A dynamic partnership: exploring the function and control of extracellular 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 are even fundamentally important for the nutrition, development of immunity of the animals with which they interact; for example, many of the bacteria that dwell in our digestive tracts are crucial to our health and well-being. Despite increasing research interest in recent years, 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, form associations, and maintain good relationships over the long term? My research program seeks to understand how animals and beneficial bacteria interact, cooperate and exchange resources. To do so, I study small, cold-water clams called thyasirids that burrow in sulfur-containing sediments in many harbours, fjords, and offshore locations around Canada. Thyasirids are considered as ecosystem engineers because their burrowing activities help 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 the sulfur they need by burrowing in sediments using their extensible foot, a behaviour called sulfur mining; in turn, symbionts are a supplementary food source for the clams. Thyasirid sulfur mining changes the chemistry of sediments around burrows and may enhance the colonization of certain bacteria around burrow linings. My students and I will keep symbiotic thyasirids in aquaria while manipulating environmental conditions and monitoring their behaviour. We will also perform detailed observations of host tissues (for example, using specialized 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. Further, we will use genome sequencing 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 and fine-tuning symbiotic relationships. Further, 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 transform these impacted sediments.