A comprehensive review of microbial electrochemical systems as a platform technology

Microbial electrochemical systems (MESs) use microorganisms to covert the chemical energy stored in biodegradable materials to direct electric current and chemicals. Compared to traditional treatment-focused, energy-intensive environmental technologies, this emerging technology offers a new and transformative solution for integrated waste treatment and energy and resource recovery, because it offers a flexible platform for both oxidation and reduction reaction oriented processes. All MESs share one common principle in the anode chamber, in which biodegradable substrates, such as waste materials, are oxidized and generate electrical current. In contrast, a great variety of applications have been developed by utilizing this in situ current, such as direct power generation (microbial fuel cells, MFCs), chemical production (microbial electrolysis cells, MECs; microbial electrosynthesis, MES), or water desalination (microbial desalination cells, MDCs). Different from previous reviews that either focus on one function or a specific application aspect, this article provides a comprehensive and quantitative review of all the different functions or system constructions with different acronyms developed so far from the MES platform and summarizes nearly 50 corresponding systems to date. It also provides discussions on the future development of this promising yet early-stage technology.     

Graphite anode surface modification with controlled reduction of specific aryl diazonium salts for improved microbial fuel cells power output

Graphite electrodes were modified with reduction of aryl diazonium salts and implemented as anodes in microbial fuel cells. First, reduction of 4-aminophenyl diazonium is considered using increased coulombic charge density from 16.5 to 200 mC/cm(2). This procedure introduced aryl amine functionalities at the surface which are neutral at neutral pH. These electrodes were implemented as anodes in "H" type microbial fuel cells inoculated with waste water, acetate as the substrate and using ferricyanide reduction at the cathode and a 1000 Ω external resistance. When the microbial anode had developed, the performances of the microbial fuel cells were measured under acetate saturation conditions and compared with those of control microbial fuel cells having an unmodified graphite anode. We found that the maximum power density of microbial fuel cell first increased as a function of the extent of modification, reaching an optimum after which it decreased for higher degree of surface modification, becoming even less performing than the control microbial fuel cell. Then, the effect of the introduction of charged groups at the surface was investigated at a low degree of surface modification. It was found that negatively charged groups at the surface (carboxylate) decreased microbial fuel cell power output while the introduction of positively charged groups doubled the power output. Scanning electron microscopy revealed that the microbial anode modified with positively charged groups was covered by a dense and homogeneous biofilm. Fluorescence in situ hybridization analyses showed that this biofilm consisted to a large extent of bacteria from the known electroactive Geobacter genus. In summary, the extent of modification of the anode was found to be critical for the microbial fuel cell performance. The nature of the chemical group introduced at the electrode surface was also found to significantly affect the performance of the microbial fuel cells. The method used for modification is easy to control and can be optimized and implemented for many carbon materials currently used in microbial fuel cells and other bioelectrochemical systems.     

Engineering microbial technologies for environmental sustainability: choices to make

Microbial technologies have provided solutions to key challenges in our daily lives for over a century. In the debate about the ongoing climate change and the need for planetary sustainability, microbial ecology and microbial technologies are rarely considered. Nonetheless, they can bring forward vital solutions to decrease and even prevent long-term effects of climate change. The key to the success of microbial technologies is an effective, target-oriented microbiome management. Here, we highlight how microbial technologies can play a key role in both natural, i.e. soils and aquatic ecosystems, and semi-natural or even entirely human-made, engineered ecosystems, e.g. (waste) water treatment and bodily systems. First, we set forward fundamental guidelines for effective soil microbial resource management, especially with respect to nutrient loss and greenhouse gas abatement. Next, we focus on closing the water circle, integrating resource recovery. We also address the essential interaction of the human and animal host with their respective microbiomes. Finally, we set forward some key future potentials, such as microbial protein and the need to overcome microphobia for microbial products and services. Overall, we conclude that by relying on the wisdom of the past, we can tackle the challenges of our current era through microbial technologies.     

