The Aquatic and Stable Isotope Biogeochemistry Group studies biogeochemical cycles of biologically important elements and their isotopic composition in freshwater and marine ecosystems. Of particular interest is the microbial generation and consumption of green house gases, such as methane (CH4) and nitrous oxide (N2O). Given the integrative nature of biogeochemical research, we often combine different analytical tools, including stable isotope mass spectrometry (natural abundance and isotope label), biomarker analysis, radio-tracer rate measurements, and DNA/RNA-based approaches. One of the main goals of our research is to understand the environmental factors that modulate rates of organotrophic or lithotrophic microbial processes (e.g., methane oxidation, denitrification, nitrification, anammox), and the isotope signatures they leave behind in the natural environment. Towards this goal, we often study the distribution, chemical speciation and stable isotope composition of various elements and compounds, providing information as to the mechanisms and microbial community structures that underlie the observed patterns. Observations in the field are often calibrated by studying the relevant biogeochemical processes in the laboratory using incubation experiments, and put into a quantitative framework using numerical models.