Protein biomethanation: insight into the microbial nexus

A scientific version of understanding "Why did the chickens cross the road"? - A guided journey through Bacillus spp. towards sustainable agriculture, circular economy and biofortification

The world, a famished planet with an overgrowing population, requires enormous food crops. This scenario compelled the farmers to use a high quantity of synthetic fertilizers for high food crop productivity. However, prolonged usage of chemical fertilizers results in severe adverse effects on soil and water quality. On the other hand, the growing population significantly consumes large quantities of poultry meats. Eventually, this produces a mammoth amount of poultry waste, chicken feathers. Owing to the protein value of the chicken feathers, these wastes are converted into protein hydrolysate and further extend their application as biostimulants for sustained agriculture. The protein profile of chicken feather protein hydrolysate (CFPH) produced through Bacillus spp. was the maximum compared to physical and chemical protein extraction methods. Several studies proved that the application of CFPH and active Bacillus spp. culture to soil and plants results in enhanced plant growth, phytochemical constituents, crop yield, soil nutrients, fertility, microbiome and resistance against diverse abiotic and biotic stresses. Overall, "CFPH - Jack of all trades" and "Bacillus spp. - an active camouflage to the surroundings where they applied showed profound and significant benefits to the plant growth under the most adverse conditions. In addition, Bacillus spp. coheres the biofortification process in plants through the breakdown of metals into metal ions that eventually increase the nutrient value of the food crops. However, detailed information on them is missing. This can be overcome by further real-world studies on rhizoengineering through a multi-omics approach and their interaction with plants. This review has explored the best possible and efficient strategy for managing chicken feather wastes into protein-rich CFPH through Bacillus spp. bioconversion and utilizing the CFPH and Bacillus spp. as biostimulants, biofertilizers, biopesticides and biofortificants. This paper is an excellent report on organic waste management, circular economy and sustainable agriculture research frontier.

Breakdown of hardly degradable carbohydrates (lignocellulose) in a two-stage anaerobic digestion plant is favored in the main fermenter

The yield and productivity of biogas plants depend on the degradation performance of their microbiomes. The spatial separation of the anaerobic digestion (AD) process into a separate hydrolysis and a main fermenter should improve cultivation conditions of the microorganisms involved in the degradation of complex substrates like lignocellulosic biomass (LCB) and, thus, the performance of anaerobic digesters. However, relatively little is known about such two-stage processes. Here, we investigated the process performance of a two-stage agricultural AD over one year, focusing on chemical and technical process parameters and metagenome-centric metaproteomics. Technical and chemical parameters indicated stable operation of the main fermenter but varying conditions for the open hydrolysis fermenter. Matching this, the microbiome in the hydrolysis fermenter has a higher dynamic than in the main fermenter. Metaproteomics-based microbiome analysis revealed a partial separation between early and common steps in carbohydrate degradation and primary fermentation in the hydrolysis fermenter but complex carbohydrate degradation, secondary fermentation, and methanogenesis in the main fermenter. Detailed metagenomics and metaproteomics characterization of the single metagenome-assembled genomes showed that the species focus on specific substrate niches and do not utilize their full genetic potential to degrade, for example, LCB. Overall, it seems that a separation of AD in a hydrolysis and a main fermenter does not improve the cleavage of complex substrates but significantly improves the overall process performance. In contrast, the remaining methanogenic activity in the hydrolysis fermenter may cause methane losses.

Recovery capability of anaerobic digestion from ammonia stress: Metabolic activity, energy generation, and genome-centric metagenomics

Excessive ammonia stresses anaerobic digestion (AD) significantly. Although there has been progress in understanding AD under ammonia exposure, investigations on AD liberated from ammonia exposure are limited. Here, the recovery capability of AD from ammonia stress was evaluated, by examining specific methanogenic activity, energy-conserving capability, microbial community succession, and metabolic pathway reconstruction. The findings demonstrated that ammonia stress relief resulted in < 50% methane recovery, with propionate conversion identified as the critical impediment to AD reactivation. Energy generation could not recovered either. Efforts to mitigate ammonia stress failed to restore acetoclastic methanogens, e.g., Methanothrix soehngenii, and proved futile in awakening propionate oxidizers, e.g., Desulfobulbus. Interestingly, a symbiotic metabolism emerged, prevailing in stress-relieved AD due to its energy-conserving advantage. This study underscores the importance of targeted interventions, including stimulating acetoclastic methanogenesis, propionate oxidation, and energy generation, as priorities for AD recovery following ammonia stress, rather than focusing solely on ammonia level management.

Microbiome-functionality in anaerobic digesters: A critical review

Microbially driven anaerobic digestion (AD) processes are of immense interest due to their role in the biovalorization of biowastes into renewable energy resources. The function-versatile microbiome, interspecies syntrophic interactions, and trophic-level metabolic pathways are important microbial components of AD. However, the lack of a comprehensive understanding of the process hampers efforts to improve AD efficiency. This study presents a holistic review of research on the microbial and metabolic "black box" of AD processes. Recent research on microbiology, functional traits, and metabolic pathways in AD, as well as the responses of functional microbiota and metabolic capabilities to optimization strategies are reviewed. The diverse ecophysiological traits and cooperation/competition interactions of the functional guilds and the biomanipulation of microbial ecology to generate valuable products other than methane during AD are outlined. The results show that AD communities prioritize cooperation to improve functional redundancy, and the dominance of specific microbes can be explained by thermodynamics, resource allocation models, and metabolic division of labor during cross-feeding. In addition, the multi-omics approaches used to decipher the ecological principles of AD consortia are summarized in detail. Lastly, future microbial research and engineering applications of AD are proposed. This review presents an in-depth understanding of microbiome-functionality mechanisms of AD and provides critical guidance for the directional and efficient bioconversion of biowastes into methane and other valuable products.

