Microbial Protein out of Thin Air: Fixation of Nitrogen Gas by an Autotrophic Hydrogen-Oxidizing Bacterial Enrichment

Dinitrogen fixation by open purple non-sulfur bacteria cultures for protein production: Diazotrophy boosts photoheterotrophic uptake rates

Purple non-sulfur bacteria (PNSB) offer a sustainable alternative to current inefficient protein production systems thanks to their high yields. This study explored the potential of specialized diazotrophic PNSB open cultures for protein production, benchmarking their performance against ammonium-grown PNSB and other diazotrophs. While diazotrophic yields (0.85-0.93 gCODbiomass·gCODsubstrate-1; COD being chemical oxygen demand) were slightly lower than non-diazotrophic (∼1.0), they were over double those of heterotrophic-diazotrophic rhizobacteria, with full N recovery as biomass (∼1.0 gNbiomass·gNfixed-1). Photoheterotrophic-diazotrophic uptake rates were the fastest ever reported for PNSB and any other diazotroph (e.g., 5.20 ± 0.83 vs. 2.64 ± 0.34 gCODsubstrate·gCODbiomass-1·d-1 for PNSB on NH4+). Optimal rates required high light intensities, aligning with diazotrophic energy demands. Photoheterotrophic-diazotrophic conditions were highly selective, enriching a specialized Rhodopseudomonas palustris strain. Biomass protein contents and essential amino acid metrics confirmed nutritional suitability for humans. This work lays the background for exploiting PNSB's potential to address global protein demands through sustainable nitrogen fixation.

Carbon and ammonium limitation and nitrogen fixation alter the nutritional profile of purple non-sulphur bacteria grown on H2 and CO2

Purple non-sulphur bacteria biotechnology represents a promising microbial platform to support the protein transition towards more sustainable food systems, particularly when cultivated photoautohydrogenotrophically using carbon dioxide and green hydrogen. This study explored how carbon and/or nitrogen limitations, as well as biological nitrogen fixation can steer the nutritional composition and growth kinetics of purple non-sulphur bacteria. The essential amino acids fraction remained stable at ∼41 %. Under carbon limitation, carbon-rich isoleucine decreased (-27 %) while nitrogen-rich histidine increased (+18 %), and phosphorus content rose by 40 %. Carbon resp. nitrogen limitation reduced bacteriochlorophyll a and carotenoids by 52 % resp. 34 %. Biological nitrogen fixation raised phenylalanine and tyrosine content by 39 %, but reduced the maximum growth rate 2.8-fold; however, final biomass levels were comparable to NH4+-based cultures. These findings highlight the nutritional tunability and metabolic versatility of purple non-sulphur bacteria, reinforcing their potential as a circular, low-carbon platform for sustainable microbial protein production.

Enrichment of nitrogen-fixing hydrogen-oxidizing bacteria community for efficient microbial protein production in airlift reactor

Autotrophic nitrogen-fixing hydrogen-oxidizing bacteria (NF-HOB) have the capability to directly utilize carbon dioxide and dinitrogen for the sustainable production of microbial protein (MP), offering a promising alternative for food and feed applications. However, the low production rate of MP remains a major bottleneck for the practical implementation of NF-HOB. This study enriched a highly active and stable NF-HOB functional community for enhanced MP production. Utilizing an airlift reactor and implementing a two-stage gas supply strategy achieved a maximum cell dry weight (CDW) of 4.8 g/L and a biomass yield of 0.25 g CDW/L/day, surpassing previously reported values. The produced MP exhibited an essential amino acid profile superior to soybean meal and comparable to fish meal, with a notable accumulation of branched-chain amino acids (BCAAs). These findings provide new insights into NF-HOB enrichment strategies and further highlight their potential as a sustainable platform for MP production.

XanthoMoClo─A Robust Modular Cloning Genetic Toolkit for the Genera Xanthobacter and Roseixanthobacter

Interest in Xanthobacter species is increasing due to their unique metabolic capabilities. They can grow in both heterotrophic and fully autotrophic environments, including carbon dioxide, dinitrogen gas, and hydrogen as the sole carbon, nitrogen, and energy sources, respectively. Academic and industrial groups looking to leverage these metabolic properties are already using Xanthobacter strains for the sustainable production of food and commodities. However, only a handful of genetic parts and protocols exist in scattered genetic backgrounds, and there is an unmet need for reliable genetic engineering tools to manipulate Xanthobacter species. Here, we developed XanthoMoClo, a robust modular cloning genetic toolkit for Xanthobacter and Roseixanthobacter species and strains, providing extensive tools to transform them, manipulate their metabolism, and express genes of interest. The toolkit contains plasmid parts, such as replication origins, antibiotic selection markers, fluorescent proteins, constitutive and inducible promoters, a standardized framework to incorporate novel components into the toolkit, and a conjugation donor to transform Xanthobacter and Roseixanthobacter strains easily with no or minimal optimization. We validated these plasmid components in depth in three of the most commonly studied Xanthobacter strains: X. versatilis Py2, X. autotrophicus GZ29, and X. flavus GJ10, as well as in R. finlandensis VTT E-85241. Finally, we demonstrate robust toolkit functionality across 21 different species of Xanthobacter and Roseixanthobacter, comprising 23 strains in total. The XanthoMoClo genetic toolkit is available to the research community (through AddGene) and will help accelerate the genetic engineering of Xanthobacter to further their applications in sustainability and bioremediation efforts.

