Determining the N and O isotope effects of microbial nitrite reduction: the global N cycle implications of an enzyme-dependent isotope effect

Abstract:

The marine nitrogen (N) cycle is a dynamic system of critical importance, since nitrogen is the limiting nutrient in over half of the world’s oceans. Denitrification and anammox, the main N loss processes from the ocean, have different effects on carbon cycling and greenhouse gas emission. Understanding the balance between the two processes is vital to understanding the role of the N cycle in global climate change. One approach for investigating these processes is by using stable isotope analysis to estimate the relative magnitudes of N fluxes, particularly for biologically mediated processes. In order to make the most of the currently available isotope analysis techniques, it is necessary to know the isotope effects for each processes occurring in the environment. Nitrite reduction is an important step in denitrification. Previous work had begun to explore the N isotope effects for nitrite reduction, but no oxygen (O) isotope effect has been measured. Additionally, no consideration has been given to the type of nitrite reductase carrying out the reaction. There are two main types of respiratory nitrite reductase, one that is Cu-based and another that is Fe-based. We performed batch culture experiments with denitrifier strains possessing either a Cu-type or Fe-type nitrite reductase. Both N and O isotope effects for nitrite reduction were determined for each of these experiments by measuring the NO₂^- concentration, as well as the N and O isotopes of nitrite and applying a Rayleigh fractionation model. Both the N and O isotope effects were found to be significantly different between the two types of enzymes. This enzyme-linked difference in isotope effects emphasizes the importance of microbial community composition within the global N cycle.