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 Fe 3ϩ complexed by natural organic ligands have been detected in coastal marine sediments with voltammetric microelectrodes. The role of soluble Fe 3ϩ complexes in diagenesis is unknown. In anoxic conditions, soluble Fe 3ϩ can effectively oxidize FeS 2 and recycle iron and sulfur for use as terminal electron acceptors during natural organic matter (NOM) degradation. Alternatively, soluble Fe 3ϩ complexes can catalyze the formation of FeS and FeS 2 through the rapid chemical reduction of Fe 3ϩ by dissolved sulfide. To better understand the role of soluble Fe 3ϩ 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 Fe 3ϩ complexes in reduced sediments show that soluble Fe 3ϩ 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 Fe 3ϩ is much higher than previously reported in the literature, suggesting that soluble Fe 3ϩ might promote alternative pathways for microbial degradation of NOM that ultimately results in the immobilization of Fe and S as reduced iron sulfide minerals.

Sulfur biogeochemical cycling and novel Fe–S mineralization pathways in a tidally re-flooded wetland

“…FeS and aqueous inorganic S species), and the actual in-situ rates is consistent with several previous studies of pyrite formation in more complex systems. For example, Canfield et al (1998), Carey and Taillefert (2005) and Burton et al (2007) all found that pyrite formation in complex systems (ferrihydrite/bacteria suspensions, estuarine sediments, and Fe(III)-rich soils, respectively) exceeded theoretical rates by several orders of magnitude.…”

Siderophores Are Not Involved in Fe(III) Solubilization during Anaerobic Fe(III) Respiration by Shewanella oneidensis MR-1

“…The voltammetric conditions used in triplicate cathodic square wave voltammetry (CSWV) measurements included preconditioning at Ϫ0.9 V for 10 s and deposition at Ϫ0.1 V for 10 s to concentrate Fe(III) species at the electrode surface. A scan rate of 200 mV s Ϫ1 from Ϫ0.1 to Ϫ1.8 V with a pulse height of 0.05 V was used as previously described (10,59,60,62 ) by CSWV in degassed medium, and the pilot ion method was used to quantify concentrations of Fe(II). Voltammograms were integrated using a semiautomated software program developed for these applications (6).…”

The effect of riverine discharge on biogeochemical processes in estuarine sediments

“…The extensive presence of iron oxides across the Satilla River estuary (not shown) suggests that both processes may have been responsible for the production of these complexes in the sediment. FeS (aq) was detected in the transition zone between iron and sulfate reduction in over 60% of the profiles that contain dissolved sulfides, indicating that iron was chemically precipitated by sulfide in these sediments (Taillefert et al 2002;Carey and Taillefert 2005). Although sediment diagenesis in estuarine sediments typically transitions from manganese and iron reduction (Aller 1980) or even methanogenesis (Martens and Goldhaber 1978) to sulfate reduction along salinity gradients, manganese and iron reduction were significant in most sediments along the Satilla River during the 3 yr of this study.…”