Revising the Nitrogen Cycle

One of the great things about science is that it is what we know about the world is always changing. Until recently, nitrification (the process of oxidizing ammonia, NH4, to nitrate, NO3) was thought to require two different microbes – one to convert ammonia to nitrite, then another to convert nitrite to nitrate. However, performing both pathways in a single organism is theoretically possible and thought to be more energy efficient than separating the two steps, so why had no microbes been discovered that could perform complete nitrification?

Two papers concurrently published in Nature found that the genus Nitrospira, already known to convert nitrite to nitrate, can also convert ammonia to nitrite. This is the first known instance of complete nitrification. The reason it hadn’t been discovered before was because the enzyme Nitrospira uses to oxidize ammonia is quite different from other bacterial enzymes that perform the same function – different enough that it did not show up in PCR surveys for the gene encoding the enzyme (called ammonia monooxygenase). In fact, once Nitrospira’s ammonia monooxygenase gene had been identified, it was found to be most similar to a gene in another bacterium, Crenothrix polyspora, labeled as an “unusual” methane monooxygenase. It’s possible that this gene actually encodes an ammonia-oxidizing enzyme instead of a methane-oxidizing enzyme.

So why is this finding important? Complete nitrification could be very helpful in wastewater treatment. Removing ammonia from water is a major goal of wastewater treatment, and finding a single microbe that can perform two steps of that removal makes the process more efficient. In freshwater and other natural environments, we know that nitrogen cycling is important both for the bacterial communities and for the ecosystem as a whole. High levels of ammonia pollution in freshwater can be toxic to animals such as mussels and snails, and the main mechanism of its removal from the environment is nitrification, followed by either release to the atmosphere by denitrification or assimilation into biomass. The newly revealed ability of Nitrospira to perform nitrification by itself could change how we look at the nitrogen cycle in these systems!


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