Transcriptomic, Proteomic, and Bioelectrochemical Characterization of an Exoelectrogen Geobacter soli Grown With Different Electron Acceptors

Xixi Cai,Shungui Zhou,Lingyan Huang,Junlin Wen,Guiqin Yang,Zhen Yu

doi:10.3389/fmicb.2018.01075

Abstract

The ability of Geobacter species to transfer electrons outside cells enables them to play an important role in biogeochemical and bioenergy processes. Our knowledge of the extracellular electron transfer (EET) process in the genus Geobacter is mainly from the study of G. sulfurreducens, and in order to fully investigate the EET mechanisms in the genus Geobacter, other Geobacter species should also be considered. This study focused on the EET of Geobacter soli GSS01, which exhibited a capability of reducing insoluble Fe(III) oxides and generating electrical current comparable with G. sulfurreducens PCA. Electrochemical characterization, including cyclic voltammetry, differential pulse voltammetry, and electrochemical in situ FTIR spectra, revealed that different redox proteins contributed to the electrochemical behaviors of G. soli and G. sulfurreducens. Based on comparative transcriptomic and proteomic analyses, OmcS was the most upregulated protein in both G. soli and G. sulfurreducens cells grown with insoluble Fe(III) oxides vs. soluble electron acceptor. However, the proteins including OmcE and PilA that were previously reported as being important for EET in G. sulfurreducens were downregulated or unchanged in G. soli cells grown with insoluble electron acceptors vs. soluble electron acceptor, and many proteins that were upregulated in G. soli cells grown with insoluble electron acceptors vs. soluble electron acceptor, such as OmcN, are not important for EET in G. sulfurreducens. We also identified 30 differentially expressed small RNAs (sRNAs) in G. soli cells grown with different acceptors. Taken together, these findings help to understand the versatile EET mechanisms that exist in the genus Geobacter and point to the possibility of sRNA in modulating EET gene expression.

Highlights

MATERIALS AND METHODSThe Geobacter genus has a remarkable respiratory versatility that includes the dissimilatory reduction of insoluble metal oxides in natural habitats and electron transfer to electrode surfaces from which electricity can be harvested (Mahadevan et al, 2006; Lovley, 2008)
Proteins Potentially Involved in electron transport (EET) Cytochromes In the transcriptomic analysis, a total of 62 c-type cytochromes (c-Cyts)-encoding genes had significantly differential transcript levels in cells grown with FH or indium tin oxide electrode (ITO) compared to with FC, among which, 23 c-Cytencoding genes showed higher transcript levels simultaneously in cells grown with FH and ITO (Figure 4, Table 1, and Supplementary Table S2)
The electrical current generation by either strain GSS01 or PCA (2.2 A/m2 and 1.7 A/m2, respectively) was higher than that in previous reports (1.4 A/m2 and 1.1 A/m2, respectively) (Yang et al, 2017), and that is because the electrical current generation by the same strain differs when different electrode materials, electrode surfaces, or reactor structures are used (Wei et al, 2011; Zhu et al, 2014; Sun et al, 2015)

Summary

Introduction

MATERIALS AND METHODSThe Geobacter genus has a remarkable respiratory versatility that includes the dissimilatory reduction of insoluble metal oxides in natural habitats and electron transfer to electrode surfaces from which electricity can be harvested (Mahadevan et al, 2006; Lovley, 2008). The expression levels of proteins associated with tricarboxylic acid (TCA) cycle, oxidative phosphorylation, and translation were compared in cells grown with FC, FH, and ITO (Figures 4, 5 and Supplementary Tables S2, S3).

Results

Conclusion