Microbiological and Geochemical Characterization of Microbial Fe(III) Reduction in Salt Marsh Sediments

Abstract

Population densities of anaerobic Fe(III)-reducing bacteria (FeRB) and aerobic heterotrophs were inversely correlated in the surficial (0-2 cm) layers of Sapelo Island, Georgia, salt marsh sediments. In surficial sediments where densities of aerobic heterotrophs were low, the density of culturable FeRB correlated positively with the concentration of amorphous Fe(III) oxyhydroxides 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-targeted rRNA signals were also detected in the upper 6-cm depth. The disappearance of FeRB below 6 cm, however, coincided with a large increase in the abundance of SRB. Thus, when FeRB are not limited by the availability of readily reducible amorphous Fe(III) oxyhydroxides, FeRB may outcompete SRB for growth substrates. Shewanella putrefaciens- and Geobacteraceae-targeted rRNA signals were at or below detection limits in all sediment samples, indicating that these FeRB are not predominant members of the active FeRB populations. The ubiquitous presence of FeRB at the sites studied challenges the traditional view that dissimilatory Fe(III) reduction is not an important pathway of organic carbon oxidation in salt marsh sediments.