Nitrogen (N2) fixation by moss-associated cyanobacteria is one of the main sources of new N input in pristine ecosystems such as boreal forests and arctic tundra. Given the non-vascular physiology of mosses, they are especially sensitive to e.g. increased N input and heavy metal deposition. While the effects of increased N input on moss-associated N2 fixation has been comprehensively assessed, hardly any reports exist on the effects of increased heavy metal load on this key ecosystem function. To address this knowledge gap, we made use of an extreme metal pollution gradient in boreal forests of Northern Sweden originating from a metal mine and its associated smelters. We collected the common moss Pleurozium schreberi, known to host cyanobacteria, along a distance gradient away from the metal source of pollution and measured moss-metal content (Fe, Cu, Zn, Pb) as well as N2 fixation. We found a strong distance gradient in moss-metal content for all investigated metals: a sharp decline in metal content with distance away from the metal pollution source. However, we found a similarly steep gradient in moss-associated N2 fixation, with highest activity closest to the metal source of pollution. Hence, while mosses may be sensitive to increased heavy metal inputs, the activity of colonising cyanobacteria seem to be unaffected by heavy metals, and consequently, ecosystem function may not be compromised by elevated metal input.

中文翻译：

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高重金属负载不会抑制苔藓-蓝细菌缔合中的固氮作用。

苔藓相关的蓝细菌固定氮（N2）是原始生态系统（如北方森林和北极冻原）中新氮输入的主要来源之一。考虑到苔藓的非血管生理性，它们对例如增加的氮输入和重金属沉积特别敏感。虽然已全面评估了氮输入增加对与苔藓相关的N2固定的影响，但几乎没有关于重金属负荷增加对这一关键生态系统功能的影响的报道。为了解决这一知识鸿沟，我们在瑞典北部的寒带森林中利用了一个极端的金属污染梯度，这种污染源自金属矿山及其相关的冶炼厂。我们沿远离金属污染源的距离梯度收集了常见的苔藓Pleurozium schreberi（已知为蓝细菌），并测量了苔藓金属含量（Fe，Cu，Zn，Pb）以及N2固定。我们发现所有调查的金属的苔藓金属含量都具有很强的距离梯度：随着距金属污染源的距离增加，金属含量急剧下降。但是，我们在与苔藓相关的N2固定中发现了类似的陡峭梯度，其最高活性最接近金属污染源。因此，虽然苔藓可能对增加的重金属输入敏感，但定居的蓝细菌的活动似乎不受重金属的影响，因此，生态系统功能可能不会因输入的金属增加而受到损害。我们在苔藓相关的N2固定中发现了类似的陡峭梯度，其最高活性最接近金属污染源。因此，虽然苔藓可能对增加的重金属输入敏感，但定居的蓝细菌的活动似乎不受重金属的影响，因此，生态系统功能可能不会因输入的金属增加而受到损害。我们在苔藓相关的N2固定中发现了类似的陡峭梯度，其最高活性最接近金属污染源。因此，虽然苔藓可能对增加的重金属输入敏感，但定居的蓝细菌的活动似乎不受重金属的影响，因此，生态系统功能可能不会因输入的金属增加而受到损害。