David Thistle

Other Research Interests

The ecology of a ciliate - bacterial symbiosis in a chemocline

Symbioses involving sulfide-oxidizing bacteria and invertebrate or protist hosts are found in habitats such as hydrothermal vents and mangrove peat. These habitats are characterized by a spatially and temporally complex chemistry, where water that is rich in oxygen and lacks sulfide alternates with water that is anoxic and rich in sulfide at scales of millimeters and seconds.

The degree of symbiotic association varies. For example, hydrothermal-vent tubeworms have intracellular symbionts. As in most endosymbioses, the relationship involves special structures and complex biochemical pathways. In contrast, in ectosymbioses, the symbiotic bacteria are on the host's external surfaces. The host is little modified, and the physiological interdependence is less strict. Such ectosymbioses may serve as model systems for the study of the early stages of the evolution of symbiosis.

Dr. Kay Vopel (National Institute of Water and Atmospheric Research, New Zealand), Prof. Dr. Jörg Ott (University of Vienna) , and I are interested in the evolution of symbiosis and have chosen to study the symbiosis between the ciliate Zoothamnium niveum (Figure 1) and sulfide-oxidizing bacteria on its surface, because it appears to be in an early stage of symbiosis evolution.

Figure 1: A colony of the ciliate Zoothamnium niveum on a peat wall in Belize. The black object is a hydrogen-sulfide microelectrode.

The Z. niveum symbiosis occurs in red mangrove (Rhizophora mangle) islands near the Smithsonian Research Station on Carrie Bow Cay, Belize (Fig. 2). The stalked colonies of Z. niveum grow on vertical walls of submarine mangrove peat banks around the openings of cm-scale conduits. The conduits form when mangrove rootlets die and decay. The large (up to 15 mm), feather-shaped colonies of Z. niveum occur in groups of as many as 100 around the opening of a conduit. Sulfur-oxidizing bacteria cover most of the colonies. The ciliate suspension feeds, using its cilia to create a feeding current. Filtering is interrupted by the rapid contraction of the stalk. During contraction, the mass of zooids bunches together and whips downward. Dr. Vopel is investigating whether this behavior allows the ciliate to capture bacteria that have been sheared off its surface during the contraction.

Figure 2: Field laboratory of the Caribbean Coral Reef Ecosystem Program of the Smithsonian Institution on Carrie Bow Cay, Belize.

In our most recent research, we investigated how the bacterial ectosymbionts of Z. niveum obtain the reduced sulfur that they use as their source of energy. We used microelectrodes in situ to measure H₂S release from the decaying tissue of the rootlets (Figure 3). Our data suggest that oscillating flows along the wall of mangrove peat create low pressure near the opening of the conduit from time to time, causing pulses of H₂S-rich water to emerge from the peat. We think that the feeding current of the ciliate pulls this sulfide-rich water over the bacteria on its surface. In our next field season, we plan to test this hypothesis.

Figure 3: Dr. Kay Vopel making measurements of [H₂S] and [O₂] near a ciliate colony.

Figure 4: Dr. Kay Vopel (left) and I on the way to the study site near Carrie Bow Cay, Belize.

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In general, I am interested in the organization of soft-bottom communities at all depths in the ocean, with a bias toward the study of the effects of physical factors in community organization. In shallow water, I combine laboratory experiments with SCUBA-supported field sampling and experimentation. My favorite venue is the sandy habitat offshore of the FSU Marine Laboratory (about 1 hour from the main campus), although recent projects have been done off Panama City, Florida, and at Carrie Bow Cay, Belize.

FSU Marine Lab from the air.

Field laboratory of the Caribbean Coral Reef Ecosystem program (National Museum of Natural History, Washington, DC) Carrie Bow Cay, Tobacco Reef section of the Belize barrier reef.

During most of my career, I have been intrigued by the possibility that natural disturbances can be an organizing force in soft-bottom communities. For example, stingrays dig pits when they feed. Some species colonize such patches quickly. I am interested in how the exploitation of such local disturbances is incorporated into the ecology of these species. Are they attracted to a region where their predators or competitors are absent for a time, giving them a chance to go through part of their lives without being troubled by other species? Or is the pit the location of a resource that is otherwise not available?

My interest in disturbance at small scales has led me to think about larger-scale disturbances. During winter storms, the seabed is put into motion, and small benthic animals are threatened. For example, they risk abrasion from the moving sediment and being suspended in the water column. I used a laboratory flume to simulate storm-induced erosion to test the hypothesis that such animals sense coming storms and move into the seabed to avoid their consequences. I found that they did not do so and thus would be suspended by storms. When I experimentally suspended them, I found that males exhausted most of their energy reserves, but females did not. In a follow-up experiment, one of my graduate students showed that the females were able to maintain their energy reserves, not by reducing their metabolism, but by feeding, a highly unanticipated result.

I have become interested in anthropogenous disturbances. For his doctoral research, my student Keith Suderman studied the environmental impact of simulated spills of fuel oil #6 (the fuel presently used for electricity generation in Florida) and Orimulsion® (a new fuel proposed for this purpose) and found little impact of either on the benthic invertebrates.

In the deep sea, I studied the impact of benthic storms on a soft-bottom community at 4600 m depth in the Atlantic and found that the fauna differed conspicuously from those of more typical, quiescent deep-sea areas.

The science team during an Alvin cruise. Three graduate students participated: Lori Bouck (far left), Susan Boa (center), and Keith Suderman (far right).

One of the most compelling questions in biological oceanography is how the extraordinarily high local diversity is maintained in the deep sea. A hypothesis that interests me states that in this physically calm environment animals make structures that other animals use as their habitat, so the complexity of the habitat reaches extraordinarily levels. I think that this complexity provides the opportunity for large numbers of similar species to coexist. To investigate this possibility, I used the research submarine Alvin to do experiments at 1000 m depth off San Diego. The results suggested that the idea has merit.

My former graduate student, Mike Foy, emerges from ALVIN after a dive to 1,000 meters in San Diego Trough.

In the deep sea, as in most habitats, a guild of large animals move across the sediment surface, disturbing it by their passage and feeding on the infauna. In shallow water, these animals have been shown to be important in community organization. I have been interested in determining their role in the deep sea. The most straightforward approach is to exclude them experimentally from areas of the seabed for a time and then to quantify any change in the fauna. I have done a preliminary study, again using Alvin, and found that cages can be used to exclude these animals without creating experimental artifacts. I look forward to using this tool to test for the importance of this guild for deep-sea communities.

I am also interested the biology, taxonomy, and phylogeny of harpacticoid copepods.