Marine Microbial Biogeochemistry Overview

Nitrogen is an essential element for all living organisms, and its biogeochemical cycle is connected to the cycling of carbon, sulfur, phosphorous, oxygen, and trace metals. Nitrification and denitrification are key microbial processes that, in part, determine the distribution of nitrate, the primary form of bioavailable nitrogen in the ocean. As such, the activities of these organisms play important roles in the Earth's present, past, and future climate by modifying the distribution and overall abundance of nitrate available for primary production. Nitrifying and denitrifying bacteria are also the main producers of N₂O in terrestrial and marine ecosystems. N₂O is a climatically important trace gas whose tropospheric concentrations have undergone dramatic changes on glacial/interglacial timescales and are currently increasing at a rapid rate. Understanding N₂O production is important not only from a climate perspective but also for what it tells us about overall balance of nitrification and denitrification and the availability of nitrate in present and past ecosystems. Furthermore, understanding the relative roles of nitrification and denitrification in the N₂O budget is important to understanding the potential routes of anthropogenic impact on atmospheric N₂O concentrations.

Specific research questions that I am pursuing include:

1. How does the production of N₂O fit into the normal physiology and genetics of nitrifying bacteria?
2. How is N₂O production by nitrifying bacteria controlled in response to changes in the environment?
3. What role do nitrifying bacteria play in determining the ¹⁸O signatures of NO₃^- in the ocean?
4. 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?
5. From an understanding of current N₂O budgets, what can we determine about the sources of N₂O in past ocean ecosystems?
6. Are the evolutionary relationships of denitrification enzymes in nitrifying and denitrifying bacteria consistent with chemical arguments for the order of the evolution of nitrification and denitrification processes in Earth's history?

One common thread that ties these topics together is the combined use of genetic and stable isotopic markers to assess the activity of these specific bacterial groups. My approach is to develop genetic (link to "facilities" for now) and stable isotopic (link to "facilities" for now) tools with which to study the biological controls on nitrogen cycling and N₂O production in cultures and in the natural world.

Large kinetic isotope effects associated with enzymatic reactions make the use of natural abundance stable isotopes a valuable tool for studying the biogeochemical cycling of important environmental constituents and detecting biological activity in the geologic record. Isotope fractionation in biological systems can provide information about the genetic background and metabolic pathways of organisms carrying out the reactions. In order to fully interpret stable isotope distributions as tracers of underlying biological activity, the effects of genetics and physiology on isotopic fractionation need to be understood. I am currently focused on understanding biological controls on stable isotope fractionation in nitrification and denitrification, comparing the genetic diversity of key enzymes and their isotope effects. This research will improve our understanding of important aspects of the nitrogen cycle and will have a broad impact on many aspects of biogeochemistry.

The use of stable isotopes to study biogeochemical processes requires knowledge of the biological controls on isotope fractionation, which may arise through genetic and physiological differences among bacteria. I have found that biological factors can play a key role in the isotope systematics of nitrification and denitrification. The discovery of denitrifying genes in nitrifying bacteria has raised many basic questions about the metabolism, ecology, and evolution of nitrifying and denitrifying bacteria. I plan to pursue these questions as they relate to production of climatically important trace gases (NO and N₂O) and global-scale nitrogen biogeochemistry. The oxygen isotopic analysis of seawater nitrate has opened up many new and exciting avenues of biogeochemical research. I plan to investigate the unusual isotopic behavior in oxygen isotopic fractionation during denitrification, which has potential analogies in other systems and may have a broad impact on the way we interpret isotopic distributions. Through the combination of state-of-the-art genetic and stable isotopic methods, I hope to improve the understanding of isotope fractionation in biological systems.

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 the genes involved have been identified and most of the substrates and products in question are now isotopically accessible. 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. Using isotopic measurements may be an excellent way to extend genetic characterization of natural systems.

Students in my lab will have the opportunity to work on projects ranging from evolution and genetic diversity to stable isotope fractionation and modeling of cellular-scale to global ocean processes. They will learn from me and from each other in an interdisciplinary environment and will be encouraged to explore the questions they're interested in from a variety of perspectives.

For more information on educational opportunities:
Undergraduate Research Opportunities (including Minority Fellowships, Summer Student Fellowships and Guest Student Appointments)
Graduate Programs (including the MIT/WHOI Joint Program, Goephysical Fluid Dynamics (GFD) Program, and Guest Student Appointments)
Postdoctoral Programs (including the Scholar Fellowship Program, Marine Policy Fellowship, Postdoctoral Fellowship, and Postdoctoral Investigator)

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