McKay, Elizabeth2021-09-242021-09-242021-06https://hdl.handle.net/11299/224482University of Minnesota M.S. thesis. 2021. Major: Water Resources Science. Advisor: Ted Ozersky. 1 computer file (PDF); 83 pages.Microorganisms play a key role in regulating the cycling of carbon, oxygen, nitrogen, sulfur, and other important elements in aquatic ecosystems. Thioploca is a giant, filamentous bacteria that oxidizes sulfide and reduces nitrate, coupling the nitrogen and sulfur cycles in its benthic habitats. Thioploca can achieve high abundances in marine sediment where it is known to alter nitrogen and sulfur dynamics by removing toxic sulfide and recycling fixed nitrogen back into the sediment and water column. Thioploca can also achieve high abundances in freshwater sediments; however, its distribution and biogeochemical function are poorly understood in freshwater environments, making it difficult to determine how it impacts elemental cycling in these habitats. To analyze Thioploca abundance, factors affecting its distribution, and its biogeochemical function in the Great Lakes, I quantified Thioploca biomass and water column and sediment characteristics at 33 sites that spanned a gradient of depth and trophic conditions in the Apostle Islands region of Lake Superior and Green Bay in Lake Michigan. Sediment cores were also collected at eight of my study sites to analyze vertical Thioploca biomass distribution and sediment chemistry. Thioploca was common in both the Apostle Islands and Green Bay and reached biomasses of up to 250 g/m2 wet weight at some sites. While PCA and logistic regression analysis indicated that Thioploca may be more likely to be present under eutrophic conditions, Thioploca was also common and abundant at some oligotrophic sites in the Apostle Islands. Thioploca was more abundant in fine-grained than coarse-grained sediment, suggesting Thioploca distribution may be linked to depositional areas of lakes. At most sites, Thioploca was most abundant in the top 5 cm of sediment. Ammonia profiles in some sediment cores appear to indicate possible ammonia consumption in sediment layers with Thioploca, which suggests these freshwater Thioploca may interact with benthic nitrogen cycling differently than marine species of Thioploca. My results, along with other reports from the Great Lakes, suggest that freshwater Thioploca may be widespread throughout the Great Lakes. At the abundances observed, Thioploca is likely significantly influencing nitrogen and sulfur cycling in these areas, although many questions remain about Thioploca’s biogeochemical functioning in freshwater environments, including how it achieves high biomass in low sulfur environments, whether it reduces nitrate to ammonia or N2, and whether it promotes the recycling of fixed nitrogen or acts as a fixed nitrogen sink.enGreat LakesNitrogenSulfurThioplocaThesis or Dissertation