Anode Biofilms of Geoalkalibacter ferrihydriticus Exhibit Electrochemical Signatures of Multiple Electron Transport Pathways

Geoalkalibacter halelectricus SAP-1 sp. nov. possessing extracellular electron transfer and mineral-reducing capabilities from a haloalkaline environment

The extracellular electron transfer (EET)-capable electroactive microorganisms (EAMs) play crucial roles in mineral cycling and interspecies electron transfer in different environments and are used as biocatalysts in microbial electrochemical technologies. Studying EAMs from extreme environments is desired to advance the electromicrobiology discipline, understanding their unique metabolic traits with implications to extreme microbiology, and develop specific bioelectrochemical applications. Here, we present a novel haloalkaliphilic bacterium named Geoalkalibacter halelectricus SAP-1, isolated from a microbial electroactive biofilm enriched from the haloalkaline lake sediments. It is a rod-shaped Gram-negative heterotrophic anaerobe that uses various carbon and energy sources and respires on soluble and insoluble terminal electron acceptors. Besides 16S-rRNA and whole-genome-based phylogeny, the GGDC values of 21.7 %, ANI of 78.5, and 2.77 % genomic DNA GC content difference with the closest validly named species Geoalkalibacter ferrihydriticus (DSM 17813^T ) confirmed its novelty. When grown with the solid-state electrode as the only electron acceptor, it produced 460±23 μA/cm² bioelectrocatalytic current, thereby confirming its electroactivity. Further electrochemical analysis revealed the presence of membrane redox components with high formal potentials, putatively involved in the direct mode of EET. These are distinct from EET components reported for any known electroactive microorganisms, including well-studied Geobacter spp., Shewanella spp. and Desulfuromonas acetexigens. Further the capabilities of G. halelectricus SAP-1 to respire soluble as well insoluble electron acceptors including fumarate, SO₄ ^2- , Fe³⁺ , and Mn⁴⁺ suggests its role in cycling these elements in haloalkaline environments. This article is protected by copyright. All rights reserved.

Tuning of Electrode Surface for Enhanced Bacterial Adhesion and Reactions: A Review on Recent Approaches

The study of bacterial adhesion and its consequences has great significance in different fields such as marine science, renewable energy sectors, soil and plant ecology, food industry, and the biomedical field. Generally, the adverse effects of microbial surface interactions have attained wide visibility. However, herein, we present distinct approaches to highlight the beneficial aspects of microbial surface interactions for various applications rather than deal with the conventional negative aspects or prevention strategies. The surface microbial reactions can be tuned for useful biochemical or bio-electrochemical applications, which are otherwise unattainable through conventional routes. In this context, the present review is a comprehensive approach to highlight the basic principles and signature parameters that are responsible for the useful microbial-electrode interactions. It also proposes various surface tuning strategies, which are useful for tuning the electrode characteristics particularly suitable for the enhanced bacterial adhesion and reactions. The tuning of surface characteristics of electrodes is discussed with a special reference to the Microbial Fuel Cell as an example.

Microbial electroactive biofilms dominated by Geoalkalibacter spp. from a highly saline-alkaline environment

Understanding of the extreme microorganisms that possess extracellular electron transfer (EET) capabilities is pivotal to advance electromicrobiology discipline and to develop niche-specific microbial electrochemistry-driven biotechnologies. Here, we report on the microbial electroactive biofilms (EABs) possessing the outward EET capabilities from a haloalkaline environment of the Lonar lake. We used the electrochemical cultivation approach to enrich haloalkaliphilic EABs under 9.5 pH and 20 g/L salinity conditions. The electrodes controlled at 0.2 V vs. Ag/AgCl yielded the best-performing biofilms in terms of maximum bioelectrocatalytic current densities of 548 ± 23 and 437 ± 17 µA/cm² with acetate and lactate substrates, respectively. Electrochemical characterization of biofilms revealed the presence of two putative redox-active moieties with the mean formal potentials of 0.183 and 0.333 V vs. Ag/AgCl, which represent the highest values reported to date for the EABs. 16S-rRNA amplicon sequencing of EABs revealed the dominance of unknown Geoalkalibacter sp. at ~80% abundance. Further investigations on the haloalkaliphilic EABs possessing EET components with high formal potentials might offer interesting research prospects in electromicrobiology.

