Department of Ecology, Environment and Plant Sciences, Stockholm University, Stockholm, Sweden.
Department of Experimental Limnology, IGB-Leibniz-Institute of Freshwater Ecology and Inland Fisheries, Berlin, Germany.
ISME J. 2020 Feb;14(2):399-412. doi: 10.1038/s41396-019-0514-9. Epub 2019 Oct 21.
Trichodesmium is an important dinitrogen (N)-fixing cyanobacterium in marine ecosystems. Recent nucleic acid analyses indicate that Trichodesmium colonies with their diverse epibionts support various nitrogen (N) transformations beyond N fixation. However, rates of these transformations and concentration gradients of N compounds in Trichodesmium colonies remain largely unresolved. We combined isotope-tracer incubations, micro-profiling and numeric modelling to explore carbon fixation, N cycling processes as well as oxygen, ammonium and nitrate concentration gradients in individual field-sampled Trichodesmium colonies. Colonies were net-autotrophic, with carbon and N fixation occurring mostly during the day. Ten percent of the fixed N was released as ammonium after 12-h incubations. Nitrification was not detectable but nitrate consumption was high when nitrate was added. The consumed nitrate was partly reduced to ammonium, while denitrification was insignificant. Thus, the potential N transformation network was characterised by fixed N gain and recycling processes rather than denitrification. Oxygen concentrations within colonies were ~60-200% air-saturation. Moreover, our modelling predicted steep concentration gradients, with up to 6-fold higher ammonium concentrations, and nitrate depletion in the colony centre compared to the ambient seawater. These gradients created a chemically heterogeneous microenvironment, presumably facilitating diverse microbial metabolisms in millimetre-sized Trichodesmium colonies.
束毛藻是海洋生态系统中一种重要的固氮蓝藻。最近的核酸分析表明，具有多种附生生物的束毛藻群体除了固氮作用外，还支持多种氮（N）转化。然而，这些转化的速率和束毛藻群体中 N 化合物的浓度梯度在很大程度上仍未得到解决。我们结合同位素示踪培养、微观分析和数值模拟，探索了单个现场采集的束毛藻群体中的碳固定、N 循环过程以及氧气、铵和硝酸盐浓度梯度。群体为净自养型，碳和 N 固定主要发生在白天。经过 12 小时的培养，10%的固定 N 以铵的形式释放。未检测到硝化作用，但添加硝酸盐时硝酸盐消耗很高。消耗的硝酸盐部分被还原为铵，而反硝化作用不明显。因此，潜在的 N 转化网络的特征是固定 N 的增益和回收过程，而不是反硝化作用。群体内的氧气浓度约为空气饱和度的 60-200%。此外，我们的模型预测了陡峭的浓度梯度，与周围海水相比，铵的浓度高达 6 倍，并且在群体中心的硝酸盐耗尽。这些梯度创造了一个化学异质的微环境，可能有利于毫米大小的束毛藻群体中多样化的微生物代谢。
