Initial development and structure of biofilms on microbial fuel cell anodes

Abstract

BackgroundMicrobial fuel cells (MFCs) rely on electrochemically active bacteria to capture the chemical energy contained in organics and convert it to electrical energy. Bacteria develop biofilms on the MFC electrodes, allowing considerable conversion capacity and opportunities for extracellular electron transfer (EET). The present knowledge on EET is centred around two Gram-negative models, i.e. Shewanella and Geobacter species, as it is believed that Gram-positives cannot perform EET by themselves as the Gram-negatives can. To understand how bacteria form biofilms within MFCs and how their development, structure and viability affects electron transfer, we performed pure and co-culture experiments.ResultsBiofilm viability was maintained highest nearer the anode during closed circuit operation (current flowing), in contrast to when the anode was in open circuit (soluble electron acceptor) where viability was highest on top of the biofilm, furthest from the anode. Closed circuit anode Pseudomonas aeruginosa biofilms were considerably thinner compared to the open circuit anode (30 ± 3 μm and 42 ± 3 μm respectively), which is likely due to the higher energetic gain of soluble electron acceptors used. The two Gram-positive bacteria used only provided a fraction of current produced by the Gram-negative organisms. Power output of co-cultures Gram-positive Enterococcus faecium and either Gram-negative organisms, increased by 30-70% relative to the single cultures. Over time the co-culture biofilms segregated, in particular, Pseudomonas aeruginosa creating towers piercing through a thin, uniform layer of Enterococcus faecium. P. aeruginosa and E. faecium together generated a current of 1.8 ± 0.4 mA while alone they produced 0.9 ± 0.01 and 0.2 ± 0.05 mA respectively.ConclusionWe postulate that this segregation may be an essential difference in strategy for electron transfer and substrate capture between the Gram-negative and the Gram-positive bacteria used here.

Current understanding of multi‐species biofilms

“…In MFCs, electrochemically active bacterial species capture the chemical energy from organic compounds and convert it to electrical energy Bacteria develop multi-species biofilms on the MFC electrodes, which enable conversion of electricity and opportunities for extracellular electron transfer (EET) [61]. Read et al reported that interactions of Gram-positive Enterococcus faecium and other Gramnegative organisms lead to development of different structures in MFC anode biofilms and enhancement of electricity generation by 30%-70% relative to the cultures of single species [62]. Besides electricity production, MFCs are also used to power desirable reactions in the cathode chamber.…”

Resilience, Dynamics, and Interactions within a Model Multispecies Exoelectrogenic-Biofilm Community

“…The interactions are most likely as diverse as they are in natural environments, in which, for instance, syntrophic associations between microorganisms play a dominant role in the overall fitness of the communities (13)(14)(15). The positive effect from coculturing in bioelectrochemical systems was already studied with synthetic communities (16)(17)(18). Nevertheless, an in-depth molecular analysis of biofilm interactions between organisms that must share the anode as a terminal electron acceptor has not yet been conducted.…”

The mechanisms of extracellular electron transfer

“…Although not studied in this thesis, the structure of biofilms has been observed to change during microbial development on anodes of BESs [39,81]. In a continuous time trial experiment of pure cultures over 72 hours, differences and similarities of anodic biofilm development between species was detected [81].…”

“…In a continuous time trial experiment of pure cultures over 72 hours, differences and similarities of anodic biofilm development between species was detected [81]. Over time it was found that biofilms of S. oneidensis and G. sulfurreducens became less dense, forming tower structures resulting in less coverage of the electrode, with the formation of channels and a reduction in biofilm mass [81]. The development of G. sulfurreducens and S. oneidensis biofilms differed in the formation of towers, which were observed more frequently throughout the biofilm of G.…”

“…Bacteria form biofilms on electrodes of BESs, allowing significant exchange capabilities and opportunities for EET [81]. in 'microbial electrolysis cell' mode, in which power is invested to increase the kinetics of the reactions and/or to drive thermodynamically unfavourable reactions 8 .…”