Ammonia oxidising microorganisms perform a vital step in the nitrogen cycle. This step is carried out by a specific enzyme, the ammonia monooxygenase. Very little is known about the ammonia monooxygenase, despite its central role in the global nitrogen cycle. We characterised the ammonia monooxygenase from ammonia oxidising archaea using substrate analogues. Ammonia oxidising archaea are wide-spread globally and among the most numerous living organisms on Earth. Substrate analogues are chemicals, which are structurally similar to the actual substrate, in this case ammonia. Substrate analogues can be very useful tools to obtain information about an enzyme without the need to purify it. We demonstrated, using model microorganisms cultured from soil, that the archaeal ammonia monooxygenase has a more restricted substrate range than that of bacterial ammonia oxidisers. This is important because it means different groups of ammonia oxidisers can be selectively inhibited. This supports the use of these substrate analogues to quantify relative contributions of bacterial and archaeal ammonia oxidisers from soils, as well as developing them as labelling tools.
In addition to gaining crucial missing information about key enzymes, another outstanding question is on linking culture-independent molecular data to process measurements in the nitrogen cycle. In collaboration with colleagues from the University of Tartu and the Quadram Institute, we have identified that ammonia oxidising archaea are important for nitrous oxide emission from the global wetlands. Nitrous oxide a potent greenhouse gas with the global warming potential 265 times that of CO2. In our study using over 600 wetlands of different land-use patterns, vegetation and latitudes, ammonia oxidising archaea emerged as the key predictor of N2O fluxes. This finding is of importance for sustainable land use management, including drainage and conversion of wetlands for cultivated land.
