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Independent measurements of dissolved sulfide (DS) production in and release from mesohaline Chesapeake Bay sediments underlying anoxic bottom water were made during summer. DS accumulated under benthic chambers at a rate of 27.3rt8.2 mmol m-2 d-l. Rates of DS release ranged from 60 to 80% ofdepth-integrated@-12 cm) sulfate reduction (SR) rates (average 39.Ok9.0 mmol m-2 d-l) determined with 35S0, 2 -. The balance between DS production, accumulation in pore waters, and sediment-water exchange was examined by steady siate and transient state transport-reaction modeling, with a DS sediment diffusion coefficient derived from that determined for sulfate by the instantaneous source technique. The results indicate that DS transport is influenced by processes other than vertical molecular diffusion, most likely gas bubble ebullition driven by methane production beneath the SR zone. Although such processes may significantly reduce porewater DS accumulation (i.e. promote DS release), the models show that most (65-95%) DS formation during summer is not stored in the DS pool regardless of the transport processes occurring. The observed rates of DS production and release correspond to a potential oxygen demand of l-2 g 0, m-z d-l, equal to or greater than estimated rates of eddy-diffusive bottom-water reaeration during summer stratification. Thus, sulfur cycling alone can maintain anoxia in these subpycnocline waters during summer under quiescent water conditions. Anoxic sediment metabolism is a quantitatively significant component of the carbon cycle in shallow-water aquatic ecosystems (Jorgensen 1983), and sulfate reduction (SR) is the principal terminal microbial process in anoxic sediments where sulfate is abundant (Capone and Kiene 1988). The reoxidation of reduced sulfur formed by SR influences oxygen balance in shallow coastal systems, accounting for at least half of sediment oxygen consumption (SOC) under waters <20 m deep (Jorgensen 1982). In coastal systems subject to vertical water stratification, SOC linked to sulfur cycling I Current address: 430 National Center, U.S. Geological Survey, Reston, Virginia 22092. AcknowledgmentsThis research was supported by NSF grant OCE 82-08032 and NOAA Sea Grant Project R/DO-9 (awarded to J. H. Tuttle), and NSFgrant SR 88-14272 (awarded to D. G. Capone).We are indebted to W. M. Kemp, W. R. Boynton, P. A. Sampou, and J. M. Barnes for deploying the benthic chambers, to C. L. Divan for performing sulfide analyses during the flux measurements, and to D. Capone and M. Marvin for permission to present unpublished data. We also thank Jeffrey Chanton and two anonymous reviewers for reviewing the manuscript.Contribution 2276 from the Center for Environmental and Estuarine Studies, University of Maryland.

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Iron and sulfur oxidation by Thiobacillus ferrooxidans as well as growth on ferrous iron were inhibited by a variety of low molecular weight organic compounds. The influences of chemical structure of the organic inhibitors, pH, temperature, physical treatment of cells, and added inhibitory or stimulatory inorganic ions and iron oxidation suggest that a major factor contributing to the inhibitory effects on iron oxidation is the relative electronegativity of the organic molecule. The data also suggest that inhibitory organic compounds may (i) directly affect the iron-oxidizing enzyme system, (ii) react abiologically with ferrous iron outside the cell, (iii) interfere with the roles of phosphate and sulfate in iron oxidation, and (iv) nonselectively disrupt the cell envelope or membrane.