The role of sulfur in salt marsh metabolism

Abstract

The rate of sulfate reduction in stands of dwarf Spartina alterniflora in the Great Sippewissett Salt Marsh is approximately 75 moles S04 m2 year-1. This is the highest rate reported for any natural ecosystem. Sulfate reduction is the most important form of respiration in the marsh and results in the annual consumption of 1800 g C m-2, approximately equivalent to net primary production. Sulfate reduction rates in the peat are high for at least three reasons: 1) the below-ground production of Spartina alterniflora provides a large, annual input of organic substrates over a depth of some 20 cm, 2) sulfate is rapidly resupplied to the peat in infiltrating tidal waters, so low sulfate concentrations never limit the rate of sulfate reduction, and 3) sulfide concentrations remain below toxic levels. The stable mineral pyrite is a major end-product of sulfate reduction in salt marsh peat while iron mono-sulfides are not. This is unlike most anoxic marine sediments and apparently results because iron mono-sulfides are undersaturated. The iron mono-sulfides are undersaturated in part because of the relatively low concentration of total soluble sulfides and in part because of the fairly low pH of the peat. Both of these conditions probably result from the activity of the Spartina roots. If the incorporation of 35S into pyrite were not measured, the S3504 reduction measurements would greatly underestimate the true rate of sulfate reduction. Pyrite acts largely as a temporary store of reduced. sulfur. The pyrite concentration of the peat undergoes seasonal changes. On an annual basis, the reduced sulfur which results from sulfate reduction is either re-oxidized to sulfate within the peat or is exported, much of it as thiosulfate or a similar intermediately reduced compound. Most of the energy which is originally in organic matters is stored in reduced sulfur compounds when the organic matter is respired by sulfate reducing bacteria. Consequently, the export of reduced sulfur compounds from the peat represents an energy export. The export of energy as reduced inorganic sulfur compounds is probably larger than the net above-ground production by Spartina. This is an important vector for moving some of the energy trapped by the below-ground production of Spartina to zones where it is available for coastal food webs.