Research Interests

Marine microbial food webs

While the ocean harbors many fascinating and exotic animals, the majority of the ocean's living biomass is microbial. These microbes include algae, bacteria, viruses, and protozoa. The interactions between these microbes have profound effects, not only on higher trophic levels (such as sharks, dolphins and whales), but on the global balance of atmospheric gases.

Thus, the interactions among the microbial producers and consumers plays major role determining whether the carbon fixed by autotrophic organisms is shuttled to higher trophic levels or respired as carbon dioxide. My research is focused on the role of heterotrophic protists in regulating the abundance and production of algae and bacteria in marine systems. I am particularly interested in the composition of the microbial community and how this makeup (e.g. food web structure) relates to the food web function.

Here are some examples of microzooplankton, including dinoflagellates (right) and ciliates (middle and right). Cells were stained with Lugol's Iodine and with phase constrast microscopy.

For example,  heteotrophic protists within a certain size range (0.02 - 0.2 mm in cell diameter) are referred to as microzooplankton. However, this size range includes some organisms that injest only bacteria-sized microbes, others that can injest microbes equal to or bigger than their own cell size, and some that can both injest other cells and fix carbon through photosynthesis (mixotrophs). The relative abundance of different groups with different metabolisms, feeding strategies, etc. will play an important role in determining the number of trophic transfers and food web efficiency. Trophic interactions between heterotrophic microbial groups can play arole in formation of algal blooms. Some of these algal blooms may be detrimental to marine life and human health.  

This ciliate resides in a 
cylindrical container (a lorica). 25 x 18 µm

The Microbial Loop

Hetetrophic protists play a key role in the microbial loop. This is the pathway in which dissolved organic carbon (DOC) is utilized by bacteria that are consumed by protists. Protists, in turn, are consumed by larger organisms, such as  microinvertebrates, jelatinous zooplankton and filter feeders. Because picoplankton are often too small to be utilized by these organisms, phagotrophic protists function are crucial to shuttle the dissolved organic matter to higher trophic levels. The function of the microbial loop and whether the DOC is efficiently transferred to larger organisms or lost to microbial respiration.

Estuaries are some of the ocean’s most productive areas and we depend heavily upon estuaries for food. In the United States, about 50% of the population lives within 50 miles of the ocean or Great Lakes (NOAA). Thus, estuaries are also some of the areas most impacted by upstream human activities, including: Agriculture (which through fertilization generates nutrient runoff); Coastal development (including building areas impervious to rain water and increasing demand on freshwater aquifers) and water diversions (disrupting natural processes of flooding and sedimentations). The Georgia coast is lined with barrier islands – the plants, animals and microbes of the marsh are live in the rhythm of the ebb and flow of the tide. Transient populations of birds and fish enter the marsh to feed. My interest in the marsh centers on the microbial processes which fix carbon (by photosynthetic algae or chemosynthetic bacteria) as well as processes which breakdown carbon that originates from either plant (such as the marsh grass, Spartina alterniflora) or algal (such as benthic diatoms) sources. 

View of Doboy Sound, from the lighthouse on Sapelo Island. The marsh grass, Spartina alternifora, can be seen at the water's edge.

Like in other aquatic environments, microbes are the base of the food web. The microbes that grow on the surface sediments are ingested by invertebrates such as fiddler crabs. Microbes that are washed from the sediment into the water column by tidal movements are important for filter-feeding bivalves, like oysters and mussels. 

These microbes also fuel a pelagic food web that, in turn, produces finfish and shell fish of economic importance.I am particularly interested in the role of benthic bacterivorous protists (i.e. single-celled microbes living in the sediments that live by eating bacteria). For example, how do these organisms fit in the food web and transfer energy to higher trophic levels? Benthic protists can become quite large in the sediments, how do they differ in trophic behavior from nematodes, which are also abundant in marine coastal sediments. Protists and microinvertebrates (like nematodes) are the link between the upper (including macroscopic animals, invertebrates and fish) and lower (i.e. microbial) food webs. Thus, understanding their role in the tidal and estuarine food webs will give us a better understanding of how these systems function and how we can protect them.

Differential Interference Contrast of a live (left) and Protargol-stained (top) hypotrich ciliate commonly found in coastal marine sediment. Cell is approximately 70 x 30 µm

Protist-Associated Pathogens

Human bacterial pathogens are adapted to live and grow at high temperatures of the human body; they are also adapted to utilize the abundant resources for growth found in the gastrointestinal track and blood stream. The natural environment (with colder temperatures, greater fluctuations in temperature/pH, and generally lower availability of growth substrate) does not allow for survival and growth of these pathogens. In aquatic environments, bacteria are also consumed by bactivorous protists (such as ciliates, flagellates and amoeba). In some cases, these pathogenic bacteria can resist digestion and persist as symbionts inside the host. Here, they have been shown to divide and even survive the encystment process. One of the most notable examples is Legionella, which can are harbored in the cytoplasm of amoeba. Pathogens isolated from protists have been shown to be more virulent, and perhaps better at resisting the human immune system (macrophages in particular).

Campylobacter is bacteria associated with chicken farms, but this genus has also been found in other livestock (including cows and pigs). We are investigating the ability of Campylobacter to persist inside free-living bactivorous protists. To do this, we have been using techniques such as Fluorescent In Situ Hybridization (FISH), PCR and gel electrophoresis, confocal microscopy and flow cytometry. Our main goal is to determine the mechanism (both among the protist host and bacterial symbiont) that allows the digestion not to proceed. We are also interested in measuring the length of survival and infection potential of symbionts.