Resource and energy recovery options for fermentation industry residuals

Over the last 40 years, the fermentation industry has provided facility planners, plant operators and environmental engineers with a wide range of residuals management challenges and resource/energy recovery opportunities. In response, the industry has helped pioneer the use of a number of innovative resource and energy recovery technologies. Production of animal feed supplements, composts, fertilizers, soil amendments, commercial baking additives and microbial protein materials have all been detailed in the literature. In many such cases, recovery of by-products significantly reduces the need for treatment and disposal facilities. Stable, reliable anaerobic biological treatment processes have also been developed to recovery significant amounts of energy in the form of methane gas. Alternatively, dewatered or condensed organic fermentation industry residuals have been used as fuels for incineration-based energy recovery systems. The sale or use of recovered by-products and/or energy can be used to offset required processing costs and provide a technically and environmentally viable alternative to traditional treatment and disposal strategies. This review examines resource recovery options currently used or proposed for fermentation industry residuals and the conditions necessary for their successful application.     

Development of GRAS strains for nutraceutical production using systems and synthetic biology approaches: advances and prospects

Nutraceuticals are food substances with medical and health benefits for humans. Limited by complicated procedures, high cost, low yield, insufficient raw materials, resource waste, and environment pollution, chemical synthesis and extraction are being replaced by microbial synthesis of nutraceuticals. Many microbial strains that are generally regarded as safe (GRAS) have been identified and developed for the synthesis of nutraceuticals, and significant nutraceutical production by these strains has been achieved. In this review, we systematically summarize recent advances in nutraceutical research in terms of physiological effects on health, potential applications, drawbacks of traditional production processes, characteristics of production strains, and progress in microbial fermentation. Recent advances in systems and synthetic biology techniques have enabled comprehensive understanding of GRAS strains and its wider applications. Thus, these microbial strains are promising cell factories for the commercial production of nutraceuticals.     

Ferric reduction in organic matter oxidation and its applicability for anaerobic wastewater treatment: a review and future aspects

Anaerobic treatment processes have the advantages of cost-effectiveness, energy efficiency, low sludge yield and potential of resource recovery over conventional aerobic treatment methods and have been gaining increasing attention as an approach for future wastewater management. An important feature of anaerobic processes is the use of alternative electron acceptors to oxygen, which renders treatment flexibility in using redox active elements such as iron and sulfate from other waste materials. Co-treatment of acid mine drainage and municipal wastewater, as an example, has been shown to be an effective method for removing organic materials, metals, and phosphate from the both wastes. It also suggested the applicability of ferric reduction process in wastewater treatment. Most of the previous studies on ferric reduction process and iron reducers were conducted in natural systems such as sediments, soils and groundwater. This paper reviews the significance and fundamentals of the ferric reduction process, its utility for organics oxidation, controlling factors, reaction kinetics, microbial processes of iron reduction and its ecology. The paper also evaluates the suitability and discusses future aspects of using iron reduction for wastewater treatment. Knowledge gaps are identified in this paper for developing such innovative wastewater technology and process optimization.     

Does plant community plasticity mediate microbial homeostasis?

Microbial homeostasis—constant microbial element ratios along resource gradients—is a core ecological tenet, yet not all systems display homeostasis. We suggest investigations of homeostasis mechanisms must also consider plant–microbial interactions. Specifically, we hypothesized that ecosystems with strong plant community plasticity to changing resources will have homeostatic microbial communities, with less microbial resource cost, because plants reduce variance in resource stoichiometry. Using long‐term nutrient additions in two ecosystems with differing plant response, we fail to support our hypothesis because although homeostasis appears stronger in the system with stronger plant response, microbial mechanisms were also stronger. However, our conclusions were undermined by high heterogeneity in resources, which may be common in ecosystem‐level studies, and methodological assumptions may be exacerbated by shifting plant communities. We propose our study as a starting point for further ecosystem‐scale investigations, with higher replication to address microbial and soil variability, and improved insight into microbial assimilable resources.     

Factors influencing the degradation of garbage in methanogenic bioreactors and impacts on biogas formation

Anaerobic digestion of garbage is attracting much attention because of its application in waste volume reduction and the recovery of biogas for use as an energy source. In this review, various factors influencing the degradation of garbage and the production of biogas are discussed. The surface hydrophobicity and porosity of supporting materials are important factors in retaining microorganisms such as aceticlastic methanogens and in attaining a higher degradation of garbage and a higher production of biogas. Ammonia concentration, changes in environmental parameters such as temperature and pH, and adaptation of microbial community to ammonia have been related to ammonia inhibition. The effects of drawing electrons from the methanogenic community and donating electrons into the methanogenic community on methane production have been shown in microbial fuel cells and bioelectrochemical reactors. The influences of trace elements, phase separation, and co-digestion are also summarized in this review.     

