细胞速率的比较表明，氮和碳的转移紧密耦合，即使在营养状况变化的情况下也能保持平衡。然而，似乎定鞭藻向UCYN - A的碳转移需要UCYN - A的氮转移。这种紧密耦合表明这种全球重要的固氮联合体存在专性共生关系。
