Abiotic and Biotic Drivers of Microbial Community Structure and Function

Abstract

Microbial communities play fundamental roles in ecosystem functioning, including organic matter decomposition, nutrient cycling, and carbon transformations such as respiration. These processes are susceptible to global environmental change, particularly rising temperatures and increased nutrient loading, which alter microbial diversity and function locally and globally. However, microbial responses to these stressors remain poorly understood, especially when abiotic factors interact with ecological dynamics such as species interactions and trait shifts. This dissertation investigates how abiotic and biotic factors jointly shape microbial communities and the ecosystem functions they support.I address three central questions: (1) How do rising temperatures and nutrient enrichment affect microbial community structure and ecosystem function? (2) How do ecological interactions, particularly microbial predation, mediate these responses? (3) Can trait shifts in interacting species link environmental change to functional outcomes?To answer these questions, I used a combination of field surveys and laboratory experiments across natural and synthetic ecosystems. In natural systems, I surveyed microbial communities in the aquatic pitchers of the carnivorous plant \textit{Sarracenia purpurea}, a system in which microbial symbionts are responsible for decomposing captured prey and mineralizing nutrients. Samples were collected across a latitudinal and elevational gradient in North Carolina and Virginia. I found that microbial diversity declined in warmer, smaller, and higher-elevation habitats. These effects were primarily indirect: rising temperatures led to host plant morphology and habitat size changes, which structured microbial communities. These findings demonstrate that abiotic factors influence microbial communities directly and through trait-mediated effects of host organisms.In parallel, I conducted a factorial microcosm experiment to test how temperature, nutrient availability, and predator presence influence microbial community dynamics and function. I assembled synthetic freshwater communities with and without a ciliate predator community and exposed them to warming and nutrient enrichment treatments. I found that predator presence mediated the effects of temperature and nutrients on microbial biomass and community composition through direct and indirect pathways. Despite these shifts in community structure, microbial respiration—used here as a proxy for ecosystem function—remained relatively stable across treatments, suggesting functional resilience. These results highlight how biotic context can shape the direction and magnitude of microbial responses to environmental stressors.In the final chapter, I explored whether environmentally driven shifts in the functional traits of microbial predators could explain variation in community-level respiration. I found that temperature and nutrient enrichment altered the average morphological traits of ciliates, such as cell volume and surface-area-to-volume ratio, and that these shifts were associated with changes in function. These findings suggest that functional trait dynamics in microbial predators can be a link between environmental change and ecosystem function.Together, this dissertation demonstrates that microbial responses to global change are shaped not only by direct effects of abiotic stressors but also by species interactions and trait-mediated processes. By integrating observational and experimental approaches, I provide a systems-level, trait-based perspective on microbial ecology. These results underscore the importance of environmental drivers and ecological complexity to understand and predict microbial contributions to biogeochemical cycles in a rapidly changing world.