Simultaneous fermentation of cellulose and current production with an enriched mixed culture of thermophilic bacteria in a microbial electrolysis cell

Comparative genomic analysis reveals electron transfer pathways of Thermoanaerobacterium thermosaccharolyticum: Insights into thermophilic electroactive bacteria

Microbial extracellular respiration is an important energy metabolism on earth, which is significant for the elemental biogeochemical cycle. Herein, extracellular Fe(III) and electrode respiration were confirmed in Thermoanaerobacterium thermosaccharolyticum MJ2. The intra/extracellular electron transfer (IET/EET) mechanism of MJ2 was investigated by comparative genomic analysis for the first time. Morphological characterization and electrochemical properties of anode illustrated that MJ2 generated bio-electricity by forming a biofilm. The respiration chain inhibition and enzyme activity tests showed that hydrogenase with cytochrome c (Cyt-c) was involved in IET of MJ2. Noteworthily, the exogenous Cyt-c increased hydrogenase activity to promote bio-electricity generation by 92.84 %. The Cyt-c gene synteny between MJ2 and another well-known exoelectrogen (Thermincola potens JR) indicated that Cyt-c bound to the outer membrane mediated the formation of biofilm involved in EET of MJ2. This study broadened the understanding of microbial extracellular respiration diversity and provided new insights to explore the electron transfer pathways of exoelectrogens.

Polyaniline-Derived Nitrogen-Containing Carbon Nanostructures with Different Morphologies as Anode Modifier in Microbial Fuel Cells

Anode modification with carbon nanomaterials is an important strategy for the improvement of microbial fuel cell (MFC) performance. The presence of nitrogen in the carbon network, introduced as active nitrogen functional groups, is considered beneficial for anode modification. In this aim, nitrogen-containing carbon nanostructures (NCNs) with different morphologies were obtained via carbonization of polyaniline and were further investigated as anode modifiers in MFCs. The present study investigates the influence of NCN morphology on the changes in the anodic microbial community and MFC performance. Results show that the nanofibrillar morphology of NCNs is beneficial for the improvement of MFC performance, with a maximum power density of 40.4 mW/m2, 1.25 times higher than the anode modified with carbonized polyaniline with granular morphology and 2.15 times higher than MFC using the carbon cloth-anode. The nanofibrillar morphology, due to the well-defined individual nanofibers separated by microgaps and micropores and a better organization of the carbon network, leads to a larger specific surface area and higher conductivity, which can allow more efficient substrate transport and better bacterial colonization with greater relative abundances of Geobacter and Thermoanaerobacter, justifying the improvement of MFC performance.

Effect of hyperthermophilic pretreatment on methane and hydrogen production from garden waste under mesophilic and thermophilic conditions

Anaerobic digestion of garden waste was investigated in a two-stage process consisting of hyperthermophilic pretreatment followed by mesophilic or thermophilic fermentation. The greatest digestion performance was achieved when the substrates were first treated at 70 °C for 3 days with no inoculation, and then mixed with inoculum (anaerobic sludge) and subjected to anaerobic digestion at 55 °C. Under such conditions, the maximum methane and hydrogen yields from grass were 517 NmlCH₄/kgVS and 52 NmlH₂/kgVS, whereas the corresponding values for leaves were 421 NmlCH₄/kgVS and 23 NmlH₂/kgVS, and these figure were far greater than the yields obtained in experiments with no hyperthermophilic stage. A metagenomic analysis of hyperthermophilic environments revealed the appearance of thermophilic and hyperthermophilic bacteria showing hydrolytic activity against lignocellulosic materials, including Caldicellulosiruptor, Thermovenabulum, Thermoanaerobacter, Moorella, Tepimicrobium, Geobacillus and Thermobacillus at a genus level. A noticeable methane production in the hyperthermophilic stage could be linked to the presence of Methanothermobacter sp.

Enzyme biotechnology development for treating polymers in hydraulic fracturing operations

Carboxymethyl cellulose (CMC) is a polymer used in many different industrial sectors. In the oil and gas industry, CMC is often used during hydraulic fracturing (fracking) operations as a thickening agent for effective proppant delivery. Accumulations of CMC at fracture faces (known as filter cakes) can impede oil and gas recovery. Although chemical oxidizers are added to disrupt these accumulations, there is industrial interest in developing alternative, enzyme-based treatments. Little is known about CMC biodegradation under fracking conditions. Here, we enriched a methanogenic CMC-degrading culture and demonstrated its ability to enzymatically utilize CMC under the conditions that typify oil fields. Using the extracellular enzyme fraction from the culture, significant CMC viscosity reduction was observed between 50 and 80˚C, at salinities up to 20% (w/v) and at pH 5-8 compared to controls. Similar levels of viscosity reduction by extracellular enzymes were observed under oxic and anoxic conditions. This proof-of-concept study demonstrates that enzyme biotechnology holds great promise as a viable approach to treating CMC filter cakes under oilfield conditions.

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.

Thermophiles for biohydrogen production in microbial electrolytic cells

Thermophiles are promising options to use as electrocatalysts for bioelectrochemical applications including microbial electrolysis. They possess several interesting characteristics such as ability to catalyze a broad range of substrates at better rates and over a broad range of operating conditions, and better electrocatalysis/electrogenic activity over mesophiles. However, a very limited number of investigations have been carried out to explore the microbial reactions/pathways and the molecular mechanisms that contribute to better electrocatalysis/electrolysis in thermophiles. Here, we review the electroactive characteristics of thermophiles, their electron transfer mechanisms, and molecular insights behind the choice of thermophiles for bioelectrochemical/electrolytic processes.

Simultaneous fermentation of cellulose and current production with an enriched mixed culture of thermophilic bacteria in a microbial electrolysis cell

An enriched mixed culture of thermophilic (60°C) bacteria was assembled for the purpose of using cellulose to produce current in thermophilic microbial electrolysis cells (MECs). Cellulose was fermented into sugars and acids before being consumed by anode‐respiring bacteria (ARB) for current production. Current densities (j) were sustained at 6.5 ± 0.2 A m⁻² in duplicate reactors with a coulombic efficiency (CE) of 84 ± 0.3%, a coulombic recovery (CR) of 54 ± 11% and without production of CH₄. Low‐scan rate cyclic voltammetry (LSCV) revealed a mid‐point potential (E_ka) of −0.17 V versus SHE. Pyrosequencing analysis of the V4 hypervariable region of 16S rDNA and scanning electron microscopy present an enriched thermophilic microbial community consisting mainly of the phylum Firmicutes with the Thermoanaerobacter (46 ± 13%) and Thermincola (28 ± 14%) genera occupying the biofilm anode in high relative abundance and Tepidmicrobium (38 ± 6%) and Moorella (11 ± 8%) genera present in high relative abundance in the bulk medium. The Thermoanaerobacter (15 ± 16%) and Brevibacillus (21 ± 30%) genera were also present in the bulk medium; however, their relative abundance varied by reactor. This study indicates that thermophilic consortia can obtain high CE and CR, while sustaining high current densities from cellulose in MECs.