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Microbial Isotope Systematics

In the laboratory we have multiple types of nitrogen cycling microbes growing including: ammonia oxidizers, nitrite oxidizers and heterotrophic denitrifiers. These cultures are used to test the effects of growth conditions on the isotope systematics of nitrification and denitrification in the hopes of using these fractionation factors in biogeochemical models. 

Coastal Sediment N Recycling

Coastal sediments are a prime location for nutrient run off from human activities. A question that we would like to answer is what happens to this anthropogenic fixed nitrogen. Is it removed from the system as N2 gas through denitrification or anammox? Or does it get released as nitrous oxide (N2O)? Using isotopes and core incubations we have begun to answer these questions. 

N cycling in Oxygen Deficient Zones

While oxygen deficient zones (ODZs) are 0.1% the volume of the worlds oceans they account for more than 30% of nitrogen loss in the ocean. We are currently trying to determine where, what and how this nitrogen is removed and what microbial metabolisms are supported in these regions. Through the analysis and modeling of natural abundance isotope profiles of nitrite and nitrate, we have shown that there is a large amount of nitrite re-oxidation in these anoxic zones. Next step is to determine how microbes are oxidizing nitrite in the absence of oxygen. 

Nitrogen requirements for optimal kelp growth

The use of kelp for human consumption and bio-fuels is becoming increasingly popular. It is therefore essential to know what the optimal properties are to grow kelp. This particular laboratory experiment considered varying both seawater flow rate as well as nutrient concentration to examine the impacts on nutrient uptake and specific growth rate of kelp.  

Nitrification in Deep Sea Sediments

Deep-sea sediments underlying the oligotrophic ocean cover vast areas of the global seafloor. The low organic carbon content of these sediments allows exceptionally deep oxygen penetration (10’s of meters).  We have examined North Atlantic porewater profiles of nitrate concentration and N and O isotopic composition (δ15N and δ18O) to constrain the relative balance of nitrification and denitrification. We find that nitrate accumulates far above bottom seawater concentrations throughout the sediment column (up to  and persists down to the oceanic basement as deep as 90 mbsf, reflecting a predominance of aerobic nitrification/remineralization in these sediments.

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