My research program centers on the study of the transformations of organic matter in atmospheric and aquatic environments. Some general areas of interest include:
1) Organic-metal interactions
2) Transformations of organic matter in freshwater and marine sediments
3) Chemical processes at interfaces such as the sea-surface microlayer and on the surface of manganese oxides
4) Photochemical transformations of dissolved organic matter (DOM)
5) The role of photochemically-generated DOM in planktonic food web dynamics
6) Atmospheric transformations of organic matter and their impact on surface water biogeochemistry
7) Antarctic photochemistry
8) Development of trace analytical techniques suitable for environmental analyses
Examples of specific research projects are outlined below.
I am quite interested in the marine sulfur cycle, particularly in the role of dimethyl sulfide (DMS) photochemistry on the dynamics of the oceanic DMS cycle. I have determined the relative importance of photo-oxidation as a removal pathway for DMS in the photic zone compared to other removal pathways including sea-air exchange and microbial consumption. The impetus for this study is that the oceanic efflux of DMS into the troposphere is the principal source of organic sulfur in the atmosphere. From a global perspective this flux is significant because DMS is an important precursor of aerosols and cloud condensation nuclei, and thus affects the earth's radiation balance. Oceanic DMS emissions are partly a function of the concentration of DMS at the sea surface, which, in turn, is regulated by imbalances in the rates of biological, physical and chemical sources and removal processes for DMS in the water column.
Another area of research that I have pursued is on the role of manganese oxide particles in the oxidation of DOM in aquatic systems and sediments. The driving force for this study is that numerous bacteria oxidize unstable manganese (II) to manganese oxides and deposit the oxides on their cell surfaces, a process that appears to account for most manganese oxidation in natural waters and sediments. Despite its widespread occurrence, the reason(s) why bacteria oxidize dissolved manganese is unclear. Manganese-oxidizing bacteria are hypothesized to form oxides as a non-selective oxidant that allows the bacteria to breakdown biologically recalcitrant DOM (presumably high molecular weight humic substances) to low molecular weight biological substrates (e.g., organic acids and carbonyl compounds) that these bacteria can use. Preliminary evidence supports this hypothesis ( Nature. (1994) 367: 62). The initial findings regarding the oxidation of DOM by manganese oxides are very exciting given that most of the organic matter in natural waters and sediments is comprised of biologically refractory DOM that cannot be used directly by microbes. Other than photolysis, very little is known about the removal mechanisms for this large reservoir of DOM. From this perspective, the coupling of bacterially-catalyzed oxidation of dissolved manganese and the subsequent oxidation of DOM by the manganese oxides may be an important factor involved in the cycling of biologically refractory DOM in the environment.
Research has been conducted in my laboratory to understand the role of free radicals in the photochemical oxidation of organic matter in clouds, freshwater, and seawater. Radicals are important intermediates in the solar breakdown of organic matter, but it has been difficult to study free radicals in natural waters because of methodological limitations. It is for this reason that a probe technique was developed to quantify organic free radicals in natural waters (e.g., Anal. Chem. (1990) 62: 2275).
The examination of free radical processes in natural waters is a logical extension of my interests in the global carbon cycle. In this context, there are several broader questions that I address through my research: 1) Is sunlight photolysis an important removal mechanism for biologically refractory carbon in natural waters? 2) Do photochemical processes represent an important mechanism for the non-biological production of biologically labile organic substrates? 3) Can organic carbon be a limiting nutrient to bacteria, especially in Antarctic waters as previously suggested by other investigators? 4) Will increased UV-B radiation (280-320 nm) stimulate bacterial production by increasing the amount of carbon available to bacteria through photo-oxidative processes? 5) Is the photochemical production of some species (e.g., the OH radical, hydrogen peroxide) inhibitory to plankton?