Mediated electron transfer in defined microbial co-cultures for bioelectrochemical system application

  • Mediator basierter Elektronentransfer in definierten Mischkulturen für eine Anwendung in bioelektrochemischen Systemen

Schmitz, Simone; Agler-Rosenbaum, Miriam (Thesis advisor); Blank, Lars Mathias (Thesis advisor)

Aachen (2018, 2019)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2018

Abstract

Bioelectrochemical systems (BES) hold great promise for sustainable energy generation as they can convert chemical energy into electrical energy via a microbial catalyst or vice versa. BES most often rely on the utilization of undefined microbial mixed cultures as synergistic effects between different community members enable a most efficient conversion of complex substrates, e.g., wastewater into electrical energy. Within this thesis the co-operation of the phenazine redox mediator producer Pseudomonas aeruginosa with 2,3-butanediol fermenting microbes in general and Enterobacter aerogenes specifically were investigated for their application in BES. To utilize microbial co-operation in BES, the physical and chemical environments provided in the natural habitats of the co-culture play a crucial role. Especially, oxygen was identified as major factor influencing synergistic effects in co-cultures of P. aeruginosa and E. aerogenes and optimization of its supply could enhance electric current production over 400%. Furthermore, operating the co-culture in fed-batch mode enabled us to obtain very high current densities and to boost the coulombic efficiency up to 20%, which is outstanding for mediator-based electron transfer. Phenazine production is subject to a highly complex regulation network effecting the expression of the two redundant phz gene operons and the two specific genes phzM and phzS necessary to convert PCA to pyocyanin. Product and expression analysis of phenazine deletion mutants revealed a tight cross regulation between the genes. A strong dominance of operon phzA2-G2 (phz2) resulting in a 10-fold higher expression than phzA1-G1 (phz1) and an almost exclusive production of PCA from this operon was observed. Furthermore, phzM and phzS seem to act as antagonists keeping phenazine production and speciation in homeostasis. Applied to the BES, the altered and often increased phenazine production in the mutant strains was directly translated into current generation. The role of fermenters in BES is to provide the anode community with easily accessible substrates. However, here also electron transfer with synthetic phenazines could be demonstrated for the 2,3-butanediol fermenters S. marcescens, K. pneumoniae and S. aureus. Even true co-cultures were established with P. aeruginosa, however only S. aureus co-cultures performed synergistically and very similar to E. aerogenes - P. aeruginosa co-cultures. To further investigate this phenomena, spent culture supernatants of E. aerogenes were applied to P. aeruginosa, which resulted independently from 2,3-butanediol in a drastic improved phenazine generation. Thus, another synergistic effector between the fermenter and P. aeruginosa is involved, which demonstrates the importance and scope of synergistic ecological effects in BES. This thesis clearly shows the potential of applying defined microbial co-cultures for current generation in BES. The results deepen the understanding of co-culture behavior and its performance which is an important step for the advancement of BES in order to make it a compatible and flexible source for bioenergy generation.

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