Nitrogen cycling

Human activity has increased the input of dissolved nitrogen to coastal waters. Because nitrogen is a limiting nutrient it can stimulate the growth of phytoplankton. While this could be beneficial, resulting in more food for higher trophic levels, like fish, it can also have negative consequences such as harmful algal blooms and hypoxia or even anoxia at depth when decaying organic matter consumes oxygen. Sediment microbes can counteract nutrient loading by converting bioavailable nitrate (nitrogen that other organisms, such as phytoplankton, can utilize) to biologically unavailable nitrogen gas (N2). However this process, known as denitrification, is not the only nitrogen cycling process occurring in sediments. Nitrate can also undergo dissimilatory nitrate reduction to ammonium (DNRA), and because ammonium remains bioavailable, it can be recycled to surface waters where it can further contribute to eutrophication. I aim to understand the environmental factors regulating these competing processes so that we can better evaluate the susceptibility of coastal waters to the negative effects of continued nutrient loading.

Biogeochemical models

Human society is currently altering conditions on our planet in unprecedented ways. So much so that we have now entered a new geological epoch, termed the anthropocene. How this will alter the biogeochemical cycles upon which all life on Earth depends is unknown. Much of this uncertainty is because we still don’t understand the fundamental laws (if any) controlling the emergent properties of the microbial communities catalyzing biogeochemical processes. One possibility is the theory of Maximum Entropy Production (MEP). This theory, from non-equilibrium thermodynamics, suggests that microbial ecosystems will organize to dissipate free energy by the fastest available pathway subject to the constraints of nutrient availability. Using this principle, I develop optimization-based models of microbial biogeochemistry. The hope is that such models will be better at predicting the response of microbial communities to environmental perturbations than traditional kinetic-based models.

Gene-centric models

Modern molecular tools provide an invaluable window into the marine microbial world by identifying organisms and their metabolisms through the analysis of genetic material. Microbial communities are ubiquitous throughout the oceans and exert great influence over ocean chemistry, yet they are rarely included explicitly in models of marine biogeochemistry. By integrating “-omic” data from state-of-the-art molecular tools into biogeochemical models, I aim to improve the predictive power of these models, and constrain biogeochemical processes that are not apparent from chemical measurements alone. Such an approach will hopefully provide deeper insights into the linkages between microbial activity and ocean chemistry.

Hydrothermal vents

Hydrothermal systems are regions where hot hydrothermal fluids, enriched in reduced molecules (H2S, Fe2+ and H2), rise upward through the crust and escape into the deep ocean. Along the way these warm reduced fluids mix with cold oxygenated seawater and the resulting chemical disequilibrium provides energy to fuel microbial communities. These communities are unique in that it is chemoautotrophic carbon fixation, rather then photosynthesis, that provides food for higher trophic levels.   Using the modelling approaches I develop in my lab, vent chemistry, and genomic surveys of microbial communities, I try to understand chemoautotrophy in these systems and constrain the rates of carbon and nutrient cycling. The goal of this work is to place these unique ecosystems in a global context, and understand their influence on marine biogeochemical cycles. Learn about our 2015 research cruise to Axial Seamount.