The Contrasting Roles and Importance of Dispersal, Horizontal Gene Transfer and Ecological Drift in Bacterial Community Assembly

Abstract

Communities are defined as the ensemble of populations that interact with each other and with the environment in a specific time and location. Community ecology studies how communities assemble, what are the patterns of diversity, abundance, and composition of species, and the processes driving these patterns. It includes four basic mechanisms for the assembly of communities: dispersal, drift, selection, and speciation, with each mechanism influencing how the communities change in a different way. Dispersal, the movement of species from one geographical location to another, plays a major role in the recolonization of barren environments and the introduction of new species to established environments. Drift (i.e., random birth and death events within a community) could, theoretically, be negligible in bacterial communities where the high population densities are expected to buffer its effect. Conversely, horizontal gene transfer can be a strong selective force, as horizontally transferred genetic material is a source of functional traits that may provide selective advantages to the recipient cells, especially in environments where strong selection pressure occurs. In my Ph.D. thesis, I aim to examine these three contrasting mechanisms in controlled, simplified bacterial communities that are designed and studied through a synthetic ecology approach. I found that even at low dispersal rates, the species abundance of planktonic bacterial communities can be homogenized by migration. This homogenization can occur even when there are strong variable selection forces interacting in each environment. I also found strong evidence on the importance of stochasticity in communities. Drift can decrease the community similarity by up to 6.3%, and increases the probabilities that species become extinct, especially in the case of rare taxa. In contrast, I found that naturally competent bacteria are favored to uptake more DNA in communities that are highly productive and phylogenetically diverse. This pattern is explained by a potential higher availability of naked DNA for naturally competent bacteria, presumably because there are more cells and the predation systems are more effective. Altogether, our findings support the theory on the importance of stochastic forces and their interaction with deterministic forces on the shaping of microbial community assembly.