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Spatial Architecture of Extracellular Electron Shuttle Production for Iron Cycling in a Model Soil Habitat

EMSL Project ID


Iron minerals are some of the most reactive and ubiquitous components of soils, sediments, and groundwater. They contribute to the oxidation and reduction of many pollutants and natural organic matter species, in some cases with complete mineralization to CO2 that releases inorganic N and P species for plant uptake. Iron cycling is facilitated by iron reducing bacteria (IRB), that, under proper geochemical conditions, transform crystalline iron (oxyhydr)oxide minerals to highly reactive reduced mineral species. IRB can directly access exposed ferric-iron (oxyhydr)oxides and promote their reduction via outer membrane cytochromes. However, when iron (oxyhydr)oxides are not directly accessible, some IRB can produce and cyclically reduce soluble extracellular electron shuttles (EES) to access and subsequently chemically reduce these minerals. While the presence and advantages of EES have been widely demonstrated, particularly with pure cultures, the processes by which communities of physiologically diverse IRB compete for and access naturally heterogeneous matrices of iron (oxyhydr)oxides minerals in spatially structured soil environments remains unknown. The specific aims of this work are to 1) identify the spatial distribution of IRB species in a model soil system containing a gradient of solid phase ferric iron sources with variable reduction potentials, 2) identify the suite of EES that these bacteria produce and metabolize, 3) determine the genes related to EES synthesis and regulation, and 4) compare the amounts of iron (oxyhydr)oxide reduction resulting from inoculation with an individual IRB species in comparison to that from a mixed culture. The underlying hypothesis is that bacteria will spatially segregate in a model soil system as a function of the location and reduction potential of different solid phase ferric iron minerals present and the quantity/type of EES they produce. Our approach involves building a two-dimensional, diffusion-controlled microfluidic soil habitat containing a gradient of iron (oxyhydr)oxide minerals of varying reduction potential and a supply of soluble electron donor. In this environment, the IRB will be able to spatially segregate between these electron acceptor and donor sources depending on their preferred reduction potential and ability to produce EES. The pores of the microfluidic soil habitat will be filled with water or agar in replicate experiments, where the agar will enable cell isolation and probing such that single cell genomics and transcriptomics can be performed.

Project Details

Project type
Exploratory Research
Start Date
End Date


Principal Investigator

Charles Werth
University of Texas at Austin

Team Members

Emma Palmer
University of Texas at Austin