Foliar Application of Chlorella Supernatant Protects Turfgrass against Clarireedia jacksonii by Eliciting Induced Resistance and Modulating the Rhizosphere Microbiota

Large-scale culture of the microalga Chlorella produces valuable products. Cultivation also generates tons of supernatant waste that require detoxification and disposal. Recent research has focused on recycling waste supernatant as a plant protectant and biofertilizer, although, to date, most studies have considered its use as a biological control of pathogens infecting dicot plants. By contrast, the current study evaluated whether Chlorella supernatant could protect turfgrass (Agrostis stolonifera), a monocot plant widely used as a turfgrass, against dollar spot disease caused by the fungal pathogen Clarireedia jacksonii (formerly Sclerotinia homoeocarpa) under greenhouse and field conditions. Foliar application of supernatants from Chlorella sp. ABC001 and HS2 cultures reduced the incidence of dollar spot disease in turfgrass under both greenhouse and field conditions without directly inhibiting growth. The effects of supernatant application on the rhizosphere microbiome were investigated using 16S rRNA amplicon sequencing. Application of ABC001 and HS2 supernatants modulated the structure of the rhizosphere microbiome and enriched specific microbial taxa that improved turfgrass health in the presence of C. jacksonii. The application of waste Chlorella supernatant therefore offers an alternative method for protecting monocot plants against fungal pathogens, while also enhancing the composition of soil microbes in the rhizosphere.

Industrial Microbial Technologies for Feed Protein Production from Non-Protein Nitrogen

Due to the increasing global demand for feed protein, microbial protein has great potential of being able to feed sustainably. However, the application of microbial protein in the animal cultivation industry is still limited by its high cost and availability on scale. From the viewpoint of industrial production, it is vital to specify the crucial processes and components for further technical exploration and process optimization. This article presents state-of-the-art industrial microbial technologies for non-protein nitrogen (NPN) assimilation in feed protein production. Nitrogen sources are one of the main cost factors in the media used for large-scale microbial protein fermentation. Therefore, the available NPN sources for microbial protein synthesis, NPN utilization mechanisms, and fermentation technologies corresponding to the strain and NPN are reviewed in this paper. Especially, the random mutagenesis and adaptive laboratory evolution (ALE) approach combined with (ultra-) throughput screening provided the main impetus for strain evolution to increase the protein yield. Despite the underlying potential and technological advances in the production of microbial protein, extensive research and development efforts are still required before large-scale commercial application of microbial protein in animal feed.

Emerging alternative food protein sources: production process, quality parameters, and safety point of view

The rise in the global population has increased the demand for dietary food protein. Strategies to maximize agricultural and livestock outputs could strain land and freshwater supply and contribute to substantial negative environmental impacts. Consequently, there has been an emphasis on identifying alternative sources of edible proteins that are more sustainable, sustainable, ethical, and healthy. This review provides a critical report on future food protein sources including: plant, cultured meat, insect, and microbial, as alternative sources to traditional animal-based sources. The technical challenges associated with the production process of alternative protein sources are discussed. The most important quality parameters of alternative proteins, such as: protein composition and digestibility, allergenicity, functional and sensory attributes, and safety regulations have been documented. Lastly, future direction and conclusion have been made on future protein trends. However, further regulatory norms need to develop for safe consumption and distribution around the world.

Creating a Genetic Toolbox for the Carbon-Fixing, Nitrogen-Fixing and Dehalogenating Bacterium Xanthobacter autotrophicus

Xanthobacter autotrophicus is a metabolically flexible microorganism with two key features: (1) The organism has adapted to grow on a wide variety of carbon sources including CO2, methanol, formate, propylene, haloalkanes and haloacids; and (2) X. autotrophicus was the first chemoautotroph identified that could also simultaneously fix N2, meaning the organism can utilize CO2, N2, and H2 for growth. This metabolic flexibility has enabled use of X. autotrophicus for gas fixation, the creation of fertilizers and foods from gases, and the dehalogenation of environmental contaminants. Despite the wide variety of applications that have already been demonstrated for this organism, there are few genetic tools available to explore and exploit its metabolism. Here, we report a genetic toolbox for use in X. autotrophicus. We first identified suitable origins of replication and quantified their copy number, and identified antibiotic resistance cassettes that could be used as selectable markers. We then tested several constitutive and inducible promoters and terminators and quantified their promoter strengths and termination efficiencies. Finally, we demonstrated that gene expression tools remain effective under both autotrophic and dehalogenative metabolic conditions to show that these tools can be used in the environments that make X. autotrophicus unique. Our extensive characterization of these tools in X. autotrophicus will enable genetic and metabolic engineering to optimize production of fertilizers and foods from gases, and enable bioremediation of halogenated environmental contaminants.

