Rivers and streams are the major conduits for transporting of Earth's carbon from the land to the oceans. Much of the carbon is in a dissolved organic form, and it serves as food for aquatic microorganisms. The challenge is that the chemical diversity of dissolved organic carbon is very great, and the genetic diversity of aquatic microbes is also immense. Accordingly, this project will use state-of-the art genomic methods to study the diversity and specific functions of these microorganisms. The results will fill a critical gap in understanding the specific metabolic capabilities of these microbes and how they carry out the key ecosystem function of transforming and metabolizing highly complex, riverborne dissolved organic carbon. The data that will be collected are important for predicting the impacts of land-use change, nutrient use, and shifting climate on freshwater quality across the USA. The research involves a collaboration with the Yale Peabody Museum Evolution program for inner-city New Haven high-school students to train interns and develop an interactive museum exhibit on U.S. rivers. The project will also train a postdoctoral researcher, and undergraduate and graduate students, including members of underrepresented groups in science.
The goal of the project is to describe interactions and feedbacks between watershed diversity, microbial functional diversity, and dissolved organic matter (DOM) chemistry in 36 large U.S. rivers, and to experimentally link gene expression with DOM degradation in five of them (the Altamaha, Columbia, Mississippi, St. Lawrence and Yukon). River sampling will be conducted in collaboration with a U.S. Geological Survey stream network. This research will address the concept that microbial genetic mechanisms in river ecosystems are closely linked to climate and landscape features that control river environmental conditions, particularly DOM quality. This concept implies that microbial function is shaped by DOM composition, climate, and river chemistry. These shaping forces alter the functional genetic capabilities required for competitive success within riverine microbial communities, and, thus, drive shifts in functional gene expression and phylogenetic composition, that, in turn, increase the efficiency of DOM metabolism and biogeochemical processes. Three hypotheses are to be tested: (1) the functional genetic composition and gene expression patterns of riverine microbial communities are correlated with the composition of riverine DOM; (2) community gene expression patterns vary predictably in response to shifts in the available forms of DOM; and, (3) the composition of riverine bacterial communities correlates with the composition of riverine DOM over space and time, and varies with watershed-specific climatic factors. The first hypothesis will be addressed with an empirical study of microbial community gene content (metagenomics), gene expression (metatranscriptomics), and DOM diversity (high-resolution analytical chemistry) during four seasons in five rivers that encompass a broad range of DOM composition. The second hypothesis will be addressed with an experimental study to identify active organisms and genes expressed by freshwater microbial metabolism. The third hypothesis will be addressed with an empirical study of microbes, DOM, and river chemistry in 36 major U.S. rivers. Statistical approaches being developed through a Microsoft-sponsored international working group (BioGeoChemistry Data System) will be applied to link microbial and DOM data.
