A dynamic partnership: exploring the function and control of extracellular symbionts in thyasirid bivalves

在动物的一生中，它们不断地与细菌相互作用。虽然其中一些细菌是致病的病原体，但许多其他细菌是无害的，甚至是有益的。有些细菌甚至对与它们相互作用的动物的营养和免疫力的发展至关重要；例如，我们消化道中的许多细菌对我们的健康和幸福至关重要。尽管近年来研究兴趣不断增加，但我们对动物和有益细菌之间发生的复杂相互作用知之甚少。例如，这些截然不同的生物是如何找到彼此，形成联系，并长期保持良好关系的？我的研究项目旨在了解动物和有益细菌如何相互作用，合作和交换资源。为此，我研究了一种名为thyasirids的小型冷水蛤，这种蛤在加拿大许多港口、峡湾和近海地区的含硫沉积物中挖洞。袋狸被认为是生态系统工程师，因为它们的挖洞活动有助于从沉积物中去除硫（这对大多数动物都是有毒的）。袋虫的鳃内有大量以硫为能量来源的细菌（“共生体”）。袋狸纲动物用可伸展的脚在沉积物中挖洞，为它们的共生体提供所需的硫，这种行为被称为硫磺开采；反过来，共生体是蛤蜊的补充食物来源。Thyasirid硫磺开采改变了洞穴周围沉积物的化学性质，并可能增强某些细菌在洞穴衬里周围的定植。我和我的学生将在水族箱中饲养共生的袋虫，同时操纵环境条件并监测它们的行为。我们还将对宿主组织进行详细观察（例如，使用专门的显微镜来可视化和量化细菌，或荧光标记来跟踪细胞过程，如细胞分裂或细胞死亡），以了解对维持囊虫共生功能重要的因素。此外，我们将使用基因组测序方法来深入了解参与宿主相互作用的潜在共生体基因，并研究thyasirid共生体的自由生活形式，以了解它们是否被宿主蛤的洞穴衬里所吸引。这项研究对于了解所有动物如何与有益细菌相互作用非常重要；特别是，它有望为宿主行为对维持和微调共生关系的重要性提供新的见解。此外，这项工作将帮助我们更好地描述加拿大水域中袋虫的生态作用；这些知识是及时的，因为我们沿海水域的沉积物由于有机物负荷而正在经历越来越多的硫富集程度，而共生的thyasiids可以很容易地定殖并转化这些受影响的沉积物。

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.