The regulation of bacterial nitrous oxide reduction

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Humans are entirely dependent on the oxygen we breathe to support our life 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, rather than oxygen to support life. One of these life-supporting processes is denitrification, in which water-soluble nitrate is converted to gaseous products, nitric oxide, nitrous oxide (N2O) and dinitrogen. This denitrification process can take place extensively in agricultural soils where nitrogen rich fertilisers added to stimulate plant growth can also stimulate bacterial life. Soil bacteria that can denitrify need to protect themselves from the effects of NO, a potent toxin, produced through their own metabolism. They have an enzyme called 'nitric oxide reductase' that has evolved to keep NO levels low in the bacteria by converting it to the relatively benign nitrous oxide (N2O) which can sometimes be released into the atmosphere. This bacterial survival strategy has significant environmental consequences as N2O is a potent greenhouse gas which can damage the ozone layer.When discussing greenhouse gas emissions the general 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 be a cause for public and political 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 20% and this atmospheric loading is increasing further by 0.25% each year. 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 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 understood 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 depend on metals such as molybdenum, iron and copper for their activity. The enzyme that breaks down N2O to inert N2 is a copper-containing enzyme called nitrous oxide reductase. It is the major enzyme on the planet that is responsible for the destruction of the potent N2O greenhouse gas. Without it the atmospheric levels of N2O would be much greater that they currently are. The nitrous oxide reductase contains twelve atoms of copper and so its activity in the environment is highly 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. As a result of the application of nitrogenous fertilisers, agricultural soils are the largest source of anthropogenic N2O. 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 fundamental studies on model organisms in controlled laboratory environments. This programme represents just such a study focused on the mechanism by which copper regulates N2O emission.