酵母表面展示体系的构建及在纤维素降解中的应用*

纤维素是来源广泛且储量较大的低成本可再生资源,但其结构致密难以利用。目前降解纤维素需要多种纤维素酶协作,而游离纤维素酶成本高、难以重复利用等问题限制了其广泛应用。利用酵母表面展示技术,可以将多个纤维素酶分别与锚定蛋白融合后共展示在细胞表面,从而构建酵母表面展示纤维素酶体系。这一体系可高效降解纤维素,一方面可以充分发挥表面展示的优点,如易回收、稳定性好、操作简单、成本低;另一方面可以将纤维素有效地降解为葡萄糖,并具有代谢产生物乙醇的潜力。阐述了酵母表面展示体系的构建原则,总结了影响展示体系效率的因素,介绍了这一技术在降解纤维素中的应用,为构建高效酵母表面展示纤维素酶体系及其他多酶体系提供参考。     

Global Phosphorus Flows in the Industrial Economy From a Production Perspective

Human activity has quadrupled the mobilization of phosphorus (P), a nonrenewable resource that is not fully recycled biologically or industrially. P is accumulated in both water and solid waste due to fertilizer application and industrial, agricultural, and animal P consumption. This paper characterizes the industrial flows, which, although smaller than the agricultural and animal flows, are an important phosphorus source contributing to the pollution of surface waters. We present the quantification of the network of flows as constrained by mass balances of the global annual metabolism of phosphorus, based on global consumption for 2004, all of which eventually ends up as waste and in the soil and water systems. We find that on a yearly basis, 18.9 million metric tons (MMT) of P is produced, of which close to 75% goes to fertilizer and the rest to industrial and others uses. Phosphoric acid is the precursor for many of the intermediate and end uses of phosphate compounds described in this study and accounts for almost 80% of all P consumed. Eventually, all of the P goes to waste: 18.5 MMT ends up in the soil as solid waste, and 1.32 MMT is emissions to air and water. Besides quantifying P flows through our economy, we also consider some possible measures that could be taken to increase the degree of recovery and optimization of this resource and others that are closely related, such as the recovery of sulfur from gypsum and wastewater (sludge), and fluorine from wet phosphoric acid production.     

Recent progress in Bacillus subtilis spore-surface display: concept,progress, and future

With the increased knowledge on spore structure and advances in biotechnology engineering, the newly developed spore-surface display system confers several inherent advantages over other microbial cell-surface display systems including enhanced stability and high safety. Bacillus subtilis is the most commonly used Bacillus species for spore-surface display. The expression of heterologous antigen or protein on the surface of B. subtilis spores has now been practiced for over a decade with noteworthy success. As an update and supplement to other previous reviews, we comprehensively summarize recent studies in the B. subtilis spore-surface display technique. We focus on its benefits as well as the critical factors affecting its display efficiency and offer suggestions for the future success of this field.

     

Cell-Surface display of heterologous proteins: From high-throughput screening to environmental applications

A variety of expression systems for the display of either short peptides or fully folded proteins on E.coli and, to a lesser extent, on Gram-positive bacteria have been developed. The expression of proteins on the surface of microbial cells has proved extremely important for numerous applications ranging from combinatorial library screening and protein engineering, to whole cell biocatalysts and adsorbants for bioremediation purposes.     

Microbial valorization of underutilized and nonconventional waste streams

The growing burden of waste disposal coupled with natural resource scarcity has renewed interest in the remediation, valorization, and/or repurposing of waste. Traditional approaches such as composting, anaerobic digestion, use in fertilizers or animal feed, or incineration for energy production extract very little value out of these waste streams. In contrast, waste valorization into fuels and other biochemicals via microbial fermentation is an area of growing interest. In this review, we discuss microbial valorization of nonconventional, aqueous waste streams such as food processing effluents, wastewater streams, and other industrial wastes. We categorize these waste streams as carbohydrate-rich food wastes, lipid-rich wastes, and other industrial wastes. Recent advances in microbial valorization of these nonconventional waste streams are highlighted, along with a discussion of the specific challenges and opportunities associated with impurities, nitrogen content, toxicity, and low productivity.     

