Determination of dissimilatory sulfate reduction rates in marine sediment via radioactive ³⁵S tracer

Abstract

The concentration of sulfate in seawater exceeds all other dissolved electron acceptors combined by more than an order of magnitude. This allows dissimilatory sulfate reduction to persist in marine sediment long after the reactive species of more energetically favorable electron acceptors have been depleted. Thus, sulfate reduction dominates anaerobic carbon oxidation in most coastal and estuarine sediments worldwide (Goldhaber and Kaplan 1975;Jørgensen 1977 Jørgensen , 1982Canfield 1993). The relative importance of sulfate reduction decreases at low sedimentation rates because slowly deposited sediments are exposed to oxygen, nitrate and oxidized metals for a longer time after deposition, and because the degradability of dead organic matter decreases steeply during aging (Middelburg 1989). Sulfate reduction in deep-sea sediments therefore only contribute minutely to the global sulfur cycling (Canfield et al. 2005; Jørgensen and Kasten 2006) but deeply buried sediments with low metabolic activity still comprise a vast volume and the carbon mineralization in these sediments exerts major control on the climate and the chemistry of the planet.Direct determination of sulfate reduction rates by measuring the decreasing sulfate concentration over time in a closed vial is possible in limnic sediment (e.g., Bak et al. 1991), but it is not practical in marine sediments due to the high sulfate concentration in seawater (28 mmol L -1 ). Typical sulfate reduction rates in coastal surface-sediments are in the order of 20 nmol SO 4 2-cm -3 d -1 (Jørgensen 1982). With near 20 μmol SO 4 2-cm -3 in the sediment this implies that a 1% reduction in sulfate content requires impractically long incubation for days to months. The slow relative change in sulfate concentration is, however, turned to an advantage with radiotracer-based methods because the specific activity of the tracer remains nearly constant throughout the experiment. Because only the added tracer is measured, the high background concentration of sulfate and sulfides is of little concern. Here we re-evaluate the 35 SO 4 2-based measurement of sulfate reduction in marine sediments that has developed into a standard method over the last 30 years. Theoretical basisThe high activation energy of SO 4 2-makes microbial reduction through enzymatic catalysis the only pathway of sulfate reduction within the physiological temperature range in marine sediments. Various organisms are capable of assimila- AbstractRates of dissimilatory sulfate reduction in aquatic sediments have been measured over many years with 35 Sradiotracer, and the method has been continuously modified and optimized. This article discusses the sequence of procedures that constitutes the method from sediment handling before incubation, via incubation and distillation, to statistical analysis of the results. We test modifications that have been added since previous method descriptions, and we recommend sound experimental procedures. We discuss the measurement of extremely low sulfate reduction rates whereby o...

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