The role of soluble Fe(III) in the cycling of iron and sulfur in coastal marine sediments

Abstract

In marine sediments, Fe(III) is found predominantly as a solid. Recently, however, soluble species of Fe3+ complexed by natural organic ligands have been detected in coastal marine sediments with voltammetric microelectrodes. The role of soluble Fe3+ complexes in diagenesis is unknown. In anoxic conditions, soluble Fe3+ can effectively oxidize FeS2 and recycle iron and sulfur for use as terminal electron acceptors during natural organic matter (NOM) degradation. Alternatively, soluble Fe3+ complexes can catalyze the formation of FeS and FeS2 through the rapid chemical reduction of Fe3+ by dissolved sulfide. To better understand the role of soluble Fe3+ in the transformation of iron and sulfur in marine sediments, we incubated the first few centimeters of unvegetated salt marsh sediment in plug-flow reactors. Microbial iron reduction seemed to prevail in suboxic conditions, but sulfate reduction outcompeted microbial iron reduction in the presence of reactive organic metabolites. The dominance of sulfate reduction led to the complete removal of reactive iron oxides by precipitation of FeS. Experiments mimicking the enrichment of soluble Fe3+ complexes in reduced sediments show that soluble Fe3+ does not reoxidize FeS and pyrite; rather, it promotes pyrite precipitation by enhancing sulfate reduction via complex bacterial interactions. The rate of pyrite formation in the presence of soluble Fe3+ is much higher than previously reported in the literature, suggesting that soluble Fe3+ might promote alternative pathways for microbial degradation of NOM that ultimately results in the immobilization of Fe and S as reduced iron sulfide minerals.