Sustainable production of microbial protein from carbon dioxide in the integrated bioelectrochemical system using recycled nitrogen sources

Given the urgency of climate change, it is imperative to develop innovative technologies for repurposing CO2 into value-added products to achieve carbon neutrality. Additionally, repurposing nitrogen-source-derived wastewater streams is crucial, focusing on sustainability rather than conventional nitrogen removal in wastewater treatment plants. In this context, microbial protein (MP) production presents a sustainable and promising approach for transforming recovered low-value resources into high-quality feed and food. We assessed MP production by hydrogen-oxidizing bacteria (HOB) utilizing CO2 and various nitrogen sources. Specifically, we investigated MP production by two different HOB strains, Cupriavidus necator H16 and Xanthobacter viscosus 7d, within an integrated water-splitting biosynthetic system that generates in situ H2 via water electrolysis. The electroautotrophically produced MPs of C. necator H16 and X. viscosus 7d exhibited amino acid contents of 555 and 717 mg protein/g cell dry weight, with 243 and 299 mg essential amino acid/g cell dry weight, respectively. They could serve as viable alternatives to conventional food/feed sources like fishmeal or soybean protein. Ammonium-rich wastewater streams are preferable for MP production in integrated bioelectrochemical systems. This study provides valuable insights into sustainable, carbon-neutral MP production using CO2, water, renewable electricity, and recycled nitrogen sources.

Upcycling of nutrients from kitchen waste: Integration of anaerobic digestion system and microbial protein production system

To upcycle the nutrients from kitchen waste (KW), an integrated system consisting of anaerobic digestion (AD) reactor and microbial protein (MP) production reactor was established in this study. The subsystem I (AD system) demonstrated an efficient bio-energy production (282.37 mL CH4/g VS), with 553.54 mg/L of NH4+-N remained in the digestate. The subsystem II (MP production system) utilized the nitrogenous constituents of the digestate, with 2.04 g/L MP production. In order to further enhance the recovery efficiency, C/N ratio in the subsystem II was studied. NH4+-N recovery efficiency was 23.08% higher after C/N ratio optimization along with 0.24 g/L increment on MP production. Over 0.7 g/L of essential amino acids was obtained, according with the qualitative necessary for the feeds. Also, the key enzyme abundance of CO2 releasing and amino acid biosynthesis was obviously increased with max. 55.21%. Meanwhile, the integrated system was profitable via a simplified economic assessment.

Enhanced Microbial Protein Production from CO2 and Air by a MoS2 Catalyzed Bioelectrochemical System

Carbon dioxide can be relatively easily reduced to organic matter in a bioelectrochemical system (BES). However, due to insufficient reduction force from in-situ hydrogen evolution, it is difficult for nitrogen reduction. In this study, MoS2 was firstly used as an electrocatalyst for the simultaneous reduction of CO2 and N2 to produce microbial protein (MP) in a BES. Cell dry weight (CDW) could reach 0.81±0.04 g/L after 14 d operation at -0.7 V (vs. RHE), which was 108±3 % higher than that from non-catalyst control group (0.39±0.01 g/L). The produced protein had a better amino acid profile in the BES than that in a direct hydrogen system (DHS), particularly for proline (Pro). Besides, MoS2 promoted the growth of bacterial cell on an electrode and improved the biofilm extracellular electron transfer (EET) by microscopic observation and electrochemical characterization of MoS2 biocathode. The composition of the microbial community and the relative abundance of functional enzymes revealed that MoS2 as an electrocatalyst was beneficial for enriching Xanthobacter and enhancing CO2 and N2 reduction by electrical energy. These results demonstrated that an efficient strategy to improve MP production of BES is to use MoS2 as an electrocatalyst to shift amino acid profile and microbial community.

From sustainable feedstocks to microbial foods

Climate change-induced alterations in weather patterns, such as frequent and severe heatwaves, cold waves, droughts, floods, heavy rain and storms, are reducing crop yields and agricultural productivity. At the same time, greenhouse gases arising from food production and supply account for almost 30% of anthropogenic emissions. This vicious circle is producing a global food crisis. Sustainable food resources and production systems are needed now, and microbial foods are one possible solution. In this Perspective, we highlight the most promising technologies, and carbon and energy sources, for microbial food production.

A comprehensive report on valorization of waste to single cell protein: strategies, challenges, and future prospects

The food insecurity due to a vertical increase in the global population urgently demands substantial advancements in the agricultural sector and to identify sustainable affordable sources of nutrition, particularly proteins. Single-cell protein (SCP) has been revealed as the dried biomass of microorganisms such as algae, yeast, and bacteria cultivated in a controlled environment. Production of SCP is a promising alternative to conventional protein sources like soy and meat, due to quicker production, minimal land requirement, and flexibility to various climatic conditions. In addition to protein production, it also contributes to waste management by converting it into food and feed for both human and animal consumption. This article provides an overview of SCP production, including its benefits, safety, acceptability, and cost, as well as limitations that constrains its maximum use. Furthermore, this review criticizes the downstream processing of SCP, encompassing cell wall disruption, removal of nucleic acid, harvesting of biomass, drying, packaging, storage, and transportation. The potential applications of SCP, such as in food and feed as well as in the production of bioplastics, emulsifiers, and as flavoring agents for baked food, soup, and salad, are also discussed.

