Secreted Flavin Cofactors for Anaerobic Respiration of Fumarate and Urocanate byShewanella oneidensis: Cost and Role

ABSTRACTShewanella oneidensisstrain MR-1, a facultative anaerobe and model organism for dissimilatory metal reduction, uses a periplasmic flavocytochrome, FccA, both as a terminal fumarate reductase and as a periplasmic electron transfer hub for extracellular respiration of a variety of substrates. It is currently unclear how maturation of FccA and other periplasmic flavoproteins is achieved, specifically in the context of flavin cofactor loading, and the fitness cost of flavin secretion has not been quantified. We demonstrate that deletion of the inner membrane flavin adenine dinucleotide (FAD) exporter Bfe results in a 23% slower growth rate than that of the wild type during fumarate respiration and an 80 to 90% loss in fumarate reductase activity. Exogenous flavin supplementation does not restore FccA activity in a Δbfemutant unless the gene encoding the periplasmic FAD hydrolase UshA is also deleted. We demonstrate that the small Bfe-independent pool of FccA is sufficient for anaerobic growth with fumarate. Strains lacking Bfe were unable to grow using urocanate as the sole electron acceptor, which relies on the periplasmic flavoprotein UrdA. We show that periplasmic flavoprotein maturation occurs in careful balance with periplasmic FAD hydrolysis, and that the current model for periplasmic flavin cofactor loading must account for a Bfe-independent mechanism for flavin transport. Finally, we determine that the metabolic burden of flavin secretion is not significant during growth with flavin-independent anaerobic electron acceptors. Our work helps frame the physiological motivations that drove evolution of flavin secretion byShewanella.IMPORTANCEShewanellaspecies are prevalent in marine and aquatic environments, throughout stratified water columns, in mineral-rich sediments, and in association with multicellular marine and aquatic organisms. The diversity of niches shewanellae can occupy are due largely to their respiratory versatility.Shewanella oneidensisis a model organism for dissimilatory metal reduction and can respire a diverse array of organic and inorganic compounds, including dissolved and solid metal oxides. The fumarate reductase FccA is a highly abundant multifunctional periplasmic protein that acts to bridge the periplasm and temporarily store electrons in a variety of respiratory nodes, including metal, nitrate, and dimethyl sulfoxide respiration. However, maturation of this central protein, particularly flavin cofactor acquisition, is poorly understood. Here, we quantify the fitness cost of flavin secretion and describe how free flavins are acquired by FccA and a homologous periplasmic flavoprotein, UrdA.

Molecular Analysis of theIn SituGrowth Rates of Subsurface Geobacter Species

ABSTRACTMolecular tools that can provide an estimate of thein situgrowth rate ofGeobacterspecies could improve understanding of dissimilatory metal reduction in a diversity of environments. Whole-genome microarray analyses of a subsurface isolate ofGeobacter uraniireducens, grown under a variety of conditions, identified a number of genes that are differentially expressed at different specific growth rates. Expression of two genes encoding ribosomal proteins,rpsCandrplL, was further evaluated with quantitative reverse transcription-PCR (qRT-PCR) in cells with doubling times ranging from 6.56 h to 89.28 h. Transcript abundance ofrpsCcorrelated best (r2= 0.90) with specific growth rates. Therefore, expression patterns ofrpsCwere used to estimate specific growth rates ofGeobacterspecies during anin situuranium bioremediation field experiment in which acetate was added to the groundwater to promote dissimilatory metal reduction. Initially, increased availability of acetate in the groundwater resulted in higher expression ofGeobacter rpsC, and the increase in the number ofGeobactercells estimated with fluorescentin situhybridization compared well with specific growth rates estimated from levels ofin situ rpsCexpression. However, in later phases, cell number increases were substantially lower than predicted fromrpsCtranscript abundance. This change coincided with a bloom of protozoa and increased attachment ofGeobacterspecies to solid phases. These results suggest that monitoringrpsCexpression may better reflect the actual rate thatGeobacterspecies are metabolizing and growing duringin situuranium bioremediation than changes in cell abundance.

Microorganisms Associated with Uranium Bioremediation in a High-Salinity Subsurface Sediment

ABSTRACT Although stimulation of dissimilatory metal reduction to promote the reductive precipitation of uranium has been shown to successfully remove uranium from some aquifer sediments, the organisms in the family Geobacteraceae that have been found to be associated with metal reduction in previous studies are not known to grow at the high salinities found in some uranium-contaminated groundwaters. Studies with a highly saline uranium-contaminated aquifer sediment demonstrated that the addition of acetate could stimulate the removal of U(VI) from the groundwater. This removal was associated with an enrichment in microorganisms most closely related to Pseudomonas and Desulfosporosinus species.