Microbial community assemblages have been studied in a variety of hosts and environments. However, to date, most of the studies conducted on microbial community structure have been observational in nature. As a result, the underlying mechanisms shaping microbial community assembly at a given place and time remain largely unknown. In this study, we were particularly interested in understanding how a microbial community develops on animal hosts. As soon as eggs or neonatal organisms are exposed to environments, microbes colonize surfaces of eggs or the epithelium of host tissues and establish a microbial community. Such a process involving initial colonization and subsequent sequential changes in species composition is called "succession". Microbial succession is a complex process with the number of different factors involved including initial stochastic arrival of microbes at an open space via dispersal, attachment / colonization at the space, subsequent community sorting via adaptation, and continuous dispersal from neighboring spaces. In this study, we examined microbial succession on the egg surface of the Lake Sturgeon, a threatened fish species inhabiting in the Great Lakes. We sought to understand the role of both host factors (e.g. innate immunity, egg chemistry) and various environmental factors (e.g. aquatic microbes, stream flow rate and temperature) in influencing the formation of microbial communities on the egg surfaces over the course of the egg incubation period. We also sought to evaluate the effect of different microbial successional processes on host life history traits, including egg mortality and larval size at hatch. These topics were important for this system because dams constructed in the Lake Sturgeon's spawning streams can alter environmental factors such as aquatic microbes, stream flow rate, and temperature, which may in turn affect the life history of the sturgeon. To achieve these objectives, we adopted an integrative approach, which relied on manipulation of environmental factors including aquatic microbial community, aquatic microbial quantity, stream flow rate, and temperature. We also employed a combination of various analytical techniques, including 16S rRNA gene pyrosequencing, 16S rRNA based T-RFLP, 16S rRNA clone library, 16S rRNA based quantitative PCR, light and fluorescence microscopy, and culture methods. We found that egg microbial communities were distinct from source water microbial communities. Host eggs shaped egg-associated microbial communities within 60 minutes of fertilization, selecting for and against certain microbial species. In addition, the egg surface microbial communities were not constant but rather dynamic, as we observed directional changes of microbial communities along with egg developmental stages. Egg-associated microbial communities also varied with the environmental variables they were exposed to during incubation, including temperature, flow rate, and aquatic microbial community. These differences in the egg-associated microbial community composition affected host life history traits including egg mortality and larvae size at hatch. We also identified a key set of microbial species that significantly improved egg survival and could be used for probiotic treatment in this threatened fish species in the future. This study was the first microbial succession study conducted on fish eggs. Our results highlight the complexity of host-microbe-environment interactions. This study has implications for managing threatened host populations such as the Lake Sturgeon inhabiting human-altered rivers, since it demonstrates the potential effect of dams (which alter aquatic microbes and temperature) on downstream host-microbe interactions.
Read