Effects of nitrogen supply on hydrogen-oxidizing bacterial enrichment to produce microbial protein: Comparing nitrogen fixation and ammonium assimilation

To investigate the effects of nitrogen source supply on microbial protein (MP) production by hydrogen-oxidizing bacteria (HOB) under continuous feed gas provision, a sequencing batch culture comparison (N2 fixation versus ammonium assimilation) was performed. The results confirmed that even under basic cultivation conditions, N2-fixing HOB (NF-HOB) communities showed higher levels of CO2 and N2 fixation (190.45 mg/L Δ CODt and 11.75 mg/L Δ TNbiomass) than previously known, with the highest biomass yield being 0.153 g CDW/g COD-H2. Rich ammonium stimulated MP synthesis and the biomass accumulation of communities (increased by 7.4 ～ 14.3 times), presumably through the enhancement of H2 and CO2 absorption. The micro mechanism may involve encouraging the enrichment of species like Xanthobacter and Acinetobacter then raising the abundance of nitrogenase and glutamate synthase to facilitate the nitrogen assimilation. This would provide NF-HOB with ideas for optimizing their MP synthesis activity.

Potential of enriched phototrophic purple bacteria for H2 bioconversion into single cell protein

Single cell protein (SCP) has emerged as an alternative protein source, potentially based on the recovery of carbon and nutrients from waste-derived resources as part of the circular economy. From those resources, gaseous substrates have the advantage of an easy sterilization, allowing the production of pathogen-free SCP. Sterile gaseous substrates allow producing pathogen-free SCP. This study evaluated the use of an enriched phototrophic purple bacteria (PPB) consortium for SCP production using H2 and CO2 as electron and C sources. The influence of pH (6.0-8.5), temperature (15-50 °C) and light intensity (0-50 W·m-2) on the growth kinetics and biomass yields was investigated using batch tests. Optimal conditions were found at pH 7, 25 °C and light intensities over 30 W·m-2. High biomass and protein yields were achieved (~ 1 g CODbiomass·g CODH2consumed-1 and 3.9-4.4 g protein·g H2-1) regardless of the environmental conditions, being amongst the highest values reported from gaseous streams. These high yields were obtained thanks to the use of light as a sole energy source by the PPB consortium, allowing a total utilization of H2 for growth. Hydrogen uptake rates varied considerably, with values up to 61 ± 5 mg COD·d-1 for the overall H2 consumption rates and 2.00 ± 0.14 g COD·g COD-1·d-1 for the maximum specific uptake rates under optimal growth conditions. The latter value was estimated using a mechanistic model able to represent PPB growth on H2. The biomass exhibited high protein contents (>50 % w/w) and adequate amino acid profiles, showing its suitability as SCP for feed. PPB were the dominant bacteria during the experiments (relative abundance over 80 % in most tests), with a stable population dominated by Rhodobacter sp. and Rhodopseudomonas sp. This study demonstrates the potential of enriched PPB cultures for H2 bioconversion into SCP.

Microbial gas fermentation technology for sustainable food protein production

The development of novel, sustainable, and robust food production technologies represents one of the major pillars to address the most significant challenges humanity is going to face on earth in the upcoming decades - climate change, population growth, and resource depletion. The implementation of microfoods, i.e., foods formulated with ingredients from microbial cultivation, into the food supply chain has a huge potential to contribute towards energy-efficient and nutritious food manufacturing and represents a means to sustainably feed a growing world population. This review recapitulates and assesses the current state in the establishment and usage of gas fermenting bacteria as an innovative feedstock for protein production. In particular, we focus on the most promising representatives of this taxon: the hydrogen-oxidizing bacteria (hydrogenotrophs) and the methane-oxidizing bacteria (methanotrophs). These unicellular microorganisms can aerobically metabolize gaseous hydrogen and methane, respectively, to provide the required energy for building up cell material. A protein yield over 70% in the dry matter cell mass can be reached with no need for arable land and organic substrates making it a promising alternative to plant- and animal-based protein sources. We illuminate the holistic approach to incorporate protein extracts obtained from the cultivation of gas fermenting bacteria into microfoods. Herein, the fundamental properties of the bacteria, cultivation methods, downstream processing, and potential food applications are discussed. Moreover, this review covers existing and future challenges as well as sustainability aspects associated with the production of microbial protein through gas fermentation.

