Award: OCE-1140404

Nitrogen (N) is required by all living organisms, and its availability can limit ecosystem productivity on a variety of space and time scales. Humans have profoundly changed the availability of N on a global scale by manufacturing synthetic fertilizer in amounts equivalent to natural terrestrial N fixation each year. This fertilizer has increased food production throughout the world, but its runoff and atmospheric transport to the ocean have negative environmental impacts, such as stimulating harmful algal blooms and coastal hypoxia, as well as enhancing production of nitrous oxide (N2O), a potent greenhouse gas. The goals of this research project were to develop a framework for using the ratios of naturally occurring stable isotopes in marine nitrate (NO3-), nitrite (NO2-) and N2O to quantify the fluxes and to understand the mechanisms of N cycling in the marine environment. In so doing, we can better anticipate how enhanced N delivery to the ocean will be processed and what impacts it will have on ocean biogeochemistry. In order to achieve these goals, we conducted a series of experiments in the laboratory and at sea and developed biogeochemical modeling frameworks for analyzing and interpreting the data. We addressed questions such as: 1) what processes are responsible for production of N2O in the ocean, 2) how is bioavailable N cycled and lost in oceanic oxygen deficient zones, and 3) what is the distribution and biogeochemical impact of nitrification in the sea? N2O is a potent greenhouse gas that is produced through microbial nitrification and denitrification in soils, fresh water, and marine environments. Current estimates place the oceans at about 30% of the global N2O source to the atmosphere. Understanding the mechanisms of N2O production in the ocean is ultimately important for predicting how the emissions of N2O might vary in response to climate change, ocean acidification, and enhanced N delivery to the ocean. When ammonia-oxidizing archaea (AOA) were discovered in 2004-2005, a big question was whether AOA might be responsible for marine N2O production. Using a combination of isotope tracer experiments and natural abundance N2O isotope measurements, we demonstrated that marine AOA could produce N2O through nitrification with bulk d15N and oxygen isotope (d18O) signals similar to the marine source (Santoro et al., 2011). Thus, the production of N2O by marine AOA appears to dominate the flux of N2O between the ocean and atmosphere, and it resolves the long-standing discrepancy between chemical and isotopic tracers of marine N2O production. Our investigations of the isotopic systematics of nitrification also revealed an unusual inverse kinetic isotope effect associated with bacterial NO2- oxidation (Casciotti, 2009; Buchwald and Casciotti, 2010). While isotopic fractionation is common in enzyme systems, our findings provide the only known exception to the æruleÆ that the light isotope-containing molecules react preferentially in enzyme-catalyzed reactions. To compliment these experimental findings, we dove into understanding the inverse kinetic isotopic fractionation from a theoretical perspective and looking for evidence of it in the natural environment. The only plausible explanation was found in quantum and statistical mechanics, whereby the transition state in this simple bond-forming reaction is more strongly stabilized by 15N substitution (has a greater zero point energy drop) than the substrate, NO2- (Casciotti, 2009). The realization that the transition state complex could promote reaction of 15N is significant at a fundamental level for biochemistry, and it provides a unique signature with which to distinguish the fates of NO2- (oxidation to NO3- or reduction to N2 gas) in the ocean. Using this, we discovered that approximately 50% of NO2- produced by NO3- reduction had to be reoxidized to NO3- rather than being reduced to N2 gas (Casciotti et al., 2013). This result is surprising since NO2--oxidizing b...