Collaborative Research: Experimental Calibration of the Isotopic Content of Marine Sulfate

Washington University, Saint Louis MO

Investigators

Abstract

Collaborative Research: Experimental Calibration of the Isotopic Content of Marine Sulfate Alexander Bradley ID: 1536559 Sulfur is an essential element to life, and its biogeochemical cycle is of interest to a broad range of scientific disciplines. A requirement for understanding the sulfur cycle involves revealing and quantifying the mechanisms by which microbes oxidize and reduce sulfur species. This can be done by tracing stable sulfur and oxygen isotopes. Enzymes play a key role in the cycling of sulfur, but the degree to which individual enzymes preferentially use specific oxygen isotopes (fractionation) is not known and prevents complete understanding of observed oxygen isotopes patterns in marine sulfate. This project will investigate isotope fractionation (oxygen and sulfur) associated with two key enzymes, which are involved in the metabolism of both sulfate reducing microbes and some sulfur oxidizing microbes. Resolving the influence of these enzymes on the isotopic composition of marine sulfate is important to stable isotope biogeochemistry and chemical oceanography. The project will serve as a vehicle to educate and train a postdoctoral researcher and undergraduate students, and will provide a teacher training opportunity in the investigators' labs to aid teachers in the design of student field trips and hands-on exercises for their students. The cycling of sulfur between its oxidized and reduced forms is a major determinant of the oxidation of organic matter in the oceans and underlying sediments, and microbes play a key role in the biogeochemical cycling of sulfur. One of the key tracers of microbial sulfur cycling involves measuring the distribution of stable isotopes among sulfur phases. A number of studies have shown that the stable oxygen isotope composition of sulfate is an informative way to investigate the sulfur cycle. Despite its utility, there are few direct constraints on how biology impacts the oxygen isotopes of sulfate. The goal of this project is to place quantitative constraints on the enzymatic oxygen isotope fractionation associated with microbial sulfate reduction. This project will involve in vitro investigations of the kinetic isotope fractionations associated with ATP sulfurylase and APS reductase, and incorporate the resulting data into an existing model for understanding isotope fractionation during sulfate reduction and its resulting influence on the isotopic composition of marine sulfate. These models will provide critical insight into the behavior of the sulfur cycle and evolution of marine chemistry over key timescales.