- Changing biogeochemistry of Antarctica (2012-2015) Southern Ocean phytoplankton and changing CO2, temperature, and iron.
- Chesapeake SIP (2010-2013) Determining rates of group-specific phytoplankton and bacteria uptake of inorganic and organic nitrogen by means of stable-isotope techniques.
- Arctic Nitrogen (2009-2012) Does competition for nitrogen between autotrophs and heterotrophs control carbon fluxes in the western coastal Arctic?
- C-43 Water Quality Treatment Project (2009) Consulting for CH2M Hill Engineering and the South Florida Water Management District.
- Virginia EON (2009-2011) Bioavailability of effluent organic nitrogen in Virginia coastal waters.
- Flow Cytometry Stable Isotope Protocol (2008-2011) Combining flow cytometry and stable-isotope technique: A method to measure phytoplankton- and bacteria-specific nitrogen and carbon uptake.
- Bioavailability of Effluent Organic Nitrogen (2008-2010) Assessing the bioavailability of effluent organic nitrogen along a freshwater to saltwater continuum.
- VCERC Bioalgae (2007-2009) Biodiesel production from algae.
- ECONUTS (2006-2011) Karenia nutrient dynamics in the Eastern Gulf of Mexico.
- Plume Busters (2005-2008) The ecology and genomics of CO2 fixation in oceanic river plumes.
- Biotechnology Investigations – Ocean Margins Program (2003-2006) Molecular approaches for in situ study of nitrate utilization by marine bacteria.
- DOMINO (2002-ongoing) Quantification and modeling of DOC and DON release in marine systems: a study of increasing trophic complexity.
- DOTGOM (2001-2004) Fate of recently fixed nitrogen in the eastern Gulf of Mexico: Does the regeneration of nitrogen fixed by Trichodesmium support the growth development of G. breve blooms?
The quantity of nitrogen entering an ecosystem is an important determinant of the ecosystem’s productivity; the higher the nitrogen input, the more productive the region tends to be. One important caveat to this generalization, however, is that the form of nitrogen (inorganic versus organic) may be as important as the total quantity of nitrogen in shaping the plankton community, the level of coastal productivity, and the degree of autotrophic versus heterotrophic production.
Much of our work on nitrogen uptake has focused on the bioavailability of organic nitrogen. In a broad sense, this research has been geared to challenge two commonly held views—that inorganic nitrogen is used primarily by phytoplankton and that organic nitrogen is used primarily by bacteria.
To address these questions we’ve pursued a number of new approaches to
- quantify organic nitrogen uptake by producing 15N labeled organic tracers, and
- distinguish phytoplankton uptake from bacterial uptake using flow cytometry.
More recently we have turned our attention to the bulk DON pool using a bioassay approach. In specific we have been working to define what fraction of the organic nitrogen in the effluent from wastewater treatment plants is bioavailable.
The global dissolved organic carbon (DOC) pool is estimated at 685 X 1015 g C (Hansell and Carlson 1998), making it comparable in size to the reservoir of inorganic C in the atmosphere. The fraction of the DOC pool that also contains nitrogen (DON) is an especially reactive component as well as the largest pool of fixed N in most marine systems. The enormous size and reactivity of the pool of dissolved organic matter (DOM, encompassing both DOC and DON) makes it a critical component of the global carbon cycle.
The source of DOM in marine systems is ultimately primary production and there are several mechanisms responsible for its production. They include direct release from primary producers and bacterioplankton, egestion and excretion from micro and mesozooplankton, viral lysis of bacterioplankton and eukaryotic cells, and particle solubilization. We know that substantial DOM fluxes occur in aquatic systems, and that bacteria are dependent on DOM as their source of reduced C to fuel both growth and respiration. As such, release processes figure prominently in the partitioning of C between gaseous, dissolved, and particulate phases in all marine systems.