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Northern Benguela Upwelling System


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Nitrous oxide is a potent greenhouse gas that also contributes to ozone destruction in the upper atmosphere. Sources of nitrous oxide to the atmosphere involve the activity of microbes (bacteria and archaea) in the ocean, particularly where oxygen is scarce. Although marine environments low in dissolved oxygen occur in both the Atlantic and Pacific oceans, much of what we now know about microbial nitrous oxide production in the ocean is based on data from the Eastern Tropical Pacific.  Substantially less research has focused on the Eastern Tropical Atlantic, such as the northern Benguela Upwelling System (nBUS), where high nitrous oxide fluxes have been observed. The lack of measurements, combined with a limited understanding of the microbial pathways that lead to nitrous oxide production, creates uncertainty in predictions about future ocean emissions of nitrous oxide. Therefore, the overall project goal is to quantify nitrous oxide cycling rates and determine microbial pathways across an oxygen gradient in the world’s most productive upwelling system – the nBUS. Collaborator: Karen Casciotti, Stanford University

Deepwater Horizon Oil Spill

Marine microorganisms are capable of transforming their environment by, for example, rendering toxic compounds non-toxic.  Thus their actions can serve to mitigate the propagation of toxic substances up trophic levels and ultimately to humans.  Microbially mediated hydrocarbon degradation processes were essential agents of these transformations during the Deepwater Horizon oil spill .  The ecosystem service provided by resident microorganisms in consuming hydrocarbons introduced to the Gulf of Mexico during the spill was significant, but not equitable.  For example, nearly all of the methane, a potent greenhouse gas, released was consumed by microorganisms, however, other constituents, including carcinogenic PAHs detected in the sediments and in the water column were largely undegraded.  Thus microbially mediated processes determined, in part, the type and magnitude of the hydrocarbon flux at the sediment seawater interface, as well as the fate of the hydrocarbons throughout the water column and eventually in coastal environments.

Northern Gulf of Mexico "Dead Zone"

Areas of low oxygen have spread exponentially over the past 40 years, and are cited as a key stressor on coastal ecosystems. The world's second largest coastal hypoxic (≤2 mg of O2 l−1) zone occurs annually in the northern Gulf of Mexico (nGOM). The net effect of hypoxia is the diversion of energy flow away from higher trophic levels to microorganisms. This energy shunt is consequential to the overall productivity of hypoxic water masses and the ecosystem as a whole.  The Mason lab has a time series in the Gulf hypoxic zone, revealing an annual increase in abundance of Thaumarchaeota in the expansive hypoxic zone.  In particular, the abundance of ammonia-oxidizing Thaumarchaeota increases significantly in correlation with decreasing dissolved oxygen concentrations. Hypoxic archaeal hotspots might persist over time in the hypoxic area and may continue to serve as a site where energy flow is diverted away from higher trophic levels to, in this case, ammonia-oxidizing Thaumarchaeota. This population of Thaumarchaeota could carry out sustained draw down of oxygen in these water masses, as well as potentially influence the nitrogen cycle in the nGOM.  Collaborators on this project are: Nancy Rabalais and Cameron Thrash.

Microscale Trophic Interactions in Pitcher Plant Leaf Fluids

The aquatic communities found within the water-filled leaves of the pitcher plant, Sarracenia purpurea, have a simple trophic structure providing an ideal system to study microscale interactions between protozoan predators and their bacterial prey.  Specifically, we use pitcher plant leaves to carry out experiments directed at characterizing predator (protozoa)/prey (microbes) interactions, evolution and to test ecological theory.  For example, niche partitioning has been proposed as a driver of evolution in ecology, however empirical evidence remains controversial.  Replicate inquiline microbial communities are maintained with four protozoan species (in monoculture and competitive mixtures) to determine whether different predators target specific microbial species, and if competition among the protozoa affects resource use.  This experiment’s design includes an additional study analyzing microbial community change after seven weeks of protozoan evolution to assess successional patterns in niche partitioning. These tractable microcosm experiments help to clarify patterns of microbial evolution and predator resource distribution.  Collaborators on this project are: Tom Miller (FSU).  

Microbe:Plant Interactions in the Marsh Rhizosphere

Coastal marsh ecosystems provide important ecosystem services such as nutrient filtering, flood control, and carbon sequestration. They represent an important coastal-ocean interface, often referred to as hotspots in terms of plant and microbial activity and biogeochemical cycling. These ecosystems are particularly vulnerable to sea level rise, which alters in situ redox conditions in sediments. Changes in redox influences microbial composition and function, likely perturbs microbe:marsh grass interactions, and ultimately may alter biogeochemical cycles. Collaborators on this project are Behzad Mortazavi (University of Alabama, Dauphin Island Sea Lab).

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