The gut microbiota of animals include a diverse array of bacteria of different shapes and sizes ranging from about 1 to 600 μm in length. These microbes perform important functions such as influencing the host circadian rhythm and breaking down dietary polysaccharides. The first chapter of this dissertation presents a review of what is known about circadian rhythms and diel cycles in bacteria. To date, a circadian clock in bacteria has only been characterized in cyanobacteria, but daily oscillations in community composition and function have been observed in symbioses, including in the giant gut symbionts of surgeonfish, Epulopisicum spp., that produce offspring on a 24-hour cycle. In gut systems, host behavior alters the composition and function of the gut microbiota while microbes can influence the host circadian rhythm as well. Beyond host circadian rhythm, other environmental conditions such as host diet, may significantly impact gut symbionts and shape the genomic content of these bacteria. The second chapter explores the genomic diversity of the family Oscillospiraceae and correlations between isolation source and genome content. The gene content of 106 bacterial isolates differs significantly by isolation source supported by non-metric multidimensional scaling (NMDS) analyses. In addition to environmental conditions, bacterial metabolic processes may be impacted by cell size. Because bacteria are reliant on diffusion for nutrient uptake and transport, typically the size of bacteria is restricted to maintain a high surface-area-to-volume ratio. However, even the smallest cells must be large enough to include space for essential cellular components. The third chapter outlines a lesson guiding students to consider the relevance of cell size using 3D-printed cell models and presents data from a pre- and post-assessment supporting that the lesson increases students’ knowledge of and confidence in the learning objectives. Finally, the fourth chapter presents a lesson exploring whether Earth is currently undergoing a mass extinction event, providing an overview of major events in Earth’s history and past and present conservation efforts. The included appendices outline work supporting the classification of novel gut bacterial species and genera from the fish Kyphosus sydneyanus and summarize genomic analysis of whole genome sequences from 19 novel bacterial isolates including six Oscillospiraceae included in the dataset from the second chapter.