Comprehensive metabolomic analyses of anode-respiring Geobacter sulfurreducens cells: The impact of anode-respiration activity on intracellular metabolite levels

doi:10.1016/j.procbio.2015.11.012

Process Biochemistry

Volume 51, Issue 1, January 2016, Pages 34-38

https://doi.org/10.1016/j.procbio.2015.11.012 Get rights and content

Highlights

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Comprehensive metabolomic analysis of anode-respiring Geobacter sulfurreducens was conducted.
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Magnitude of the microbial current was dominant factor determining the metabolic state of cells.
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Activation of TCA cycle and passivation of gluconeogenesis corresponded with larger current flow.

Abstract

Comprehensive metabolomic analysis of anode-respiring Geobacter sulfurreducens cells revealed that intracellular levels of metabolites related to the tricarboxylic acid (TCA) cycle, gluconeogenesis, consumptions of nicotinamide adenine dinucleotide (NADH) and nicotinamide adenine dinucleotide phosphate (NADPH), and adenosine triphosphate (ATP) generation correlated well with the activity of microbial anode-respiration detected as microbial electric current. Use of a glassy carbon electrode as the anode material in a three-electrode system resulted in a higher microbial current at +0.2 V (vs. Ag/AgCl) than at −0.2 V. A larger current flow resulted in higher concentrations of TCA cycle metabolites and lower concentrations of gluconeogenesis metabolites. Metabolomic analysis also revealed that higher anode-respiration activity resulted in a higher ATP/adenosine diphosphate (ADP) ratio and lower ratios of NADH/NAD⁺ and NADPH/NADP⁺. These findings provide direct experimental evidence that microbial anode-respiration activity controlled by the anode potential influences both the flux of central metabolic pathways and the redox balance in microbial fuel cells.

Graphical abstract

Introduction

Members of the deltaproteobacterial genus Geobacter can oxidize organic compounds to carbon dioxide during respiration using solid-state terminal electron acceptors such as iron and manganese oxides via extracellular electron transfer (EET) reactions mediated by outer membrane cytochromes [1], [2], [3]. Anodes can also serve as terminal electron acceptors during EET; consequently, Geobacter species have attracted considerable attention by researchers developing microbial fuel cells (MFCs) [4], [5], [6], [7], [8]. Improvements to MFC technology require knowledge of the parameters that influence the metabolic pathways during EET. To date, model-driven simulations of the metabolism of Geobacter sulfurreducens cells have been conducted to elucidate the EET-related metabolic pathways [9,10]. Meng et al. reported that respiration activity increased as the cellular growth rate decreased in a model assuming a constant rate of consumption of acetate, used as an organic substrate [10]. This negative correlation between respiration activity and cellular growth rate is attractive when considering MFC as a technology for waste water treatment (WWT). However, the control parameters for satisfying this negative correlation are currently unknown. It has been reported that the carbon flux of G. sulfurreducens cells can be changed by genetic manipulation [11], but such genetically-engineered cells cannot be used for practical WWT plants.

On the other hand, it has been demonstrated that the magnitude of the EET-current of G. sulfurreducens cells depends on the potential at the anode [12], [13], [14]. Since the anode potential in a practical MFC can in principle be tuned by changing the external load and/or the ratio of the anode/cathode area, it is worth investigating the metabolism patterns of G. sulfurreducens under active EET conditions achieved by an appropriate choice of anode potential. We here describe comprehensive intracellular metabolomic analyses of G. sulfurreducens cells cultured at different poised potentials aimed at evaluating how the anode potential and/or the EET activity influence metabolism.

Section snippets

Preparation of Geobacter cells and cultivation in an electrochemical cell

G. sulfurreducens PCA was cultured anaerobically at 30 °C for 72 h in 50 mL PS medium, as described previously [13]. Cells were collected by centrifugation for 5 min at 5000 × g, washed three times with PS medium, then injected into an electrochemical cell (described below) using a 1-mL syringe (Terumo, Tokyo, Japan) equipped with a 21-gauge needle. The concentration of the cell suspension in the electrochemical cell was determined by measuring the optical density at 600 nm (OD₆₀₀) and was adjusted to

Results and discussion

Fig. 1(a) shows representative time courses of the microbial current generated by G. sulfurreducens cells cultured at −0.2 V (black lines) and +0.2 V (gray lines) on a GC anode. The microbial catalytic current was 16.5 ± 11.0% lower at −0.2 V than at +0.2 V. As a reference for later discussion, the current vs. time curves obtained on an ITO anode are shown in Fig. 1(b) and show that the potential of the microbial current tended to be opposite to that obtained using the GC anode. Consumptions of

Conclusion

Our experimental results indicate that activation of the TCA cycle, passivation of gluconeogenesis, production of ATP, and utilization of NAD(P)H, corresponded well with larger current generation in G. sulfurreducens. Although the pattern of the microbial current generation depended on the anode material, the above correlation held under all experimental conditions studied, indicating that the metabolism of G. sulfurreducens can be controlled by an appropriate choice of the poised potential of

Competing interests

The authors declare that they have no competing interests.

Acknowledgments

We are grateful to Yasuko Koura and Ayami Fujino for providing analytical support during the experiments. This work was financially supported by a Grant-in-Aid for Specially Promoted Research (24000010), and also partly by a Special Coordination Fund for Promoting Science and Technology, Creation of Innovative Centers for Advanced Interdisciplinary Research Areas (Innovative Bioproduction, Kobe) from the Ministry of Education, Culture, Sports, and Science and Technology (MEXT), Japan.

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¹: Both authors equally contributed to this work.

²: Current address: Research Center for Solar Energy Chemistry, Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka 560-8531, Japan.

View full text

Comprehensive metabolomic analyses of anode-respiring Geobacter sulfurreducens cells: The impact of anode-respiration activity on intracellular metabolite levels

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Preparation of Geobacter cells and cultivation in an electrochemical cell

Results and discussion

Conclusion

Competing interests

Acknowledgments

FEMS Microbiol. Lett.

Metab. Eng.

Bioelectrochemistry

J. Biosci. Bioeng.

J. Chromatogr. A

J. Chromatogr. A

J. Biosci. Bioeng.

Bioresour. Technol.

Bug juice: harvesting electricity with microorganisms

Nat. Rev. Microbiol.

Respiration of metal (hydr) oxides by Shewanella and Geobacter: a key role for multihaem c-type cytochromes

Mol. Microbiol.

Microorganisms pumping iron: anaerobic microbial iron oxidation and reduction

Nat. Rev. Microbiol.

Electricity production by Geobacter sulfurreducens attached to electrodes

Appl. Environ. Microbiol.