In nature, different bacteria have evolved strategies to transfer electrons far beyond the cell surface. This electron transfer enables the use of these bacteria in bioelectrochemical systems (BES), such as microbial fuel cells (MFCs) and microbial electrosynthesis (MES). The main feature of electroactive bacteria (EAB) in these applications is the ability to transfer electrons from the microbial cell to an electrode or vice versa instead of the natural redox partner. In general, the application of electroactive organisms in BES offers the opportunity to develop efficient and sustainable processes for the production of energy as well as bulk and fine chemicals, respectively. This review describes and compares key microbiological features of different EAB. Furthermore, it focuses on achievements and future prospects of genetic manipulation for efficient strain development.

Electroactive bacteria—molecular mechanisms and genetic tools

Carbon-mineral association between iron oxides and dissolved organic carbon (DOC) occurs ubiquitously in natural environments, and is thus an important control on the transport and sequestration of organic carbon. However, the stability of C-mineral association under reducing conditions and its implication on carbon turnover and cycling is unclear. In this study, anaerobic incubation experiments were conducted to investigate the biodegradability of ferrihydrite (Fe-oxide)-adsorbed natural DOC with known Fe-reducing bacterium, Shewanella oneidensis MR-1. Two types of bulk DOC were used to form the carbon-ferrihydrite association –forest fresh leaf layer (LDOC) and forest humified layer (HDOC). Under controlled pH and redox condition, DOC concentrations increased by 11.2 (±0.4)% and 13.3 (±0.3)% for LDOC and HDOC after anaerobic microbial iron reduction. Coupled to the DOC release, 28% and 14% of Fe(III) were reduced with LDOC and HDOC, respectively. Our results demonstrated that under controlled pH conditions, the C release was driven by dissimilatory iron reduction. Likely microbial iron reduction decrease the carbon sequestration potential via reducing the Fe-oxides surface area and therefore release DOC to the solution and make it more susceptible to microbial degradation. Further, dissimilatory iron reduction also shifted DOC spectroscopic properties (UV and fluorescence) and pronounced increase in humification index (HIX) values was observed after the incubation. Our results suggest dissimilatory iron reduction is an important mechanism for Fe dissolution and C mobilization, which impacts the long-term carbon transformation, storage and turnover in soil environments.

Dissimilatory microbial iron reduction release DOC (dissolved organic carbon) from carbon-ferrihydrite association

Reduction of jarosite by Shewanella oneidensis MR-1 and secondary mineralization

Shewanella species are isolated from the oxic/anoxic regions of seawater and aquatic sediments where redox conditions fluctuate in time and space. Colonization of these environments is by virtue of flexible respiratory chains, many of which are notable for the ability to reduce extracellular substrates including the Fe(III) and Mn(IV) contained in oxide and phyllosilicate minerals. Shewanella oneidensis MR-1 serves as a model organism to consider the biochemical basis of this flexibility. In the present paper, we summarize the various systems that serve to branch the respiratory chain of S. oneidensis MR-1 in order that electrons from quinol oxidation can be delivered the various terminal electron acceptors able to support aerobic and anaerobic growth. This serves to highlight several unanswered questions relating to the regulation of respiratory electron transport in Shewanella and the central role(s) of the tetrahaem-containing quinol dehydrogenase CymA in that process.

/pdf/the-roles-of-cyma-in-support-of-the-respiratory-flexibility-4rxdr057is.pdf

The roles of CymA in support of the respiratory flexibility of Shewanella oneidensis MR-1

Hypothetical and conserved hypothetical genes account for >30percent of sequenced bacterial genomes. For the sulfate-reducing bacterium Desulfovibrio vulgaris Hildenborough, 347 of the 3634 genes were annotated as conserved hypothetical (9.5percent) along with 887 hypothetical genes (24.4percent). Given the large fraction of the genome, it is plausible that some of these genes serve critical cellular roles. The study goals were to determine which genes were expressed and provide a more functionally based annotation. To accomplish this, expression profiles of 1234 hypothetical and conserved genes were used from transcriptomic datasets of 11 environmental stresses, complemented with shotgun LC-MS/MS and AMT tag proteomic data. Genes were divided into putatively polycistronic operons and those predicted to be monocistronic, then classified by basal expression levels and grouped according to changes in expression for one or multiple stresses. 1212 of these genes were transcribed with 786 producing detectable proteins. There was no evidence for expression of 17 predicted genes. Except for the latter, monocistronic gene annotation was expanded using the above criteria along with matching Clusters of Orthologous Groups. Polycistronic genes were annotated in the same manner with inferences from their proximity to more confidently annotated genes. Two targeted deletion mutants were used as test cases to determine the relevance of the inferred functional annotations.

Expression profiling of hypothetical genes in Desulfovibrio vulgaris leads to improved functional annotation

Although little is known of potential function for conserved signaling proteins, it is hypothesized that such proteins play important roles to coordinate cellular responses to environmental stimuli. In order to elucidate the function of a putative sensory box protein (PAS domains) in Shewanella oneidensis MR-1, the physiological role of SO3389 was characterized. The predicted open reading frame (ORF) encodes a putative sensory box protein that has PAS, GGDEF, and EAL domains, and an in-frame deletion mutant was constructed (ΔSO3389) with approximately 95% of the ORF deleted. Under aerated conditions, wild-type and mutant cultures had similar growth rates, but the mutant culture had a lower growth rate under static, aerobic conditions. Oxygen consumption rates were lower for mutant cultures (1.5-fold), and wild-type cultures also maintained lower dissolved oxygen concentrations under aerated growth conditions. When transferred to anoxic conditions, the mutant did not grow with fumarate, iron(III), or dimethyl sulfoxide (DMSO) as electron acceptors. Biochemical assays demonstrated the expression of different c-type cytochromes as well as decreased fumarate reductase activity in the mutant transferred to anoxic growth conditions. Transcriptomic studies showed the inability of the mutant to up-express and down-express genes, including c-type cytochromes (e.g., SO4047/SO4048, SO3285/SO3286), reductases (e.g., SO0768, SO1427), and potential regulators (e.g., SO1329). The complemented strain was able to grow when transferred from aerobic to anoxic growth conditions with the tested electron acceptors. The modeled structure for the SO3389 PAS domains was highly similar to the crystal structures of FAD-binding PAS domains that are known O2/redox sensors. Based on physiological, genomic, and bioinformatic results, we suggest that the sensory box protein, SO3389, is an O2/redox sensor that is involved in optimization of aerobic growth and transitions to anoxia in S. oneidensis MR-1.

Shewanella oneidensis MR-1 Sensory Box Protein Involved in Aerobic and Anoxic Growth

GrowthInvolved in Aerobic and Anoxic Shewanella oneidensis MR-1 Sensory Box

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Shewanella oneidensis MR-1 Sensory Box Protein Involved in Aerobic and Anoxic Growth

GrowthInvolved in Aerobic and Anoxic Shewanella oneidensis MR-1 Sensory Box