Thesis (Ph.D.) - Indiana University, Biology/University Graduate School, 2020Microorganisms are the most diverse organisms on the planet. Understanding the processes by which they are assembled into communities across space and time is a primary goal of microbial ecology. I employed empirical and simulation studies to investigate the effects of dispersal and dormancy on patterns of microbial biodiversity. Microorganisms are thought to have high rates of dispersal, linking communities across space to form a metacommunity. In Chapter 1, I investigated the importance of local- versus regional-scale processes for the assembly of planktonic and sediment-associated bacterial communities in a stream network. Using phylogenetic and taxonomic null models, I found habitat-specific spatial patterns of community assembly in the network, demonstrating the potentially overlooked importance of vertical habitat structure for microbial diversity in stream metacommunities. In Chapter 2, I investigated the roles of biotic interactions and dormancy for the maintenance of microbial biodiversity in University Lake, Indiana, USA. By comparing metabolically active and total diversity in a high-resolution time series, I found evidence that stabilizing biotic interactions allow taxa to persist at the local scale, aided by a dormant seed bank. In Chapter 3, I synthesized the roles of dispersal and dormancy in metacommunity ecology by analyzing empirical data and simulation models. In Chapter 4, I tested predictions about the effects of dormancy and dispersal in University Lake. Dispersal from the neighboring terrestrial ecosystem influenced diversity near the terrestrialaquatic interface. However, most terrestrial-derived bacteria were apparently dormant, with only a few taxa reaching high abundances in the metabolically active portion of the aquatic community. Taken together, this dissertation provides empirical demonstrations of how dispersal and dormancy affect microbial communities in nature. More broadly, it develops novel insights into the roles of dispersal and dormancy in metacommunities.