24 Radiotracer Assays of Sulfate Reduction Rates in Marine and Freshwater Sediments Gary M King Darling Marine Center,University of Maine, Walpole, ME 045 73, USA CONTENTS Introduction Brief history of ~SO~ ~ methodologies Single-step chromium reduction for assay of sulfate reduction rates Modifications of the single-step procedure INTRODUCTION Sulfate dominates the dissolved electron acceptors in marine sediments, with concentrations (up to 28 raM) much greater than those of all others (e.g. oxygen, nitrate) combined. Freshwater sulfate concentrations are also relatively high, often exceeding 100 btM. In contrast, oxygen and nitrate concentrations seldom exceed 300 ~M and 100 btM, respectively, in fresh- waters. Thus, once oxygen is depleted, dissimilatory sulfate reduction dominates anaerobic metabolism in marine systems (predominantly in sediments) and often plays an important role in freshwater systems (Capone and Kiene, 1988) and algal mats (Frtind and Coben, 1992). In addition, transformations of sulfate reduction endproducts (e.g. dissolved and metal sulfides) play an important role in oxygen budgets and can limit oxygen availability for aerobic heterotrophs (Jorgensen, 1982; Fenchel et al., 1998). Much of the current understanding of sulfate reduction has been derived from assays based on ~SO4 e , a readily available and relatively inexpensive radiotracer that can be purchased in a carrier-free form with very high specific activity. However, prior to routine use of radiolabeled sulfate, sulfate reduction rates in marine sediments were assayed success- fully with depletion methods and diagenetic modeling (Berner, 1964; Martens and Berner, 1977; Jorgensen, 1978a,b; Canfield, 1989). Depletion approaches, which simply measure loss of sulfate over time, have proven METHODS IN MICROBIOLOGY, VOLUME 30 Copyright © 2001 Academic Press Ltd 1SBN 0 12 521530-4 All rights of reproduction in any form reserved e" ,m