Display of proteins on bacteria

Display of heterologous proteins on the surface of microorganisms, enabled by means of recombinant DNA technology, has become an increasingly used strategy in various applications in microbiology, biotechnology and vaccinology. Gram-negative, Gram-positive bacteria, viruses and phages are all being investigated in such applications. This review will focus on the bacterial display systems and applications. Live bacterial vaccine delivery vehicles are being developed through the surface display of foreign antigens on the bacterial surfaces. In this field, 'second generation' vaccine delivery vehicles are at present being generated by the addition of mucosal targeting signals, through co-display of adhesins, in order to achieve targeting of the live bacteria to immunoreactive sites to thereby increase immune responses. Engineered bacteria are further being evaluated as novel microbial biocatalysts with heterologous enzymes immobilized as surface exposed on the bacterial cell surface. A discussion has started whether bacteria can find use as new types of whole-cell diagnostic devices since single-chain antibodies and other type of tailor-made binding proteins can be displayed on bacteria. Bacteria with increased binding capacity for certain metal ions can be created and potential environmental or biosensor applications for such recombinant bacteria as biosorbents are being discussed. Certain bacteria have also been employed for display of various poly-peptide libraries for use as devices in in vitro selection applications. Through various selection principles, individual clones with desired properties can be selected from such libraries. This article explains the basic principles of the different bacterial display systems, and discusses current uses and possible future trends of these emerging technologies.     

Versatile microbial surface-display for environmental remediation and biofuels production

Surface display is a powerful technique that uses natural microbial functional components to express proteins or peptides on the cell exterior. Since the reporting of the first surface-display system in the mid-1980s, a variety of new systems have been reported for yeast, Gram-positive and Gram-negative bacteria. Non-conventional display methods are emerging, eliminating the generation of genetically modified microorganisms. Cells with surface display are used as biocatalysts, biosorbents and biostimulants. Microbial cell-surface display has proven to be extremely important for numerous applications, ranging from combinatorial library screening and protein engineering to bioremediation and biofuels production.     

微生物表面展示技术及其在环境污染治理中的应用

微生物表面展示技术是通过基因工程手段,将短的外源肽或蛋白质表达在微生物细胞表面,该技术可以应用于开发活的细菌疫苗、筛选抗体库、生产生物细胞吸附剂以及制备整细胞生物催化剂。通过金属高效结合肽的肽库筛选和微生物展示技术,将金属结合肽直接展示在微生物的表面,用于处理环境中的重金属污染,为环境中重金属污染的防治提供了一条崭新的途径。利用微生物表面展示技术制备整细胞催化剂,用于有毒有机污染物的处理,可以极大地加快污染物的降解速率。简要介绍了微生物表面展示技术及其在重金属污染治理和毒性有机污染物的脱毒等环境生物修复方面的最新研究进展。     

Let's Be Clear(er) about Substitution: A Reporting Framework to Account for Product Displacement in Life Cycle Assessment

The multifunctional character of resource recovery in waste management systems is commonly addressed through system expansion/substitution in life cycle assessment (LCA). Avoided burdens credited based on expected displacement of other product systems can dominate the overall results, making the underlying assumptions particularly important for the interpretation and recommendations. Substitution modeling, however, is often poorly motivated or inadequately described, which limits the utility and comparability of such LCA studies. The aim of this study is therefore to provide a structure for the systematic reporting of information and assumptions expected to contribute to the substitution potential in order to make substitution modeling and the results thereof more transparent and interpretable. We propose a reporting framework that can also support the systematic estimation of substitution potentials related to resource recovery. Key components of the framework include waste‐specific (physical) resource potential, recovery efficiency, and displacement rate. End‐use–specific displacement rates can be derived as the product of the relative functionality (substitutability) of the recovered resources compared to potentially displaced products and the expected change in consumption of competing products. Substitutability can be determined based on technical functionality and can include additional constraints. The case of anaerobic digestion of organic household waste illustrates its application. The proposed framework enables well‐motivated substitution potentials to be accounted for, regardless of the chosen approach, and improves the reproducibility of comparative LCA studies of resource recovery.     