Syntrophic growth of alkaliphilic anaerobes controlled by ferric and ferrous minerals transformation coupled to acetogenesis

Redox-active iron minerals can act as energy sources or electron-transferring mediators in microbial syntrophic associations, being important means of interspecies metabolic cooperation in sedimentary environments. Alkaline conditions alter the thermodynamic stability of iron minerals, influencing their availability for interspecies syntrophic interactions. We have modeled anaerobic alkaliphilic microbial associations in ethanol-oxidizing co-culture of an obligate syntroph Candidatus "Contubernalis alkalaceticum" and a facultative lithotroph Geoalkalibacter ferrihydriticus, which is capable of dissimilatory Fe(III) reduction and homoacetogenic oxidation of Fe(II) with CO₂. The co-cultures were cultivated with thermodynamically metastable ferric-containing ferrihydrite, or ferrous-containing siderite, or without minerals. Mössbauer spectral analysis revealed the transformation of both minerals to the stable magnetite. In the presence of ferrihydrite, G. ferrihydriticus firstly reduced Fe(III) with ethanol and then switched to syntrophic homoacetogenesis, providing the growth of obligate syntroph on ethanol. The ability of G. ferrihydriticus to accept hydrogen from its syntrophic partner and produce extra acetate from carbonate during ethanol oxidation was confirmed by co-culture growth without minerals. In the presence of siderite, G. ferrihydriticus performed homoacetogenesis using two electron donors simultaneously- siderite and hydrogen. Pieces of evidence for direct and indirect hydrogen-mediated electron exchange between partner organisms were obtained. Relative abundancies of partner organisms and the rate of acetate production by their co-cultures were strongly determined by thermodynamic benefits, which G. ferrihydriticus got from redox transformations of iron minerals. Even the minor growth of G. ferrihydriticus sustained the growth of the syntroph. Accordingly, microbe-to-mineral interactions could represent underestimated drivers of syntrophic interactions in alkaline sedimentary environments.

Thermophiles; or, the Modern Prometheus: The Importance of Extreme Microorganisms for Understanding and Applying Extracellular Electron Transfer

Approximately four billion years ago, the first microorganisms to thrive on earth were anaerobic chemoautotrophic thermophiles, a specific group of extremophiles that survive and operate at temperatures ∼50 - 125°C and do not use molecular oxygen (O2) for respiration. Instead, these microorganisms performed respiration via dissimilatory metal reduction by transferring their electrons extracellularly to insoluble electron acceptors. Genetic evidence suggests that Gram-positive thermophilic bacteria capable of extracellular electron transfer (EET) are positioned close to the root of the Bacteria kingdom on the tree of life. On the contrary, EET in Gram-negative mesophilic bacteria is a relatively new phenomenon that is evolutionarily distinct from Gram-positive bacteria. This suggests that EET evolved separately in Gram-positive thermophiles and Gram-negative mesophiles, and that EET in these bacterial types is a result of a convergent evolutionary process leading to homoplasy. Thus, the study of dissimilatory metal reducing thermophiles provides a glimpse into some of Earth's earliest forms of respiration. This will provide new insights for understanding biogeochemistry and the development of early Earth in addition to providing unique avenues for exploration and discovery in astrobiology. Lastly, the physiological composition of Gram-positive thermophiles, coupled with the kinetic and thermodynamic consequences of surviving at elevated temperatures, makes them ideal candidates for developing new mathematical models and designing innovative next-generation biotechnologies. KEY CONCEPTS Anaerobe: organism that does not require oxygen for growth. Chemoautotroph: organism that obtains energy by oxidizing inorganic electron donors. Convergent Evolution: process in which organisms which are not closely related independently evolve similar traits due to adapting to similar ecological niches and/or environments. Dissimilatory Metal Reduction: reduction of a metal or metalloid that uses electrons from oxidized organic or inorganic electron donors. Exoelectrogen: microorganism that performs dissimilatory metal reduction via extracellular electron transfer. Extremophiles: organisms that thrive in physical or geochemical conditions that are considered detrimental to most life on Earth. Homoplasy: a character shared by a set of species that is not shared by a common ancestor Non-synonymous Substitutions (K a ): a substitution of a nucleotide that changes a codon sequence resulting in a change in the amino acid sequence of a protein. Synonymous Substitutions (K s ): a substitution of a nucleotide that may change a codon sequence, but results in no change in the amino acid sequence of a protein. Thermophiles: a specific group of extremophiles that survive and operate at temperatures ∼50-125°C.

