S Read

Cathodic oxygen reduction catalyzed by bacteria in microbial fuel cells.

Microbial fuel cells (MFCs) have the potential to combine wastewater treatment efficiency with energetic efficiency. One of the major impediments to MFC implementation is the operation of the cathode compartment, as it employs environmentally unfriendly catalysts such as platinum. As recently shown, bacteria can facilitate sustainable and cost-effective cathode catalysis for nitrate and also oxygen. Here we describe a carbon cathode open to the air, on which attached bacteria catalyzed oxygen reduction. The bacteria present were able to reduce oxygen as the ultimate electron acceptor using electrons provided by the solid-phase cathode. Current densities of up to 2.2 A m−2 cathode projected surface were obtained (0.303±0.017 W m−2, 15 W m−3 total reactor volume). The cathodic microbial community was dominated by Sphingobacterium, Acinetobacter and Acidovorax sp., according to 16S rRNA gene clone library analysis. Isolates of Sphingobacterium sp. and Acinetobacter sp. were obtained using H2/O2 mixtures. Some of the pure culture isolates obtained from the cathode showed an increase in the power output of up to three-fold compared to a non-inoculated control, that is, from 0.015±0.001 to 0.049±0.025 W m−2 cathode projected surface. The strong decrease in activation losses indicates that bacteria function as true catalysts for oxygen reduction. Owing to the high overpotential for non-catalyzed reduction, oxygen is only to a limited extent competitive toward the electron donor, that is, the cathode. Further research to refine the operational parameters and increase the current density by modifying the electrode surface and elucidating the bacterial metabolism is warranted.

Initial development and structure of biofilms on microbial fuel cell anodes

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.

Bacterial community structure corresponds to performance during cathodic nitrate reduction.

Microbial fuel cells (MFCs) have applications other than electricity production, including the capacity to power desirable reactions in the cathode chamber. However, current knowledge of the microbial ecology and physiology of biocathodes is minimal, and as a result more research dedicated to understanding the microbial communities active in cathode biofilms is required. Here we characterize the microbiology of denitrifying bacterial communities stimulated by reducing equivalents generated from the anodic oxidation of acetate. We analyzed biofilms isolated from two types of cathodic denitrification systems: (1) a loop format where the effluent from the carbon oxidation step in the anode is subjected to a nitrifying reactor which is fed to the cathode chamber and (2) an alternative non-loop format where anodic and cathodic feed streams are separated. The results of our study indicate the superior performance of the loop reactor in terms of enhanced current production and nitrate removal rates. We hypothesized that phylogenetic or structural features of the microbial communities could explain the increased performance of the loop reactor. We used PhyloChip with 16S rRNA (cDNA) and fluorescent in situ hybridization to characterize the active bacterial communities. Our study results reveal a greater richness, as well as an increased phylogenetic diversity, active in denitrifying biofilms than was previously identified in cathodic systems. Specifically, we identified Proteobacteria, Firmicutes and Chloroflexi members that were dominant in denitrifying cathodes. In addition, our study results indicate that it is the structural component, in terms of bacterial richness and evenness, rather than the phylogenetic affiliation of dominant bacteria, that best corresponds to cathode performance.

Biofilm stratification during simultaneous nitrification and denitrification (SND) at a biocathode.

The aeration of the cathode compartment of bioelectrochemical systems (BESs) was recently shown to promote simultaneous nitrification and denitrification (SND). This study investigates the cathodic metabolism under different operating conditions as well as the structural organization of the cathodic biofilm during SND. Results show that a maximal nitrogen removal efficiency of 86.9 ± 0.5%, and a removal rate of 3.39 ± 0.08 mg NL(-1)h(-1) could be achieved at a dissolved oxygen (DO) level of 5.73 ± 0.03 mg L(-1) in the catholyte. The DO levels used in this study are higher than the thresholds previously reported as detrimental for denitrification. Analysis of the cathodic half-cell potential during batch tests suggested the existence of an oxygen gradient within the biofilm while performing SND. FISH analysis corroborated this finding revealing that the structure of the biofilm included an outer layer occupied by putative nitrifying organisms, and an inner layer where putative denitrifying organisms were most dominant. To our best knowledge this is the first time that nitrifying and denitrifying microorganisms are simultaneously observed in a cathodic biofilm.

Microbial Resource Management revisited: successful parameters and new concepts

In the twenty-first century, scientists will want to steer the microbial black box in (engineered) ecosystems, rather than only study and describe them. This strategy led to a new way of thinking: Microbial Resource Management (MRM). For the last few years, MRM has been utilized to consolidate and communicate our acquired knowledge of the microbiome to many areas of the scientific community. This shared knowledge has brought us closer to formulating a plan toward the analysis, and at a later stage, the management of our varied microbial communities and to look at ways of harnessing their unique abilities for future practices. We require this acquired knowledge for a more sustainable solution to our ongoing global challenges such as our diminishing energy and water supply. Like any successful concept, MRM must be updated to adapt to new molecular technologies, and thus, in this review, MRM has been reengineered to encompass these changes. This review reports how MRM has been used successfully over the last few years within various environments and how we can broaden its capabilities to increase its compliance in the face of state of the art ever changing technologies. Not only have we reengineered and improved MRM, but also we have discussed how newly formed relationships between technologies can provide the full picture of these complex microbial communities and their interactions for future opportunities.