Understanding nitrous oxide emission from denitrifying bacteria: integrating chemostat and soil studies

我们人类完全依赖我们呼吸的氧气来支持我们的代谢过程。值得注意的是，对于许多细菌物种来说，情况并非如此。面对环境中氧气的短缺，许多细菌物种能够切换到使用硝酸盐（NO3-），而不是氧气来支持呼吸。这些能量产生过程之一，称为反硝化，将水溶性硝酸盐转化为气态产物，一氧化氮（NO），一氧化二氮（N2 O）和二氮（N2）。这种反硝化过程可以广泛地发生在农业土壤中，其中为了刺激植物生长而添加的富氮肥料也可以刺激细菌氮循环。土壤细菌通过自身代谢产生的NO对自身产生自毒作用，需要保护自身不受其影响。它们有一种酶，一氧化氮还原酶（NOR），它通过将内源性NO转化为相对温和的一氧化二氮（N2 O），从而将其释放到大气中，从而使内源性NO水平保持在较低水平。从细菌代谢的角度来看，当细胞毒性NO转化为N2 O时，解毒的工作就完成了，但从环境的角度来看，已经产生了一种环境毒素，一种温室气体。在讨论温室气体排放时，公众敏锐地意识到二氧化碳和甲烷带来的问题。然而，N2 O的排放，也许最有名的是牙科麻醉剂的“笑气”，也应该引起关注。一氧化二氮最早是由英国化学家约瑟夫·普里斯特利于1793年发现的，当时大气中的一氧化二氮含量已经稳定了数千年。然而，在过去的100年里，大气中的N2 O增加了十亿分之50，而且这种大气负荷每年还以0.25%的速度增加，大多数评论家将这种增加与世纪为提高农田生产力而密集使用化肥有关。尽管其大气水平仅为二氧化碳的一小部分，但其全球变暖潜力却高出300倍。因此，当以二氧化碳当量表示时，它约占全球温室气体排放总量的10%。由于它在大气中的寿命约为150年，今天产生的N2 O将可能影响我们的曾曾孙经历的气候，因此现在制定战略以减少这些排放是很重要的。从分子水平理解了硝化细菌从硝酸盐产生NO的途径，其中已知将硝酸盐转化为亚硝酸盐（硝酸盐还原酶）和将亚硝酸盐转化为一氧化氮（亚硝酸盐还原酶）的酶的结构。这些酶是金属蛋白，其活性依赖于过渡金属如钼、铁和铜。将N2 O分解为惰性N2的酶是含铜酶（Nos）。它是地球上主要的酶，是负责强大的N2 O温室气体。如果没有它，大气中的N2 O水平将比现在高得多。Nos的分子结构是已知的。它含有12个铜原子，因此它在环境中的活性取决于铜的生物利用度。它还对pH值和氧气敏感，因此其在环境中的活性取决于许多不同的环境变量。人类活动产生的N2 O排放的最大来源是农业土壤，这是因为在土壤中施用了氮肥。自英国签署《京都议定书》以来，许多非生物来源的N2 O排放量已经减少，但生物来源的排放量不太容易管理。努力提高农业N2 O排放的预测和管理将受益于更好地了解影响细菌N2 O净产量的因素。这就需要结合在受控实验室环境中对模式生物的研究和在土壤中的实地研究。本项目将提供这样一个综合研究。

We humans we are entirely dependent on the oxygen we breath to support our metabolic processes. Significantly, this is not so for many species of bacteria. Faced with a shortage of oxygen in their environment many bacterial species are able to switch to using nitrate (NO3-), rather than oxygen to support respiration. One of these energy yielding processes, known as denitrification, converts water-soluble nitrates to gaseous products, nitric oxide (NO), nitrous oxide (N2O) and dinitrogen (N2). This denitrification process can take place extensively in agricultural soils where nitrogen rich fertilisers added to stimulate plant growth can also stimulate bacterial nitrogen cycling. Soil bacteria which can denitrify need to protect themselves from the autotoxic effects of NO produced through their own metabolism. They have an enzyme, nitric oxide reductase (NOR) that has evolved to keep endogenous NO levels low by converting it to the relatively benign nitrous oxide (N2O) which can be released into the atmosphere. From the perspective of bacterial metabolism the job of detoxifying the cytoxic NO is done when it is converted to N2O, but from an environmental perspective an envirotoxin, a greenhouse gas, has been produced. When discussing greenhouse gas emissions the public are acutely aware of the problems posed by carbon dioxide and methane. However, emissions of N2O, perhaps best known as the dental anaesthetic 'laughing gas', should also cause concern. N2O was first discovered by the British chemist Joseph Priestley in 1793 when its atmospheric levels had been steady for millennia. However, over the last 100 years N2O in the atmosphere has increased by 50 parts per billion and this atmospheric loading is increasing further by 0.25% each year, with most commentators linking this increase to intensive use of fertiliser to increase farmland productivity in the 20th Century. Although its atmospheric levels are only a fraction of that of CO2 it has a 300-fold greater global warming potential. Thus when expressed in terms of CO2 equivalents it represents around 10% of total global emissions of greenhouse gases. Since it has an atmospheric lifetime of some 150 years the N2O produced today will potentially influence the climate experienced by our great-great grandchildren thus it is important to devise strategies to mitigate these releases now. The pathways by which denitrifying bacteria produce NO from nitrate are undertstood from a molecular level with structures of enzymes that convert nitrate to nitrite (nitrate reductases) and nitrite to nitric oxide (nitrite reductases) being known. These enzymes are metalloproteins that depend on transition metals such as molybdenum, iron and copper for activity. The enzyme that breaks down N2O to inert N2 is a copper-containing enzyme (Nos). It is the major enzyme on the planet that is responsible for the potent N2O greenhouse gas. Without it the atmospheric levels of N2O would be much greater that they currently are. The molecular structure of Nos is known. It contains twelve atoms of copper and so its activity in the environment is dependent on the bioavailability of copper. It is also sensitive to pH and oxygen and so its activity in the environment is dependent on a number of different environmental variables. The largest source of anthropogenic N2O emissions is agricultural soils because of the application of nitrogenous fertilisers to soils. Since the UK signed up to the Kyoto Protocol, many non-biological sources of N2O emissions have been reduced, but emissions from biological sources are less easy to manage. Efforts to improve the prediction and management of agricultural N2O emissions will benefit from a better understanding of the factors that influence the net production of N2O by bacteria. This requires a combination of studies on model organisms in controlled laboratory environments and on studies in situ in soils. This project will provide such an a integrated study.