Influence of Microbially Reducible Fe(III) on the Bacterial Community Structure of Estuarine Surface Sediments

We analyzed PCR-amplified 16S rRNA genes from native and Fe(III)-enriched surface sediments of a major tidal channel in the Tijuana River Estuary, California, USA. Clones from native sediments were most closely affiliated with photosynthetic taxa (Cyanobacteria, Chloroflexi, and Halochromatium) and microorganisms known to reduce (Desulfatibacillus, Desulfobacterium, and Desulfuromusa) or oxidize (Microcoleus, Phormidium, and Halochromatium) various sulfur species, reflective of the fluctuating redox conditions in the tidal zone. Fe(III) was rapidly reduced in anaerobic microcosms amended with 2-line ferrihydrite, with or without the sulfate reduction inhibitor sodium molybdate. The addition of ferrihydrite without molybdate caused a major shift in community structure to a dominance of the Fe(III)-reducing genus Shewanella, while at the same time the sulfate-reducing and sulfide-oxidizing populations were replaced by taxa known to cycle elemental sulfur. Sediments amended with both ferrihydrite and molybdate were again populated by Shewanella clones, but also numerically important were clones most similar to Marinobacterium, Pseudomonas, and Bacillus, suggesting a role for these taxa in Fe(III) reduction in marine habitats.