Oceanic Dissolved Organic Matter

One main research areas is the biogeochemistry of dissolved organic matter (DOM), and in particular dissolved organic nitrogen (DON). In the open sea dissolved organics make up a huge and complex soup of molecules . In the surface ocean DOM cycles rapidly, sustaining the oceanic microbial food webs which form the basis of open ocean ecosystems, while at the same time complexing inorganic species, and influencing a multitude of seawater properties. In the deep ocean, however, dissolved materials persist over thousands of years and multiple oceanic mixing cycles, in some way having become resistant to biological utilization. These dissolved molecules which accumulate in the ocean's interior represent one of the Earth's largest active reservoirs of reduced carbon and fixed nitrogen, comparable to total carbon in the atmospheric CO₂ pool.

Despite the central roles that DOM plays, its composition on the molecular level remains largely unknown. My research interests are particularly focused on the nitrogen-containing molecules in DOM. Nitrogen is a central limiting nutrient in much of the ocean, yet in the same huge regions where nutrients are most in demand, dissolved organic nitrogen persists in abundance. The identity and transformations of DON thus represent key controls on biological and biogeochemical cycles in the upper ocean, and DON may represent an especially sensitive tracer for key alteration processes which affect the entire DOM reservoir.

Some basic research questions include:

What is the molecular identity of the organic Nitrogen pool, and what organisms produce it? How important are bacterial sources and how can we identify their molecular remnants?
What molecular changes or processes control the preservation of dissolved organics in the deep ocean? In particular, how are amino acids, among the most labile of all biomolecules, apparently sequestered over long time scales?
What is the time-scale of cycling of major dissolved organic N species in ocean basins? How does this differ from major non-N containing biomolecules?
Are there compositional differences between DOM produced in a highly dynamic upwelling regime, such as central CA Coast, and oligotrophic blue-water regions, such as off Hawaii? What might such differences tell us about origin and cycling of DOM and DON in the ocean's as a whole?

Many molecular-level organic analyses, in particular molecular-level isotopic work, requires very large water samples coupled with ultrafiltration to isolate sufficient organic material. Oceanographic cruises can collect up to several thousand liter samples via CTD casts. However, for the largest samples needed unusual sampling arrangements are required. We have established two sample sites in contrasting oceanographic locations for ongoing collection of the large-volume DOM samples needed for the most sample-intensive experiments. Pipelines at the Natural Energy Laboratory of Hawaii, at Keahole Pt. near Kona, provide an essentially unlimited supply of surface and deeper water from the oligotrophic North- Central Pacific. Closer to home, similar intake systems at the Long Marine Lab near UCSC campus can provide similar access to shallow, coastal water from a highly productive upwelling-driven regime.