Sulfur Cycling in the Water Columns of Lakes and Oceans

Sulfur is a critical bioelement central to many of Earth’s biogeochemical cycles. Studies of sulfur have overwhelmingly focused on sediments, where transformations between organic and inorganic sulfur phases drive short-term biological reactions and long-term climate cycles. However, sulfur cycling in the water column is just as dynamic and exerts similar controls over biogeochemical cycles in lakes and oceans – although the exact dynamics are only beginning to be understood. This thesis provides new understanding of sulfur cycling in aquatic environments through three chapters that span laboratory developments and field observations. Chapter 1 presents a time-series in enigmatic Mono Lake, CA, where the temporal dynamics of sulfur cycling microbes was investigated. This study, published in Geobiology, highlights the dependency of sulfate reduction and oxidation on lake chemistry and the need for studies to move beyond “snapshots” of microbial diversity. Chapter 2, published in Rapid Communications in Mass Spectrometry, presents development of a highly sensitive (1-10 µg S) mass spectrometry technique that allows, for the first time, sulfur isotope measurements of amino acids. These new measurements permitted discovery of new connections between metabolism and sulfur isotope signatures. Chapter 3 further applies these novel methods, making the first sulfur isotope measurements of marine dissolved organic matter. The data indicated that marine organic sulfur is entirely produced by phytoplankton and implied that heterotrophic bacteria rapidly and efficiently recycle reduced sulfur compounds, even in the water column. Taken together, these three chapters significantly advanced available tools for studying sulfur in the environment and expanded our understanding of modern aquatic sulfur cycling. The final chapter represents a departure from oceans, lakes, mass spectrometry, and sulfur. Here, I evaluate the success and impacts of my outreach project, the popular Women Doing Science Instagram, in portraying diverse, international women scientists, noting the powerful potential for social media to bolster STEM identity for graduate students.