Abstract
AFM and confocal resonance Raman microscopy (CRRM) of single-cells were used to study the transition of anode-grown Geobacter sulfurreducens biofilms from lag phase (initial period of low current) to exponential phase (subsequent period of rapidly increasing current). Results reveal that lag phase biofilms consist of lone cells and tightly packed single-cell thick clusters crisscrossed with extracellular linear structures that appear to be comprised of nodules approximately 20 nm in diameter aligned end to end. By early exponential phase cell clusters expand laterally and a second layer of closely packed cells begins to form on top of the first. Abundance of c-type cytochromes (c-Cyt) is > 3-fold greater in 2-cell thick regions than in 1-cell thick regions. The results indicate that early biofilm growth involves two transformations. The first is from lone cells to 2-dimensionally associated cells during lag phase when current remains low. This is accompanied by formation of extracellular linear structures. The second is from 2- to 3-dimensionally associated cells during early exponential phase when current begins to increases rapidly. This is accompanied by a dramatic increase in c-Cyt abundance.
Highlights
IntroductionAnode-grown Geobacter sulfurreducens biofilms attract significant attention because they are considered as model bio-electrochemical systems (BES) in which microorganisms are used to catalyze desirable electrode reactions such as oxidation of organic matter in wastewater for power generation (Richter et al, 2009; Katuri et al, 2010; Marsili et al, 2010; Rollefson et al, 2011; Tender, 2011; Bond et al, 2012; Hasan et al, 2012; Katuri et al, 2012; Snider et al, 2012; Strycharz-Glaven and Tender, 2012a; Zhu et al, 2012; Lebedev et al, 2014)
Figure 1 depicts optical microscopy of a typical G. sulfurreducens biofilm grown on a gold electrode during lag phase
Upon transition to early exponential phase, when current begins to rapidly increase, a second layer of cells begins to appear on top of the first, accompanied by a pronounced increase in local abundance of c-type cytochromes (c-Cyt)
Summary
Anode-grown Geobacter sulfurreducens biofilms attract significant attention because they are considered as model bio-electrochemical systems (BES) in which microorganisms are used to catalyze desirable electrode reactions such as oxidation of organic matter in wastewater for power generation (Richter et al, 2009; Katuri et al, 2010; Marsili et al, 2010; Rollefson et al, 2011; Tender, 2011; Bond et al, 2012; Hasan et al, 2012; Katuri et al, 2012; Snider et al, 2012; Strycharz-Glaven and Tender, 2012a; Zhu et al, 2012; Lebedev et al, 2014). All cells in an anode-grown G. sulfurreducens biofilm, which can grow to more than 20 cells thick, appear to be catalytically active, coupling acetate oxidation with transport of their respired electrons to the underlying anode resulting in current (Franks et al, 2010). It is not clear how electron transport occurs through a G. sulfurreducens biofilm to the underlying electrode surface (referred to here as extracellular electron transport, EET) (Bond et al, 2012; Strycharz-Glaven and Tender, 2012b). The ability to perform CRRM of single cells demonstrated here enables relative quantification of local abundance of c-Cyt at each step, which indicates that lone cells and one-cell thick biofilm regions have a similar local abundance of c-Cyt, whereas two-cell thick biofilm regions have www.frontiersin.org
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