Non-enzymatic palladium recovery on microbial and synthetic surfaces

The use of microorganisms as support for reduction of dissolved Pd(II) to immobilized Pd(0) nanoparticles is an environmentally friendly approach for Pd recovery from waste. To better understand and engineer Pd(0) nanoparticle synthesis, one has to consider the mechanisms by which Pd(II) is reduced on microbial surfaces. Escherichia coli, Shewanella oneidensis, and Pseudomonas putida were used as model organisms in order to elucidate the role of microbial cells in Pd(II) reduction under acidic conditions. Pd(II) was reduced by formate under acidic conditions, and the process occurred substantially faster in the presence of cells as compared to cell-free controls. We found no difference between native (untreated) and autoclaved cells, and could demonstrate that even a non-enzymatic protein (bovine serum albumin) stimulated Pd(II) reduction as efficiently as bacterial cells. Amine groups readily interact with Pd(II), and to specifically test their role in surface-assisted Pd(II) reduction by formate, we replaced bacterial cells with polystyrene microparticles functionalized with amine or carboxyl groups. Amine-functionalized microparticles had the same effect on Pd(II) reduction as bacterial cells, and the effect could be hampered if the amine groups were blocked by acetylation. The interaction with amine groups was confirmed by infrared spectroscopy on whole cells and amine-functionalized microparticles. In conclusion, bio-supported Pd(II) reduction on microbial surfaces is possibly mediated by a non-enzymatic mechanism. We therefore suggest the use of amine-rich biomaterials rather than intact cells for Pd bio-recovery from waste.     

Microbial selective plugging and enhanced oil recovery

Summary The ability of indigenous populations of microorganisms in Berea sandstone to improve the volumetric sweep efficiency and increase oil recovery by in situ growth and metabolism following the injection of nutrients was studied. Cores of differing permeabilities connected in parallel without crossflow and slabs of sandstone with differing permeabilities in capillary contact to allow crossflow were used. The addition of a sucrosenitrate mineral salts medium stimulated the growth and metabolism of microorganisms in the sandstone systems. This resulted in a preferential decrease in permeability in the core or slab with the higher initial permeability, diverted flow into the lower-permeability core or slab and improved the volumetric sweep efficiency. Injectivity into the slab with the lower initial permeability in the crossflow system increased during subsequent nutrient injections. Thus, microbial selective plugging does occur in laboratory systems that have the complex flow patterns observed in petroleum reservoirs without losing the ability to inject fluids into the formation. In situ microbial growth and metabolism increased oil recovery 10 to 38% of the original oil in place. Biogenic gas production accompanied oil production, and much of the gas was entrained within the produced oil suggesting that gas production was an important factor leading to increased oil recovery. Quantitation of the amount of phospholipid in the core confirmed that microbial growth preferentially occurred throughout the core with the higher initial permeability. These data showed that in situ microbial growth in the high-permeability regions improved not only the volumetric sweep efficiency but also the microscopic oil displacement efficiency.     

Improving the ‘tool box’ for robust industrial enzymes

The speed of sequencing of microbial genomes and metagenomes is providing an ever increasing resource for the identification of new robust biocatalysts with industrial applications for many different aspects of industrial biotechnology. Using ‘natures catalysts’ provides a sustainable approach to chemical synthesis of fine chemicals, general chemicals such as surfactants and new consumer-based materials such as biodegradable plastics. This provides a sustainable and ‘green chemistry’ route to chemical synthesis which generates no toxic waste and is environmentally friendly. In addition, enzymes can play important roles in other applications such as carbon dioxide capture, breakdown of food and other waste streams to provide a route to the concept of a ‘circular economy’ where nothing is wasted. The use of improved bioinformatic approaches and the development of new rapid enzyme activity screening methodology can provide an endless resource for new robust industrial biocatalysts.This mini-review will discuss several recent case studies where industrial enzymes of ‘high priority’ have been identified and characterised. It will highlight specific hydrolase enzymes and recent case studies which have been carried out within our group in Exeter.