Long-term effect of biochar amendment on the biodegradation of petroleum hydrocarbons in soil microbial fuel cells

Biochar is extensively applied in amendment of contaminated soils. However, the effect of biochar on the biodegradation of petroleum hydrocarbons and electricity generation in soil microbial fuel cells (MFCs) remains unclear. Here, three biochars respectively derived from poultry (chicken manure, CB), agriculture (wheat straw, SB) and forestry industries (wood sawdust, WB) were investigated after 223 days of amendment. Consequently, high removal for alkanes was in CB with the mineral nutrition and phosphorus while aromatics were in SB with the most N content and the highest molecular polarity. The lowest removal efficiency of total petroleum hydrocarbons was observed in WB with the highest surface area, whereas the most charge was obtained. The different performance of soil MFCs was due to physicochemical properties of biochar and colonized microbial communities of bacteria and archaea. The abundance of Actinotalea increased by 144-263% in SB and CB while that of Desulfatitalea distinctly increased in WB. Meanwhile, species from Methanosarcina, Methanoculleus, Halovivax and Natronorubrum exerted probably a methanogenic degrading role. This study revealed that the degrader, azotobacter and electricigens exhibited a close relationship in order to degrade hydrocarbons and generate electricity in soil bioelectrochemical remediation systems.

Bidirectional extracellular electron transfers of electrode-biofilm: Mechanism and application

The extracellular electron transfer (EET) between microorganisms and electrodes forms the basis for microbial electrochemical technology (MET), which recently have advanced as a flexible platform for applications in energy and environmental science. This review, for the first time, focuses on the electrode-biofilm capable of bidirectional EET, where the electrochemically active bacteria (EAB) can conduct both the outward EET (from EAB to electrodes) and the inward EET (from electrodes to EAB). Only few microorganisms are tested in pure culture with the capability of bidirectional EET, however, the mixed culture based bidirectional EET offers great prospects for biocathode enrichment, pollutant complete mineralization, biotemplated material development, pH stabilization, and bioelectronic device design. Future efforts are necessary to identify more EAB capable of the bidirectional EET, to balance the current density, to evaluate the effectiveness of polarity reversal for biocathode enrichment, and to boost the future research endeavors of such a novel function.

Biofilms, active substrata, and me

Having worked with biofilms since the 1970s, I know that they are ubiquitous in nature, of great value in water technology, and scientifically fascinating. Biofilms are naturally able to remove BOD, transform N, generate methane, and biodegrade micropollutants. What I also discovered is that biofilms can do a lot more for us in terms of providing environmental services if we give them a bit of help. Here, I explore how we can use active substrata to enable our biofilm partners to provide particularly challenging environmental services. In particular, I delve into three examples in which an active substratum makes it possible for a biofilm to accomplish a task that otherwise seems impossible. The first example is the delivery of hydrogen gas (H₂) as an electron donor to drive the reduction and detoxification of the rising number of oxidized contaminant: e.g., perchlorate, selenate, chromate, chlorinated solvents, and more. The active substratum is a gas-transfer membrane that delivers H₂ directly to the biofilm in a membrane biofilm reactor (MBfR), which makes it possible to deliver a low-solubility gaseous substrate with 100% efficiency. The second example is the biofilm anode of a microbial electrochemical cell (MxC). Here, the anode is the electron acceptor for anode-respiring bacteria, which "liberate" electrons from organic compounds and send them ultimately to a cathode, where we can harvest valuable products or services. The anode's potential is a sensitive tool for managing the microbial ecology and reaction kinetics of the biofilm anode. The third example is intimately coupled photobiocatalysis (ICPB), in which we use photocatalysis to enable the biodegradation of intrinsically recalcitrant organic pollutants. Photocatalysis transforms the recalcitrant organics just enough so that the products are rapidly biodegradable substrates for bacteria in a nearby biofilm. The macroporous substratum, which houses the photocatalyst on its exterior, actively provides donor substrate and protects the biofilm from UV light and free radicals in its interior. These three well-developed topics illustrate how and why an active substratum expands the scope of what biofilms can do to enhance water sustainability.

