My overarching research interests center on geomicrobiology, the interactions between microbial communities, their geochemical environment and the resulting modifications of the rock substrate and environmental chemistry. I use a combination of modern molecular microbiological (PCR, per, etc.), geochemical (wet chemistry and electrochemistry) and isotopic (natural abundances nitrogen, tracer, and isotope pairing) techniques to answer the how, who, where and how fast questions of geomicrobiology. More specifically I am interested in examining the direct and intimate relationship between the activity of the biological community and the geochemical signature of the activity that may endure into the rock record. Often these environments may serve as modern analogs of ancient Earth systems, such as laminated sediments, bare rock or hydrothermal deposits.
Nitrogen and Sulfer Cycling at Deep-sea Hydrothermal Vents
Hydrothermal vents are one of a few unique environments on earth in that the energy that drives the biological communities is not derived from sunlight but rather based on the geochemistry of the deep earth. The interaction of hydrothermal fluids and cold seawater at hydrothermal systems results in the precipitation of mineral deposits (hydrothermal deposits), primarily metal sulfides (Tivey et al. 1995). Hydrothermal deposits occur as chimney structures, brecciated rubble, particulate material (black smoke), and as metalliferous sediment formed by settling particles. These deposits are composed mostly of pyrite (FeS2), chalcopyrite (CuFeS2), sphalerite (ZnS), marcasite (FeS2) and pyrrhotite (FeS). Hydrothermal deposits apart from the fluids, and principally the sulfur moiety, account for a large portion of the geochemical energy available for microbial respiration and biomass accumulation in deep-sea hydrothermal ecosystems (McCollom 2000). These deposits provide a variety of habitats due to changing physical and chemical conditions and steep gradients aqueous and mineral chemistry that relate to the flow or lack of flow of hydrothermal source waters. The microbial communities these habitats support are as diverse as the chemical setting, ranging from highly diverse consortia of Bacteria and Archaea found on thermally and chemically stratified chimney structures (Harmsen et al. 1997; Reysenbach et al. 2000; Schrenk et al. 2003) to lower diversity communities that may harbor metabolic specialists as the deposit age in absence of hydrothermal fluids (Rogers et al. 2003; Suzuki et al. 2004). The presence and activity of microorganisms contribute the weathering process, exploiting the disequilibria between the mineral and the surrounding seawater for the conservation of cellular energy. Microorganisms able to respire the sulfide minerals found in hydrothermal environments use this energy for the formation of biomass. Numerous studies have focused on the microbial respiration of the sulfur moiety of the deep-sea sulfur minerals (eg. Jannasch 1985; Wirsen et al. 1993; Wirsen et al. 1998). However, relatively little work has examined the role of microorganisms in the weathering of the deposit after the sulfur rich, high temperature source waters cease flowing.
Microbial Controls on Coastal Nutrient Cycles
Nutrient addition to coastal waters, known as eutrophication, has broad deleterious effects on coastal ecosystems including increased hypoxia and anoxia, altered local food webs, and degraded sea grass beds, kelp beds and coral reefs. These ecosystem changes result in decreased biodiversity, increased frequency and duration of harmful algal blooms, and compromised fin-fish and shellfish populations. Three decades after the Clean Water Act (1972), the nitrogen flux into coastal waters has increased four- to five-fold and as much as ten-fold in some areas as a result of human activity. Despite these efforts, two thirds of coastal waterways are moderately to severely impacted by nutrient pollution. With rapid population growth projected, mainly concentrated within 50 miles of the coastline, we can only expect N-loading in coastal systems to increase.
Nitrogen enters the coastal system through freshwater inputs and atmospheric deposition. Interest in groundwater as a freshwater input to coastal waters has increased in recent years because of the projected magnitude of the water flux, chemical constituents associated with this flux, and the potential impact of those constituents compared to overland runoff (rivers and stormwater). At our study site, a groundwater estuary in Waquoit, MA, groundwater inputs account for up to 50% of the total freshwater input. Unlike overland runoff, the groundwater at this site is suboxic and flows through a matrix with a lot of surface area, thus providing an environment that is expected to favor certain microbial activities that are capable of nitrogen transformation and removal.