Enrichment of hydrogen-oxidizing bacteria using a hybrid biological-inorganic system

Hybrid biological-inorganic (HBI) systems comprising inorganic water-splitting catalysts and aerobic hydrogen-oxidizing bacteria (HOB) have previously been used for CO₂ conversion. In order to identify new biocatalysts for CO₂ conversion, the present study used an HBI system to enrich HOB directly from environmental samples. Three sediment samples (from a brackish water pond, a beach, and a tide pool) and two activated sludge samples (from two separate sewage plants) were inoculated into HBI systems using a cobalt phosphorus (Co-P) alloy and cobalt phosphate (CoPi) as inorganic catalysts with a fixed voltage of 2.0 V. The gas composition of the reactor headspaces and electric current were monitored. An aliquot of the reactor medium was transferred to a new reactor when significant consumption of H₂ and CO₂ was detected. This process was repeated twice (with three reactors in operation for each sample) to enrich HOB. Increased biomass concomitant with increased H₂ and CO₂ consumption was observed in the third reactor, indicating enrichment of HOB. 16S rRNA gene amplicon sequencing demonstrated enrichment of sequences related to HOB (including bacteria from Mycobacterium, Hydrogenophaga, and Xanthobacter genera) over successive sub-cultures. Finally, four different HOB belonging to the Mycobacterium, Hydrogenophaga, Xanthobacter, and Acidovorax genera were isolated from reactor media, representing potential candidates as HBI system biocatalysts.

Biotechnological Production of Sustainable Microbial Proteins from Agro-Industrial Residues and By-Products

Microbial proteins, i.e., single-cell proteins or microbial biomass, can be cultivated for food and animal feed due to their high protein content and the fact that they represent a rich source of carbohydrates, minerals, fats, vitamins, and amino acids. Another advantage of single-cell proteins is their rapid production due to the growth rate of microorganisms and the possibility of using agro-industrial waste, residues and by-products for production through this renewable technology. Agro-industrial residues and by-products represent materials obtained from various processes in agriculture and agriculture-related industries; taking into account their composition and characteristics, as well as vast amounts, they have an enormous potential to generate sustainable bioproducts, such as microbial proteins. This review aims to summarize contemporary scientific research related to the production of microbial proteins on various agro-industrial residues and by-products, as well as to emphasize the current state of production of single-cell proteins and the importance of their production to ease the food crisis and support sustainable development.

Hydrogen Oxidizing Bacteria as Novel Protein Source for Human Consumption: An Overview

The increasing threat of climate change combined with the prospected growth in the world population puts an enormous pressure on the future demand for sustainable protein sources for human consumption. In this review, hydrogen oxidizing bacteria (HOB) are presented as a novel protein source that could play a role in fulfilling this future demand. HOB are species of bacteria that merely require an inflow of the gasses hydrogen, oxygen, carbon dioxide, and a nitrogen source to grow in a conventional bioreactor. Cupriavidus necator is proposed as HOB for industrial cultivation due to its remarkably high protein content (up to 70% of mass), suitability for cultivation in a bioreactor, and the vast amount of available background information. A broad overview of the unique aspects of the bacteria will be provided, from the production process, amino acid composition, and source of the required gasses to the future acceptance of HOB into the market.

Riboflavin synthesis from gaseous nitrogen and carbon dioxide by a hybrid inorganic-biological system

Microbes can provide a more sustainable and energy-efficient method of food and nutrient production compared to plant and animal sources, but energy-intensive carbon (e.g., sugars) and nitrogen (e.g., ammonia) inputs are required. Gas-fixing microorganisms that can grow on H₂ from renewable water splitting and gaseous CO₂ and N₂ offer a renewable path to overcoming these limitations but confront challenges owing to the scarcity of genetic engineering in such organisms. Here, we demonstrate that the hydrogen-oxidizing carbon- and nitrogen-fixing microorganism Xanthobacter autotrophicus grown on a CO₂/N₂/H₂ gas mixture can overproduce the vitamin riboflavin (vitamin B₂). We identify plasmids and promoters for use in this bacterium and employ a constitutive promoter to overexpress riboflavin pathway enzymes. Riboflavin production is quantified at 15 times that of the wild-type organism. We demonstrate that riboflavin overproduction is maintained when the bacterium is grown under hybrid inorganic-biological conditions, in which H₂ from water splitting, along with CO₂ and N₂, is fed to the bacterium, establishing the viability of the approach to sustainably produce food and nutrients.

Hydrogen-oxidizing bacteria and their applications in resource recovery and pollutant removal

Hydrogen oxidizing bacteria (HOB), a type of chemoautotroph, are a group of bacteria from different genera that share the ability to oxidize H₂ and fix CO₂ to provide energy and synthesize cellular material. Recently, HOB have received growing attention due to their potential for CO₂ capture and waste recovery. This review provides a comprehensive overview of the biological characteristics of HOB and their application in resource recovery and pollutant removal. Firstly, the enzymes, genes and corresponding regulation systems responsible for the key metabolic processes of HOB are discussed in detail. Then, the enrichment and cultivation methods including the coupled water splitting-biosynthetic system cultivation, mixed cultivation and two-stage cultivation strategies for HOB are summarized, which is the critical prerequisite for their application. On the basis, recent advances of HOB application in the recovery of high-value products and the removal of pollutants are presented. Finally, the key points for future investigation are proposed that more attention should be paid to the main limitations in the large-scale industrial application of HOB, including the mass transfer rate of the gases, the safety of the production processes and products, and the commercial value of the products.

