Reactive Fe has a profound influence on the cycling of nutrients in coastal marine and aquatic environments. Though most studies have concentrated on Fe cycling in porewaters, most reactive Fe is present in the solid phase of sediments. Further, much of the release or solubilization of solid phase reactive Fe is thought to be mediated by organisms. This study was initiated to develop and implement methods to study the biogeochemical cycling of Fe in coastal marine environments and to assess the role of metal-reducing bacteria in the coupling of Fe and C cycles.

A calibrated chemical extraction scheme was developed for partitioning reactive Fe minerals in marine sediments. Amorphous, highly reactive Fe(III) minerals were shown to comprise a large fraction (>45% of total Fe) and crystalline, less reactive minerals were also abundant (20-33% of total Fe) in the root zone of saltmarsh sediments. Iron sulfides, pyrite and acid-volatile sulfide (AVS), were measured to completely partition oxidized and reduced Fe fractions of the sediment. This extraction scheme was combined with an analysis of porewater chemistry to examine the seasonal cycling of Fe in sediments sampled from the root zone of Spartina alterniflora and in sediments overlain by a cyanobacterial mat. In 10 sediment cores taken over a 13 month period, a majority of solid Fe in vegetated sediments was observed to completely cycle between oxidized reactive Fe and reduced Fe as pyrite. In mat sediments, no seasonal trend was apparent and the speciation of reactive Fe revealed that a majority was reduced. Solid phase and porewater data supported the dominant role of Spartina roots and sediment bacteria in controlling the reactivity of Fe and its interaction with S cycling.

Sediment slurries, amended with microbial inhibitors, showed biological reduction of solid Fe(III) over formaldehyde-killed controls, exclusive of Fe reduction which may be attributed to biogenic sulfide. Microbial Fe(III) reduction was demonstrated in continental slope sediments and in summer saltmarsh sediments at in situ temperatures. The data suggested that sediment temperature and the presence of labile organic matter limit microbial Fe(III) reduction in these sediments.

The coupling of microbial growth to the respiration of Fe and S was studied in pure cultures of the Fe-reducing bacterium, Shewanella putrefaciens strain MR-4, isolated from the anoxic water column of the Black Sea. Growth data showed that a broad range of organic carbon compounds (C1 to C6) were oxidized during dissimilatory Fe(III) and thiosulfate reduction. Novel growth yields were similar with Fe(III) or S(IV) as electron acceptor and yields were comparable to those of other anaerobic respiratory bacteria. Because of their abundance in the marine environment, their carbon versatility, and their ability to transform large amounts of Fe or S, the S. putrefaciens group was suggested to play an important role in biogeochemical cycles.