Lake Huron Sinkholes - Microbial Composition and Processes in Biogeochemical HotspotsBopi A. Biddanda1 and Stephen C. Nold21Annis Water Resources Institute, Grand Valley State University,Muskegon, MI, USA2Biology Department, University of Wi

        Karst sinkholes discharging groundwater onto the Lake Huron floor  through   Paleozoic bedrock have created unique habitats characterized by steep  environmental   gradients and conspicuous benthic microbial mats and organic-rich  sediments.  These   ecosystems feature high microbial biomass and intense activity -  biogeochemical "Hot   Spots" (Biddanda et al. Ecosystems 9:828-842, 2  6).  We have  identified three sinkhole   communities along a depth gradient in Thunder Bay National Marine  Sanctuary.  The   principal goals of this 2-year (9/ 6-8/ 8) study are to: (1) Describe  the abundance,   diversity and activities of the microbial community in the submerged  sinkhole ecosystems   located along the depth gradient, and (2) Determine how the changing  environmental   gradients in submerged sinkhole ecosystems impact microbial  composition and processes.    Microbial communities are being characterized using molecular  techniques that target   different molecules.  First, membrane lipid profiling is combined with  measurements of   stable carbon isotope (13C) incorporation into individual fatty acids  to provide an   integrated view of community composition and activity.  Second,  genetic diversity profiles   (ARISA) and DNA sequence data provide a high-resolution view of community   composition.  Third, fluorescence microscopy and radioisotopic tracers  are used to directly   assess microbial abundance and growth rates.  We are using these tools  to test the   hypothesis that photosynthesis-dominated microbial communities in  shallow sunlit   sinkholes give way to chemosynthesis-dominated sinkhole communities in  deep aphotic   water.  In these model sinkhole habitats, we will test the hypotheses  about the role of   microbial diversity in determining community function including carbon  flow and nutrient   cycling.  These analyses provide the first detailed picture of  microbial life in sinkhole   ecosystems of the Laurentian Great Lakes.