Fermentation liquid as a carbon source for wastewater nitrogen removal reduced nitrogenous disinfection byproduct formation potentials of the effluent

Sludge alkaline fermentation liquid (SAFL) is an alternative to sodium acetate (NaAc) in enhancing wastewater nitrogen removal. Upon SAFL addition, dissolved organic nitrogen (DON) can be externally introduced or biologically synthesized during nitrogen removal, which is an important precursor to toxic nitrogenous disinfection by-products (N-DBPs). This study aims to evaluate the effects of different carbon source addition on effluent DON concentration, composition, and N-DBP formation potentials. A lab-scale A²O system treating real municipal wastewater was operated with NaAc or SAFL as external carbon sources. DON molecules and potential N-DBP precursors were identified by Orbitrap mass spectrometry. Subsequently, major microorganisms contributing to DON biosynthesis were suggested based on metagenomics. It was found that effluent DON was higher with SAFL as the carbon source than NaAc (1.51 ± 0.24 v.s. 0.56 ± 0.08 mg N/L, p < 0.05). Nevertheless, dichloroacetonitrile and nitrosamine formation potentials (7.14 ± 1.02 and 1.57 ± 0.07 μg/mg DON-N, respectively) of the effluent with SAFL addition were 42.79 ± 2.42% and 54.89 ± 1.70% lower than those of NaAc. Protein- and lignin-like compounds were the most abundant DON molecules in the effluent, where alanine, glycine and tyrosine were important precursors to N-DBPs. Azonexus and Flavobacterium spp. were positively correlated with these precursors, and possessed key genes involved in precursor synthesis. SAFL is a promising carbon source, not only for achieving efficient inorganic nitrogen and DON removals, but also for reducing N-DBP formation potentials of chlorinated effluent.

Improving electroautotrophic ammonium production from nitrogen gas by simultaneous carbon dioxide fixation in a dual-chamber microbial electrolysis cell

Microbial electrosynthesis is a promising technology for high-value added products generation from organic and inorganic waste. In this work, autotrophic dual-chamber microbial electrolysis cells (MECs) were set up for N2 fixation at -0.9 V vs Ag/AgCl (sat. KCl) cathodic potential under ambient conditions. Higher NH4+ production yield (average value of 0.35 µmol h-1 cm-2, normalized to cathode surface area) and higher faradaic efficiency (FE, 20.25%) were obtained with intermittent addition of N2 and CO2, while the yield and FE were only 0.018 µmol h-1 cm-2 and 4.21% in the absence of CO2. Furthermore, cyclic voltammograms (CV) explained the bioelectrochemical behavior of N2 reduction was coupled with CO2 reduction in the autotrophic MECs. Microbial community analysis and functional prediction in the cathodic chamber revealed that Xanthobacter and Hydrogenophaga played as producers for N2 and CO2 fixation and Pannonibacter acting as a decomposer for converting organic nitrogen to ammonium. This work not only provided an optional bioelectrocatalytic method for N2 fixation with negative CO2-emissions but also revealed the mechanism of simultaneous fixation of N2 and CO2 via Calvin cycle in autotrophic MECs.

Thermodynamic Constraints on Electromicrobial Protein Production

Global consumption of protein is projected to double by the middle of the 21st century. However, protein production is one of the most energy intensive and environmentally damaging parts of the food supply system today. Electromicrobial production technologies that combine renewable electricity and CO2-fixing microbial metabolism could dramatically increase the energy efficiency of commodity chemical production. Here we present a molecular-scale model that sets an upper limit on the performance of any organism performing electromicrobial protein production. We show that engineered microbes that fix CO2 and N2 using reducing equivalents produced by H2-oxidation or extracellular electron uptake could produce amino acids with energy inputs as low as 64 MJ kg-1, approximately one order of magnitude higher than any previous estimate of the efficiency of electromicrobial protein production. This work provides a roadmap for development of engineered microbes that could significantly expand access to proteins produced with a low environmental footprint.

The role of single cell protein in cellular agriculture

More food needs to be produced for the growing human population, but the possibilities of expanding the area of arable land are limited. Cellular Agriculture is an emerging field of biotechnology, aimed at finding alternatives to agricultural production of various commodities. As a part of Cellular Agriculture, the use of microbes and microalgae as food and feed with high protein content, so-called single cell protein (SCP), is gaining renewed scientific and commercial interest. In this review, we give an introduction to SCP production by heterotrophic microbial species, phototrophs, methanotrophs and autotrophic hydrogen oxidizers, as well as highlight some challenges and the latest developments in the growing SCP industry.

Sustainable treatment of nitrate-containing wastewater by an autotrophic hydrogen-oxidizing bacterium

Bacteria are key denitrifiers in the reduction of nitrate (NO₃ ^--N), which is a contaminant in wastewater treatment plants (WWTPs). They can also produce carbon dioxide (CO₂) and nitrous oxide (N₂O). In this study, the autotrophic hydrogen-oxidizing bacterium Rhodoblastus sp. TH20 was isolated for sustainable treatment of NO₃ ^--N in wastewater. Efficient removal of NO₃ ^--N and recovery of biomass nitrogen were achieved. Up to 99% of NO₃ ^--N was removed without accumulation of nitrite and N₂O, consuming CO₂ of 3.25 mol for each mole of NO₃ ^--N removed. The overall removal rate of NO₃ ^--N reached 1.1 mg L^-1 h^-1 with a biomass content of approximately 0.71 g L^-1 within 72 h. TH20 participated in NO₃ ^--N assimilation and aerobic denitrification. Results from ¹⁵N-labeled-nitrate test indicated that removed NO₃ ^--N was assimilated into organic nitrogen, showing an assimilation efficiency of 58%. Seventeen amino acids were detected, accounting for 43% of the biomass. Nitrogen loss through aerobic denitrification was only approximately 42% of total nitrogen. This study suggests that TH20 can be applied in WWTP facilities for water purification and production of valuable biomass to mitigate CO₂ and N₂O emissions.

