Microbial Activity & Diversity in High-Altitude Oligotrophic Lakes of the Sierra Nevada, California

Lakes traditionally have been considered unimportant in global and regional biogeochemical cycling, however these systems receive and process large volumes of material within their watersheds and thus function as integrators and processors of relatively large amounts of material compared with the surrounding landscape. Due to the relative concentration of biogeochemical cycling, these systems disproportionately influence global biogeochemical cycling (e.g. carbon and nitrogen cycling). This is critical as these cycling processes have the potential to be significantly influenced by anthropogenic activities --i.e., lakes are considered `sentinels' of global change. Microbial communities are integral components of these systems as archaea and bacteria -ubiquitous organisms that we still know very little about- almost exclusively perform these processes. To investigate the controls governing the abundance, activity and distribution of these microbial communities, I sampled multiple lakes (n=15) at multiple time points along an elevational transect located in Yosemite National Park, California, USA. At these sites I examined the effects of spatio-temporal and environmental variability on the abundance, activity, and composition of archaeal and bacterial communities. This was accomplished using a combination of methods that included quantification of functional nitrification genes (ammonia monooxygenase) using quantitative polymerase chain reaction, stable isotope based in-situ measurements of nitrification, chemical analyses of critical micronutrients, and next-generation high-throughput sequencing to determine community composition and diversity among microbial communities. Active nitrogen cycling and high abundances of ammonia-oxidizing bacteria (AOB) were captured by my sampling, as well as the presence of a large and diverse microbial community that exhibits a strong distance-decay relationship with increasing inter-lake distance. In my thesis I address questions regarding the dominance of ammonia-oxidizing archaea (AOA) versus AOB in nitrogen cycling processes and the effects of spatio-temporal and environmental variability on microbial community composition and diversity in these systems