Phillips, Katherine N.2012-10-012012-10-012012-07http://purl.umn.edu/135198University of Minnesota Ph.D. dissertation. July 2012. Major: Ecology, Evolution and Behavior. Advisor:James Bryan Cotner. 1 computer file (PDF); iv, 112 pages.Freshwater bacteria play a key role in the regulation of nutrients in aquatic ecosystems. Bacteria mediate the availability of nutrients tied up in organic molecules and are therefore capable of constraining both primary production and higher trophic levels in aquatic ecosystems. To assess how changes in temperature and phosphorus availability impact three diverse strains of heterotrophic bacteria (<italic>Agrobacterium sp., Flavobacterium sp.,and Arthrobacter sp.isolated from Lake Itasca State Park, MN, I investigated 1) the effects of phosphorus availability on bacterial biomass composition, 2) the effects of temperature on bacterial biomass composition and 3) the combined effects of phosphorus availability and temperature on bacterial biomass composition. To assess how phosphorus availability impacts biomass composition in photoautotrophic bacteria Synechocystis sp PCC 6803, the effect of phosphorus availability was investigated. Under phosphorus limitation I observed two distinct strategies adopted by the three heterotrophic strains. Either the cells became large, long and thin or they stayed relatively small and coccoid in shape. Both strategies observed under phosphorus limitation resulted in different patterns of cellular phosphorus allocation. Controlling for growth rate across changing temperatures, I observed that as temperature increased, all three strains decreased their cellular size, and their carbon, phosphorus, and nucleic acid content. However, a pronounced decrease in DNA content was observed in only one strain. The combined impact of increasing temperature and phosphorus limitation resulted in a more dramatic decrease in cellular size and cellular carbon than when phosphorus was not limiting. These findings suggest no one strategy is adopted by heterotrophic bacteria under phosphorus limitation or across changing temperature. Secondly, I observed that changes in phosphorus availability resulted in a much more flexible C:P biomass composition in Synechocystis than changes in nitrogen availability. These findings suggest that less carbon may be in bacterial biomass when cells are nitrogen limited than when cells are phosphorus limited. These results provide insights into the impact that both increasing temperature and phosphorus has on microbial composition and the potential effects of that composition on the microbial loop in aquatic ecosystems.en-USBiomass compositionCellular morphologyHeterotrophic bacteriaPhosphorusEcology, Evolution and BehaviorThesis or Dissertation