Effective orthophosphate removal from surface water using hydrogen-oxidizing bacteria: Moving towards applicability

Effective orthophosphate removal strategies are needed to counteract eutrophication and guarantee water quality. Previously, we established that hydrogen-oxidizing bacteria (HOB) have the ability to remove orthophosphate from artificial surface water. In the present study, we expand the application of the HOB orthophosphate removal strategy (1) to treat artificial surface water with low initial orthophosphate concentrations, (2) to treat real surface water and real wastewater effluent, and (3) to remove orthophosphate continuously. For synthetic surface water, irrespective of the initial concentration of 0.7, 0.5, 0.3, and 0.1 mg PO₄^3--P/L, ultra-low concentrations (0.0058 ± 0.0028 mg PO₄^3--P/L) were obtained. When artificial surface water was replaced by real surface water, without added nutrients or other chemicals, it was shown that over 90% orthophosphate could be removed within 30 min of operation in a batch configuration (0.031 ± 0.023 mg PO₄^3--P/L). In continuous operation, orthophosphate removal from surface water left an average concentration of 0.040 ± 0.036 for 60 days, and the lowest orthophosphate concentration measured was 0.013 mg PO₄^3-/L. Simultaneously, nitrate was continuously removed for 60 days below 0.1 mg/L. The ability to remove orthophosphate even under nitrogen limiting conditions might be related to the ability of HOB to fix nitrogen. This study brings valuable insights into the potential use of HOB biofilms for nutrient remediation and recovery.

Carbon myopia: The urgent need for integrated social, economic and environmental action in the livestock sector

Livestock have long been integral to food production systems, often not by choice but by need. While our knowledge of livestock greenhouse gas (GHG) emissions mitigation has evolved, the prevailing focus has been-somewhat myopically-on technology applications associated with mitigation. Here, we (1) examine the global distribution of livestock GHG emissions, (2) explore social, economic and environmental co-benefits and trade-offs associated with mitigation interventions and (3) critique approaches for quantifying GHG emissions. This review uncovered many insights. First, while GHG emissions from ruminant livestock are greatest in low- and middle-income countries (LMIC; globally, 66% of emissions are produced by Latin America and the Caribbean, East and southeast Asia and south Asia), the majority of mitigation strategies are designed for developed countries. This serious concern is heightened by the fact that 80% of growth in global meat production over the next decade will occur in LMIC. Second, few studies concurrently assess social, economic and environmental aspects of mitigation. Of the 54 interventions reviewed, only 16 had triple-bottom line benefit with medium-high mitigation potential. Third, while efforts designed to stimulate the adoption of strategies allowing both emissions reduction (ER) and carbon sequestration (CS) would achieve the greatest net emissions mitigation, CS measures have greater potential mitigation and co-benefits. The scientific community must shift attention away from the prevailing myopic lens on carbon, towards more holistic, systems-based, multi-metric approaches that carefully consider the raison d'être for livestock systems. Consequential life cycle assessments and systems-aligned 'socio-economic planetary boundaries' offer useful starting points that may uncover leverage points and cross-scale emergent properties. The derivation of harmonized, globally reconciled sustainability metrics requires iterative dialogue between stakeholders at all levels. Greater emphasis on the simultaneous characterization of multiple sustainability dimensions would help avoid situations where progress made in one area causes maladaptive outcomes in other areas.

Waste-to-nutrition: a review of current and emerging conversion pathways

Residual biomass is acknowledged as a key sustainable feedstock for the transition towards circular and low fossil carbon economies to supply whether energy, chemical, material and food products or services. The latter is receiving increasing attention, in particular in the perspective of decoupling nutrition from arable land demand. In order to provide a comprehensive overview of the technical possibilities to convert residual biomasses into edible ingredients, we reviewed over 950 scientific and industrial records documenting existing and emerging waste-to-nutrition pathways, involving over 150 different feedstocks here grouped under 10 umbrella categories: (i) wood-related residual biomass, (ii) primary crop residues, (iii) manure, (iv) food waste, (v) sludge and wastewater, (vi) green residual biomass, (vii) slaughterhouse by-products, (viii) agrifood co-products, (ix) C₁ gases and (x) others. The review includes a detailed description of these pathways, as well as the processes they involve. As a result, we proposed four generic building blocks to systematize waste-to-nutrition conversion sequence patterns, namely enhancement, cracking, extraction and bioconversion. We further introduce a multidimensional representation of the biomasses suitability as potential as nutritional sources according to (i) their content in anti-nutritional compounds, (ii) their degree of structural complexity and (iii) their concentration of macro- and micronutrients. Finally, we suggest that the different pathways can be grouped into eight large families of approaches: (i) insect biorefinery, (ii) green biorefinery, (iii) lignocellulosic biorefinery, (iv) non-soluble protein recovery, (v) gas-intermediate biorefinery, (vi) liquid substrate alternative, (vii) solid-substrate fermentation and (viii) more-out-of-slaughterhouse by-products. The proposed framework aims to support future research in waste recovery and valorization within food systems, along with stimulating reflections on the improvement of resources' cascading use.

