Using experiments and stable isotopes to track the fate of fixed nitrogen in the marine environment

Nitrogen*is essential for all life on this planet. In the ocean nitrogen is often the*limiting nutrient, controlling primary production and consequently carbon dioxide*uptake from the atmosphere. While nitrogen is important for life, it can also*be harmful in excess. In coastal waters nitrogen is added as a pollutant from*agriculture and wastewater run off, which in turn causes blooms of algae, which*when they die and are respired by microbes that consume oxygen from the water*column creating dead zones, where no animal can survive. Humans have created*these dead zones by doubling the amount of fixed nitrogen on the planet (Gruber*and Galloway, 2008). Increased nutrients in the environment can also stimulate*microbially mediated N2O production. N2O is a greenhouse*gas 200 times more potent than carbon dioxide, so constraining this source is*important for understanding climate change. *Microbes are an integral controller of nitrogen in the ocean; some add fixed*nitrogen naturally by turning nitrogen gas into biomass; others can remove it*back to its gaseous form through denitrification and anammox. With future human*induced changes to the environment it will be imperative to understand how*these microbes will respond and counteract the negative impacts of*eutrophication. In my research program, I have a culture facility that will*grow nitrogen cycling microbes in order to answer over-arching questions on how*microbes can respond to anthropogenic influences such as coastal nutrient*loading or climate change. *The main tools I use in my research are stable isotopes. The nitrogen (N) and*oxygen (O) isotopes of nitrite and nitrate can be used together to disentangle*co-occurring marine biogeochemical processes. The success of using any stable*isotope signature depends on an in depth understanding of isotope systematics*for key microbial processes. The lack of this basic information is currently*the major roadblock to successfully using natural abundance stable isotopes in*most marine biogeochemical systems. The major objectives of my research program*are to identify where natural abundance stable isotopes can be useful for*answering complex biogeochemical and environmentally important questions and to*develop the necessary framework for utilizing them. *The anticipated benefits of this program will be 1) an increased understanding*of the complexity of microbial nitrogen cycling processes, 2) the stable*isotope systematics of these processes, and 3) a clearly defined plan for using*nitrogen and oxygen stable isotopes as a way to monitor nitrogen in the coastal*environment and aquaculture, in order to protect local waters from future*climate and anthropog