A new dynamic for Phosphorus in RIverbed Nitrogen Cycling - PRINCe

人类已经学会了如何操纵和利用维持地球上生命的元素(碳-碳、氮-氮和磷-磷)。事实上，我们在这方面已经变得如此熟练，以至于我们几乎已经将可用于种植农作物的固定氮(N)数量增加了一倍，按照目前的耕作做法，如果没有它，我们根本无法维持人类人口。然而，这种对氮的利用对环境造成了相当大的代价，特别是对河流、河口和沿海海域，因为它影响到它们作为生态系统的质量和价值。例如，高氮负荷可能会作为农田的径流进入河流，导致藻类大量繁殖、氧气枯竭及其普遍恶化。河床可以自然地减少这些高N负荷，从而在全球范围内提供重要的“生态系统服务”，不仅是为河流--也为它们排入的河口和沿海海域。因此，河床被认为是氮循环的热点，在一个被称为反硝化的过程中，将大约40%的N-径流转化为惰性的大气氮气(N_2)。在这里，生活在河床无氧区的特殊细菌(反硝化细菌)通过一些中间体将N转化为硝酸盐，转化为N 2气体。这一过程的硝酸盐要么来自陆地径流，要么来自另一个被称为硝化作用的微生物驱动过程。硝化作用只有在氧气充足的环境中才活跃，并通过亚硝酸盐将氨转化为硝酸盐。直到最近，关于固定氮在河流中的去除方式，硝化和反硝化之间的耦合一直是人们的共识。然而，我们的发现表明，另一种过程对整个生态系统的服务也可能是必不可少的。在氮气生产中，这种反硝化的替代过程被称为厌氧氨氧化(Anammox)，在这种过程中，亚硝酸盐和氨被更简单地转化为氮气。直到最近，Anammox还不被认为在氧气充足的河流中有任何重要作用。然而，我们的工作已经表明，厌氧氨氧化在渗透性河床(砾石和沙床)中具有最重要的意义，贡献了高达58%的氮气产量，而在不透水的粘土中，这一比例仅为7%。这是非常令人惊讶的，与目前关于河流的功能以及控制和调节自然界厌氧氨氧化活动的因素的认识完全不同。我们现在还可以证明，被完全硝化为硝酸盐(生态系统氮保护)或被氧化为氮气(生态系统N损失)的铵的部分似乎依赖于磷(P)。在磷含量较高的地方，更多的铵被回收为硝酸盐，而在磷稀少的地方，更多的氨氮以氮气的形式流失--特别是通过厌氧氨氧化。最后，鉴于我们知道人类来源的氮和磷都导致了全球富营养化问题--基本上太多的植物在水中生长--在这里，我们提出了磷的新的拮抗作用，并询问：1.通过支持铵到硝酸盐的完全硝化，磷的有效性是否积极地帮助保存生物有效氮，而不是将其去除到惰性氮气中？2.旨在从淡水中去除磷的管理方案是否具有直接和间接的好处，从而降低磷积极地促进了固定氮的去除？目前，磷在去除或保存固定氮方面的作用尚不清楚，这是我们新的“蓝天”建议的主要推动力。这些可渗透的河床起到天然生物催化过滤器的作用，容纳微生物群落，它们协同有效地去除固定的氮。要充分了解和利用这一点，我们需要询问谁是主要的微生物，它们如何相互作用，以及是什么调节它们的活动？这些是我们希望在我们的项目中解决的关键问题。这种认识可转化为更有效的废水处理过程，并为更好的过程控制和水资源的一般管理制定业务最佳做法。

Humans have learnt how to manipulate and harness the elements that sustain life on Earth (Carbon - C; nitrogen - N and phosphorus - P). Indeed, we have become so skilled at this that we have practically doubled the amount of fixed nitrogen (N) available to us to grow crops and, with current farming practices, we simply couldn't sustain the human population without it. This harnessing of N has come at a considerable cost to the environment, however, particularly rivers, estuaries and coastal seas, where it affects their quality and value as ecosystems. For example, high N loads can enter rivers as run-off from agricultural land to cause algal blooms, oxygen depletion and their general deterioration. Riverbeds can naturally reduce these high N loads and thus provide an important "ecosystem service" globally, not only for the rivers - but also for the estuaries and coastal seas into which they drain. Consequently, riverbeds are recognised hotspots of N cycling, converting ~40% of N-runoff back to inert, atmospheric nitrogen gas (N2) in a process known as denitrification. Here, specialized bacteria (denitrifying bacteria), living in oxygen-free zones of the riverbed, convert N as nitrate, via a number of intermediates, to N2 gas. The nitrate for this process is provided either from terrestrial run-off or from another microbial driven process called nitrification. Nitrification is only active in well-oxygenated environments and converts ammonia to nitrate - via nitrite. This coupling between nitrification and denitrification was, until recently, the consensus view on how fixed N was removed in rivers. However, our findings suggest that another process may also be essential for this overall ecosystem service.This alternative process to denitrification in N2 production is known as anaerobic ammonium oxidation (anammox), whereby nitrite and ammonia are converted more simply to N2 gas. Up until recently, anammox was not considered to be of any importance in well oxygenated rivers. However, our work has already shown that anammox is of greatest significance in permeable riverbeds (gravel and sand-beds), contributing up to 58% of N2 production, and compared to only 7% in impermeable clays. This is very surprising and completely at odds with present knowledge on the function of rivers and factors governing and regulating anammox activity in nature. We can also now demonstrate that the fraction of ammonium that is either fully nitrified to nitrate (ecosystem N conservation) or oxidised to N2 gas (ecosystem N loss) appears to be dependent on phosphorus (P). Where P is higher, more ammonium is recovered as nitrate and where P is scarce a greater fraction is lost as N2 gas - particularly through anammox.Finally, whereas we know that both human derived N and P contribute to the global problem of eutrophication - basically too much plant growth in water - here we are proposing a new antagonistic effect of P and ask whether: 1. By supporting complete nitrification of ammonium to nitrate, does the availability of P actively help to conserve bioavailable N over its removal to inert N2 gas? 2. Could management schemes aimed at removing P from freshwater have both direct and indirect benefits, whereby lowering P actively promotes the removal of fixed N? Currently the role of P in relation to the removal or conservation of fixed N is unknown and that is the main thrust of our new, 'blue-skies' proposal. These permeable riverbeds function as natural biocatalytic filters, hosting microbial communities that, in concert, efficiently remove fixed N. To fully understand and exploit this we need to ask who the main microbes are, how they interact and what regulates their activity? These are the key questions we wish to address in our project. Such understanding could be translated into more efficient wastewater treatment processes and the development of operational best practice for better process control and general management of water resources.