How can stable isotopic signatures of NO₃^- and N₂O be used to assess the activities of nitrifying and denitrifying bacteria over multiple space and timescales?

Stable isotopes are valuable integrative tools for studying the biogeochemical cycling of nitrogen in aquatic systems (Sigman and Casciotti, 2001). A stronger basis for interpretation of NO₃^- and N₂O isotopic distributions would be provided by estimates of isotopic effects of the microbial processes underlying the production and consumption of these compounds. My work has shown that there are significant differences in the isotope effect for ammonia oxidation among closely related species of ammonia-oxidizing bacteria and that these differences are correlated with differences in the amino acid sequence of the enzyme (Casciotti et al., submitted). A similar correspondence between genetic and isotopic similarity was also found in nitrite reductase enzymes from nitrifying bacteria (Casciotti et al., in prep). These differences in the isotope effect may contribute to the differences that are observed in the isotopic signatures of NO₃^- and N₂O produced by these nitrifiers. This correspondence may provide a useful marker for identifying an appropriate isotope effect to apply in modeling isotope dynamics in a particular system. I am interested in pursuing the study of the genetic and physiological bases for differences in isotope fractionation among bacteria and in extending this connection to interpret isotopic signatures of NO₃^- and N₂O in terms of underlying microbial processes.

What we learn about isotopic behavior in nitrification and denitrification has potential applications to other systems, and I hope to expand my work to investigate isotope fractionation in other microbial processes. At this point, nitrification and denitrification, in addition to having keen biogeochemical importance, are excellent systems for addressing the connection between biology and geochemistry because many of the genes involved have been identified and the substrates and products in question are isotopically accessible.

Isotopic measurements may be an excellent way to extend genetic characterization of natural systems. Genetic characterization yields very detailed information about the organisms that are present and active in a given microenvironment, but extrapolation of these data to the larger system is hampered by spatial and temporal variability. In order to assess the contribution of different organisms to the overall N₂O budget, it is important to combine genetic characterization of the environment with broader biogeochemical measurements. Stable isotopes provide an integrative measurement of the interaction of bacterial processes over larger spatial scales. Interpretation of isotopic measurements, however, would be aided by understanding of the underlying bacterial communities, particularly where there is a close relationship between genetic diversity and the isotopic behavior of bacterial communities.

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