Microbial food from light, carbon dioxide and hydrogen gas: Kinetic, stoichiometric and nutritional potential of three purple bacteria

The urgency for a protein transition towards more sustainable solutions is one of the major societal challenges. Microbial protein is one of the alternative routes, in which land- and fossil-free production should be targeted. The photohydrogenotrophic growth of purple bacteria, which builds on the H₂- and CO₂-economy, is unexplored for its microbial protein potential. The three tested species (Rhodobacter capsulatus, Rhodobacter sphaeroides and Rhodopseudomonas palustris) obtained promising growth rates (2.3-2.7 d^-1 at 28°C) and protein productivities (0.09-0.12 g protein L^-1 d^-1), rendering them likely faster and more productive than microalgae. The achieved protein yields (2.6-2.9 g protein g^-1 H₂) transcended the ones of aerobic hydrogen oxidizing bacteria. Furthermore, all species provided full dietary protein matches for humans and their fatty acid content was dominated by vaccenic acid (82-86%). Given its kinetic and nutritional performance we recommend to consider Rhodobacter capsulatus as a high-potential sustainable source of microbial food.

From Biogas and Hydrogen to Microbial Protein Through Co-Cultivation of Methane and Hydrogen Oxidizing Bacteria

Increasing efforts are directed towards the development of sustainable alternative protein sources among which microbial protein (MP) is one of the most promising. Especially when waste streams are used as substrates, the case for MP could become environmentally favorable. The risks of using organic waste streams for MP production-the presence of pathogens or toxicants-can be mitigated by their anaerobic digestion and subsequent aerobic assimilation of the (filter-sterilized) biogas. Even though methane and hydrogen oxidizing bacteria (MOB and HOB) have been intensively studied for MP production, the potential benefits of their co-cultivation remain elusive. Here, we isolated a diverse group of novel HOB (that were capable of autotrophic metabolism), and co-cultured them with a defined set of MOB, which could be grown on a mixture of biogas and H2/O2. The combination of MOB and HOB, apart from the CH4 and CO2 contained in biogas, can also enable the valorization of the CO2 that results from the oxidation of methane by the MOB. Different MOB and HOB combinations were grown in serum vials to identify the best-performing ones. We observed synergistic effects on growth for several combinations, and in all combinations a co-culture consisting out of both HOB and MOB could be maintained during five days of cultivation. Relative to the axenic growth, five out of the ten co-cultures exhibited 1.1-3.8 times higher protein concentration and two combinations presented 2.4-6.1 times higher essential amino acid content. The MP produced in this study generally contained lower amounts of the essential amino acids histidine, lysine and threonine, compared to tofu and fishmeal. The most promising combination in terms of protein concentration and essential amino acid profile was Methyloparacoccus murrelli LMG 27482 with Cupriavidus necator LMG 1201. Microbial protein from M. murrelli and C. necator requires 27-67% less quantity than chicken, whole egg and tofu, while it only requires 15% more quantity than the amino acid-dense soybean to cover the needs of an average adult. In conclusion, while limitations still exist, the co-cultivation of MOB and HOB creates an alternative route for MP production leveraging safe and sustainably-produced gaseous substrates.

Highly efficient ammonium removal through nitrogen assimilation by a hydrogen-oxidizing bacterium, Ideonella sp. TH17

Ideonella sp. TH17, an autotrophic hydrogen-oxidizing bacterium (HOB), was successfully enriched and isolated from activated sludge in a domestic wastewater treatment plant (WWTP). Batch experiments were conducted to identify the cell growth and ammonium (NH₄⁺-N) removal, and to verify the pathways of nitrogen utilization under different conditions. At a representative NH₄⁺-N concentration of 100 mg/L in domestic wastewater, it was the first time that a HOB strain achieved a nearly 100% ammonium removal. More than 90% of NH₄⁺-N was assimilated to biomass nitrogen by strain TH17. Only a little of N₂ (<10% of initial NH₄⁺-N) was detected without N₂O emission in aerobic denitrification process. Autotrophic NH₄⁺-N assimilation contributed predominantly to biomass nitrogen production, supplemented by assimilatory nitrate (NO₃^--N) reduction under aerobic conditions. A total of 17 amino acids, accounting for 54.25 ± 1.98% of the dry biomass, were detected in the bacterial biomass harvested at 72 h. These results demonstrated that the newly isolated strain TH17 was capable of removing NH₄⁺-N and recovering nutrients from wastewater efficiently. A new solution was thus provided by this HOB strain for ammonium treatment in sustainable WWTPs of future.