Periodic Reporting for period 2 - UNITY (Linking ecology, enzymes and ecosystems in the global nitrogen cycle)
Reporting period: 2021-08-01 to 2023-01-31
The global nitrogen cycle is central to both food security and climate change. Most soils globally are nitrogen limited, which necessitates the use of ammonia-based fertilisers for high crop yield. However, up to 70% of nitrogen fertiliser is lost from agricultural fields through the activity of specific microbes. This nitrogen is emitted into the Earth’s atmosphere as greenhouse gases or leached into rivers and seas causing pollution. Key microbes responsible for the nitrogen fertiliser loss are known as ammonia oxidisers. There are several different types of ammonia oxidising microbes, the activity and distribution of which depends on soil conditions and land management. Different groups of ammonia oxidising microbes also have different relative contributions to the greenhouse gas emissions and to the nitrogen flux in the environment. There are gaps in our knowledge of mechanisms underpinning the distribution and activity of the key nitrogen cycling microorganisms, and this makes it more challenging to predict and manage nitrogen turnover in the environment. This is important for our society, because nitrogen is crucial for food production, but it is also responsible for environmental change. The aim of this research programme is to address the key knowledge gaps in this research area. Specifically, this research will generate novel mechanistic understanding of nitrification in different soil environments, and link the microbial physiology and environmental nitrogen turnover rates in soil. In doing so, this research programme will provide a major advance in our understanding of terrestrial nitrogen cycling.
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.
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.
We have demonstrated that substrate analogues are useful tools to characterise enzymes, which have not yet been purified, and they can be used to gain mechanistic information about enzymes and metabolism in ammonia oxidising microbes. Furthermore, the work currently under way is focused on further applying these tools to quantify relative contributions of different groups of ammonia oxidising microbes to nitrogen turnover in the environment. In addition, we aim to link the cellular physiology and the capacity of these microbes for nitrogen turnover. We have also provided evidence that ammonia oxidising archaea are a key factor explaining the nitrous oxide flux in certain soil ecosystems. By the end of this project, we expect have gained a more thorough, novel understanding of the links between microbial diversity, metabolism and processes, and a framework for more accurately predicting nitrogen transformations in the environment.