Title: The microbial ecology of a polluted, stratified lake
Description: Permanently stratified bodies of water present a rare opportunity to examine the structure and function of native bacterial populations and their role in the distinct geochemical processes of these systems. Such sites maintain discreet layers: the epilimnion, hypolimnion, and metalimnion, and are characterized by steep physico-chemical gradients that create unique environments over sometimes surprisingly shallow depths. This thesis presents an in depth evaluation of the microbial communities in Lake Mishawum, a permanently stratified lake, that is significantly impacted by pollutants from nearby Superfund sites. The stratification of Lake Mishawum is due to inputs of saline and contaminated groundwater rich in methane and sulfate into the hypolimnion and inputs of freshwater from an inflowing stream into the epilimnion. Linkages of aerobic and anaerobic biogeochemical processes to specific bacterial populations enable the integration of microbial ecology with environmental analyses and data. This work demonstrates that principle for a range of microbially-mediated processes in Lake Mishawum with a specific focus on aerobic methanotrophy and the biodegradation of aromatic contaminants as well as anaerobic sulfate reduction and methanogenesis. This was accomplished by evaluation of the community composition, abundance, and activity of microbial populations using quantitative and qualitative approaches and by coupling these data with physical and chemical measurements of the lake. In addition, it introduces combined measures for bacterial number and activity and reports the development of specific approaches to quantify these vast but unseen communities.

The findings suggest the interface between the oxic and anoxic layers is the center of aerobic microbial activity, particularly for methanotrophy and the degradation of aromatic pollutants, and that the resident bacteria comprise active and dynamic populations in the metalimnion that vary with season and environmental condition. Though less dynamic, the hypolimnion is characterized by high cell numbers and evidence of signature anaerobic microorganisms including sulfate reducing bacteria and methanogenic archaea that benefit from the unique character of this layer. Taken in sum, these results serve to create a comprehensive evaluation of the number and function of microbial communities in Lake Mishawum which could serve in future biogeochemical modeling as well as the design of remediative strategies for polluted ecosystems.