The role of sulfur in salt marsh metabolism

Later support was from the National Science Foundation Grant DEB 78-03557.

Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution February 1979

The rate of sulfate reduction in stands of dwarf Spartinaalterniflora in the Great Sippewissett Salt Marsh is approximately75 moles S04 m2 year-1. This is the highest rate reported forany natural ecosystem. Sulfate reduction is the most importantform of respiration in the marsh and results in the annual consumptionof 1800 g C m-2, approximately equivalent to net primaryproduction. Sulfate reduction rates in the peat are high for atleast three reasons: 1) the below-ground production of Spartinaalterniflora provides a large, annual input of organic substratesover a depth of some 20 cm, 2) sulfate is rapidly resupplied tothe peat in infiltrating tidal waters, so low sulfate concentrationsnever limit the rate of sulfate reduction, and 3) sulfideconcentrations remain below toxic levels.The stable mineral pyrite is a major end-product of sulfatereduction in salt marsh peat while iron mono-sulfides are not.This is unlike most anoxic marine sediments and apparently resultsbecause iron mono-sulfides are undersaturated. The iron mono-sulfidesare undersaturated in part because of the relatively lowconcentration of total soluble sulfides and in part because of thefairly low pH of the peat. Both of these conditions probablyresult from the activity of the Spartina roots. If the incorporationof 35S into pyrite were not measured, the S3504 reduction measurementswould 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. Onan annual basis, the reduced sulfur which results from sulfatereduction is either re-oxidized to sulfate within the peat or isexported, much of it as thiosulfate or a similar intermediately reducedcompound.Most of the energy which is originally in organic matters isstored in reduced sulfur compounds when the organic matter is respiredby sulfate reducing bacteria. Consequently, the export of reducedsulfur compounds from the peat represents an energy export. Theexport of energy as reduced inorganic sulfur compounds is probablylarger than the net above-ground production by Spartina. This is animportant vector for moving some of the energy trapped by the below-groundproduction of Spartina to zones where it is available forcoastal food webs.