Microbiological and geochemical characterization of microbial Fe(III) reduction in salt marsh sediments

Population densities of anaerobic Fe(III)-reducing bacteria (FeRB) and aero bic heterotrophs were inversely correlated in the surficial (0-2 cm) layers of Sapelo Island, Georgia, salt marsh sediments. In surficial sediments wh ere densities of aerobic heterotrophs were low, the density of culturable F eRB correlated positively with the concentration of amorphous Fe(III) oxyhy droxides extractable by ascorbate. High FeRB densities and a decrease with depth of ascorbate-extractable Fe(III) were observed in the upper 6 cm of a tidal creek core. Culturable sulfate-reducing bacteria (SRB) and SRB-targe ted rRNA signals were also detected in the upper 6-cm depth. The disappeara nce of FeRB below 6 cm, however, coincided with a large increase in the abu ndance of SRB. Thus, when FeRB are not limited by the availability of readi ly reducible amorphous Fe(III) oxyhydroxides, FeRB may outcompete SRB for g rowth substrates. Shewanella putrefaciens- and Geobacteraceae-targeted rRNA signals were at or below detection limits in all sediment samples, indicat ing that these FeRB are not predominant members of the active FeRB populati ons. The ubiquitous presence of FeRB at the sites studied challenges the tr aditional view that dissimilatory Fe(III) reduction is not an important pat hway of organic carbon oxidation in salt marsh sediments.