Microbes are responsible for most of the carbon and nutrient cycling in freshwater lakes, and influence both local water quality and global carbon budgets. However, it is not currently possible to predict which types of microbes will be found in a particular lake at a particular time, or to predict how a microbial community would respond to environmental change. Microbial biologists need to learn more about how microbes in lakes assemble into communities, the role of drivers such as competition, predation, resource availability, disturbance, and natural selection in shaping these assemblages. This project addresses the following overarching question: What forces organize microbial communities in humic lakes? Microbes and their communities respond to many forces acting at different spatial and temporal scales, and the effect of these forces can be observed at the level of both communities and populations. An interdisciplinary team of microbiologists, ecologists, engineers, and limnologists will use a suite of molecular tools and genomics to investigate (1) Interactions between bacteria and phytoplankton in the surface layers of lakes; (2) Changes in microbial communities resulting from water column disturbances such as thermal stratification and seasonal mixing and (3) The role of evolutionary processes in shaping individual microbial populations within and among lakes. The results of this work will allow us to better explain and ultimately predict the composition and dynamics of microbial communities and populations in freshwater lakes.
This project will not only advance our understanding of the microbial ecology of humic lakes, but of microbial ecology and biodiversity in a broader context. The research approach is structured within an ecological framework and reflect urgent questions in the field of microbial biology. To enhance the broader impacts of the work, the investigators will engage in outreach activities coordinated with the following organizations; the NTL-LTER SchoolYard Science program which promotes inquiry-based learning through workshops for high-school teachers; the Center for Biology Education which is developing a virtual professional development resource for middle and high school science and mathematics teachers; the UW-Madison PEOPLE program which organizes summer courses for underrepresented high-school students; the NSF-funded Center for the Integration of Teaching, Research, and Learning which promotes professional development of graduate students, post-docs, and faculty. The research team will continue to leverage these existing programs to show the general public how freshwater microbes are key components of lake ecosystems, impacting water quality and nutrient cycling in profound ways.
The overarching goal of this project was to identify and describe the forces organizing microbial communities along expanding spatial, temporal, and taxonomic scales of resolution in humic lakes. Humic lakes are hot spots for carbon processing in temperate and boreal landscapes, are characteristic in their tendency to thermally stratify strongly during summer months, and harbor unique microorganisms that cannot be found in other kinds of lakes. We sought to determine how biological interactions, disturbance, and natural selection contribute to the structure of bacterial communities and populations in these lakes. We linked the dynamics of key bacterial groups to known drivers of community composition: algal community composition, light, and temperature. We now know which lineages respond to changes in these drivers, providing us with new insight into ecological interactions and key traits for important freshwater bacterial groups. Our results have implications for understanding microbiological responses to environmental perturbations at both fine and coarse temporal scales. In this study, the microbial communities changed after the disturbance, exhibiting sensitivity to a pulse perturbation that occurred over a relatively short time scale (one week). Further, the clear connection between biogeochemical conditions and community composition suggests that there are direct linkages between changes in environmental conditions, biogeochemistry, and microbial communities. Work by our group and others has shown repeatable seasonal patterns in microbial communities inhabiting rivers, oceans, seas, and lakes, providing cumulative evidence of a consistent microbial community "baseline" that spans many aquatic systems. In ecology, increases in ecosystem variance have been suggested as indicators of ecological regime shifts. Therefore, gradual changes in aquatic microbial communities that exceed normal variability may correspond to press disturbances, such as those expected with gradual global change (e.g. increases in mean temperature). Our findings highlight the importance and feasibility of examining ecological change in microbial communities across taxonomic scales while also providing valuable insight into the ecological characteristics of ecologically coherent groups. We found that change in the population structure of a single genotype can provide additional insight into the mechanisms of community-level responses. In a fundamental study of bacterial ecology, we identified archetypal groups that are found in all freshwater lakes and proposed a structured and rigorous naming scheme to simplify how researcher communicate about them. The outcomes of this project have strong intellectual merit because they contribute significantly to our understanding of how microbial communities in freshwater lakes respond to changes in their surroundings. This has important implications for our ability to predict short and long-term change in response to altered land-use and climate. We also generated new knowledge about the mechanisms of bacterial evolution at the population scale and key factors governing interactions between aquatic bacteria and algae. The broader impacts of our work included outreach at K-12 levels, undergraduate research projects, inclusion of high school teachers in our research projects, and the use of project findings in courses taught by the PI and co-PIs.
