Sunlight provides much of the energy that drives the ecology of lake ecosystems. Current ecological models recognize microbes that harvest light energy and generate organic materials and other microbes capable of obtaining carbon and energy only from previously generated organic materials. However, new data suggest that photoheterotrophic microbes capable of using both light and dissolved organic materials (DOM) are more prevalent in aquatic systems than was previously believed. The goal of this work is to obtain quantitative data on the abundances of photoheterotrophic microbes in freshwaters that will reveal their ecological significance and biogeochemical roles. The project will focus on three types of microbes. The impact of cyanobacteria on freshwater ecosystems is well recognized, but their photoheterotrophic role has been largely overlooked. A second type of photoheterotroph, the aerobic anoxygenic phototrophic (AAP) bacteria, has been examined in laboratory experiments using cultured strains, but nothing is known about the ecology of AAP bacteria in freshwaters. A third type of photoheterotroph possessing proteorhodopsin (PR) has not even been examined in lakes, even though evidence indicates that about half of marine prokaryotes contain PR. The project will apply a unique set of analytical tools, including flow cytometry, fluorescence microscopy and environmental genomics for examining photoheterotrophs in the lakes of southern Québec, the Laurentian region north of Montreal and boreal lakes in the taiga regions of northern Québec. These lakes vary in several environmental properties (e.g. trophic status, DOC concentrations, color and turbidity) that are hypothesized to affect photoheterotroph abundance. The proposed work will test hypotheses about community dynamics and environmental controls of photoheterotrophic microbes, which potentially have unique impacts on biogeochemical processes in freshwaters.
This work is significant because it will contribute to an emerging picture of the geographical distribution of microbes that now appears to follow ecological principles like those previously thought to apply only to higher organisms. The work will contribute to understanding the biogeographical patterns of photoheterotrophs, what environmental conditions control their abundance and diversity, and the underlying ecological significance of these microbes in freshwaters. This project will support the education of graduate and undergraduate students and the general public. The project will directly support the work of a graduate student and would positively impact the NSF REU program at the College of Marine and Earth Studies at the University of Delaware (Lewes, DE). Students from Lincoln University, the nation's oldest Historical Black College, will also participate as part of another NSF-supported program. Interactions with visitors from Canada will enrich the overall experience of the summer interns and graduate students. Dr. Cottrell's results are featured during weekly tours of Lewes facilities (about 1000 visitors per year) and during Coast Day, an annual open house that attracts about 10,000 visitors. The project's web site will help to educate colleagues about photoheterotrophic microbes and the general public about the importance of microbes in freshwater ecosystems.
