The control of extracellular bacterial symbionts in thyasirid bivalves

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Throughout their lives, animals constantly interact with bacteria. While some of these bacteria are disease-causing pathogens, many others are harmless or even beneficial. Some bacteria can even be fundamentally important for the nutrition, development or immune system of the animals with which they interact; for example, many of the bacteria dwelling in our digestive tracts are crucial to our health and well being. We know very little about the complex interactions that take place between animals and beneficial bacteria: for example, how do such dramatically different organisms find each other and maintain good, long-term relationships? My research focuses on how animals and beneficial bacteria interact, cooperate and exchange resources. To study this, I use a unique model system: a group of small, cold-water clams called thyasirids that burrow in sulfur-containing sediments in many harbours, fjords, and offshore sites around Canada. Thyasirids are considered to be ecosystem engineers because their burrowing activities help to remove sulfur (which is toxic to most animals) from sediments. The gills of thyasirids harbour large populations of bacteria (“symbionts”) that use sulfur as an energy source. Thyasirids provide their symbionts with sulfur by burrowing in sediments using their extensible foot, a behaviour called sulfur mining; in turn, symbionts serve as a supplementary food source for the clams. Thyasirid sulfur mining changes the chemistry of sediments around burrows. My previous research on symbiotic thyasirids has shown that it is possible to keep these clams for long periods of time in aquaria while manipulating environmental conditions and monitoring their behaviours. My students and I will perform similar experiments, in combination with detailed observations of host tissues (e.g. using high magnification microscopes to visualize and quantify bacteria, or fluorescent markers to track cellular processes such as cell division or cell death), to understand the factors that are important for maintaining a functioning symbiosis in thyasirids. We will also use genomic approaches to gain insights on potential symbiont genes involved in host interactions, and study the free-living form of thyasirid symbionts to see if they are attracted to the burrow linings of their host clams. This research is important for understanding how all animals interact with beneficial bacteria; in particular, it is expected to provide novel insights on the importance of host behaviours for maintaining symbiotic relationships. In addition, this work will help us better characterize the ecological role of thyasirids in Canadian waters; such knowledge is timely given that sediments in our coastal waters are experiencing an increasing degree of sulfur enrichment as a result of organic matter loading, and symbiotic thyasirids could readily colonize and remediate these impacted sediments.