Microbial activity influences electrical conductivity of biofilm anode

This study assessed the conductivity of a Geobacter-enriched biofilm anode in a microbial electrochemical cell (MxC) equipped with two gold anodes (25 mM acetate medium), as different proton gradients were built throughout the biofilm. There was no pH gradient across the biofilm anode at 100 mM phosphate buffer (current density 2.38 A/m2) and biofilm conductivity (Kbio) was as high as 0.87 mS/cm. In comparison, an inner biofilm became acidic at 2.5 mM phosphate buffer in which dead cells were accumulated at ∼80 μm of the inner biofilm anode. At this low phosphate buffer, Kbio significantly decreased by 0.27 mS/cm, together with declined current density of 0.64 A/m2. This work demonstrates that biofilm conductivity depends on the composition of live and dead cells in the conductive biofilm anode.

Metatranscriptomic Evidence for Direct Interspecies Electron Transfer between Geobacter and Methanothrix Species in Methanogenic Rice Paddy Soils

The possibility that Methanothrix (formerly Methanosaeta) and Geobacter species cooperate via direct interspecies electron transfer (DIET) in terrestrial methanogenic environments was investigated in rice paddy soils. Genes with high sequence similarity to the gene for the PilA pilin monomer of the electrically conductive pili (e-pili) of Geobacter sulfurreducens accounted for over half of the PilA gene sequences in metagenomic libraries and 42% of the mRNA transcripts in RNA sequencing (RNA-seq) libraries. This abundance of e-pilin genes and transcripts is significant because e-pili can serve as conduits for DIET. Most of the e-pilin genes and transcripts were affiliated with Geobacter species, but sequences most closely related to putative e-pilin genes from genera such as Desulfobacterium, Deferribacter, Geoalkalibacter, and Desulfobacula, were also detected. Approximately 17% of all metagenomic and metatranscriptomic bacterial sequences clustered with Geobacter species, and the finding that Geobacter spp. were actively transcribing growth-related genes indicated that they were metabolically active in the soils. Genes coding for e-pilin were among the most highly transcribed Geobacter genes. In addition, homologs of genes encoding OmcS, a c-type cytochrome associated with the e-pili of G. sulfurreducens and required for DIET, were also highly expressed in the soils. Methanothrix species in the soils highly expressed genes for enzymes involved in the reduction of carbon dioxide to methane. DIET is the only electron donor known to support CO₂ reduction in Methanothrix Thus, these results are consistent with a model in which Geobacter species were providing electrons to Methanothrix species for methane production through electrical connections of e-pili.IMPORTANCEMethanothrix species are some of the most important microbial contributors to global methane production, but surprisingly little is known about their physiology and ecology. The possibility that DIET is a source of electrons for Methanothrix in methanogenic rice paddy soils is important because it demonstrates that the contribution that Methanothrix makes to methane production in terrestrial environments may extend beyond the conversion of acetate to methane. Furthermore, defined coculture studies have suggested that when Methanothrix species receive some of their energy from DIET, they grow faster than when acetate is their sole energy source. Thus, Methanothrix growth and metabolism in methanogenic soils may be faster and more robust than generally considered. The results also suggest that the reason that Geobacter species are repeatedly found to be among the most metabolically active microorganisms in methanogenic soils is that they grow syntrophically in cooperation with Methanothrix spp., and possibly other methanogens, via DIET.

Electrochemical Behavior of Anode-Respiring Bacteria on Doped Carbon Electrodes

Cultivating anodic respiring bacteria (ARB) on anodes doped with metal-enhanced biological growth and affected higher electocatalytic activity (ECA). The anode doped with calcium sulfide (CaS) proved more favorable for ARB than the magnetite (Fe₃O₄) or iron(II) sulfide (FeS). Average anodic current densities of 8.4 Am^2- (Fe₃O₄), 11.1 Am^2- (FeS), and 22.0 Am^2- (CaS) were achieved as compared to that of nondoped carbon (5.1 A m^-2). Thus, CaS-doped graphite represents a promising anode material which is suitable for highly efficient bioelectrochemical systems (BES). Electrochemical evaluation during turnover and starvation using simple cycle voltammetry (CV) and derivative cycle voltammetry (DCV) indicated several extracellular electron transfer (EET) pathways characterized with lower potentials for biofilms. However, despite the high affinity of bacteria to iron, their lower ECA was kinetically attributed to the accumulation of self-produced mediators on iron-doped anodes.