Adaptation of Anaerobic Digestion Microbial Communities to High Ammonium Levels: Insights from Strain-Resolved Metagenomics

Ammonia release from proteinaceous feedstocks represents the main inhibitor of the anaerobic digestion (AD) process, which can result in a decreased biomethane yield or even complete failure of the process. The present study focused on the adaptation of mesophilic AD communities to a stepwise increase in the concentration of ammonium chloride in synthetic medium with casein used as the carbon source. An adaptation process occurring over more than 20 months allowed batch reactors to reach up to 20 g of NH4+ N/L without collapsing in acidification nor ceasing methane production. To decipher the microbial dynamics occurring during the adaptation and determine the genes mostly exposed to selective pressure, a combination of biochemical and metagenomics analyses was performed, reconstructing the strains of key species and tracking them over time. Subsequently, the adaptive metabolic mechanisms were delineated by following the single nucleotide variants (SNVs) characterizing the strains and prioritizing the associated genes according to their function. An in-depth exploration of the archaeon Methanoculleus bourgensis vb3066 and the putative syntrophic acetate-oxidizing bacteria Acetomicrobium sp. ma133 identified positively selected SNVs on genes involved in stress adaptation. The intraspecies diversity with multiple coexisting strains in a temporal succession pattern allows us to detect the presence of an additional level of diversity within the microbial community beyond the species level.

Achieving efficient methane production from protein-rich organic waste in anaerobic digestion: Using conductive materials or regulating inoculum-to-substrate ratios?

The contribution of inoculum-to-substrate ratios (ISRs) and conductive materials (CMs) on the productivity of anaerobic digestion (AD) remains unclear, particularly for protein-rich organic waste. This study investigated whether the addition of CMs, i.e., biochar and iron powder, can overcome the limitations imposed by varying ISRs for the AD of protein as the sole substrate. Results indicate the ISR plays a decisive role in hydrolysis, acidification, and methanogenesis for protein conversion, irrespective of CMs addition. Methane production increased stepwise as the ISR escalated to 3:1. The addition of CMs provided limited improvement, and iron powder even inhibited methanogenesis at a low ISR. Bacterial community variations were contingent on the ISR, while iron powder supplementation significantly elevates the proportion of hydrogenotrophic methanogen. This study demonstrates that the addition of CMs could affect methanogenic efficiency but can not overcome the limitation of ISRs for the AD of protein.

Supplemental Sewage Scum and Organic Municipal Solid Waste Addition to the Anaerobic Digestion of Thickened Waste Activated Sludge: Biomethane Potential and Microbiome Analysis

Sewage scum (SS) is collected from sedimentation tanks in wastewater treatment plants (WWTPs). Despite its huge biogas potential, there is limited information on its potential as a co-substrate and microbial ecology, especially during anaerobic co-digestion (ACo-D) of the organic fraction of municipal solid waste (OFMSW) and thickened waste activated sludge (TWAS). In this biomethane potential (BMP) study, the bioenergy yield achieved by the supplemental addition of SS and OFMSW to TWAS was investigated, along with the microbial ecology. Compared with the digestion of TWAS alone, which produced 184.6 mLCH4 gVS−1, biomethane yield was enhanced by as much as 32.4–121.6% in trinary mixtures with SS and OFMSW, mainly due to the positive synergistic effect. Furthermore, a mixture of 40%SS + 10%TWAS + 50%OFMSW produced the highest biogas yield of 407 mLCH4 gVS−1, which is proof that existing WWTPs can produce additional energy by incorporating external bioresources, thereby reducing greenhouse gas emissions. Modified Gompertz and logistic function estimates showed that methane production rate improved by as much as 60% in a trinary mixture compared with the digestion of TWAS alone. The genus Methanosaeta, capable of generating methane by the acetoclastic methanogenic pathway among all the archaeal communities, was the most prominent, followed by hydrogenotrophic methanogen Methanospirillum.

Distinguishing responses of acetoclastic and hydrogenotrophic methanogens to ammonia stress in mesophilic mixed cultures

A shift from the acetoclastic to the hydrogenotrophic pathway in methanogenesis under ammonia inhibition is a common observation in anaerobic digestion. However, there are still considerable knowledge gaps concerning the differential ammonia tolerance of acetoclastic and hydrogenotrophic methanogens (AMs and HMs), their responses to different ammonia species (NH₄⁺, NH₃), and their recoverability after ammonia inhibition. With the successful enrichment of mesophilic AMs and HMs cultures, this study aimed at addressing the above knowledge gaps through batch inhibition/recovery tests and kinetic modeling under varying total ammonia (TAN, 0.2-10 g N/L) and pH (7.0-8.5) conditions. The results showed that the tolerance level of HMs to free ammonia (FAN, IC₅₀=1345 mg N/L) and NH₄⁺ (IC₅₀=6050 mg N/L) was nearly 11 times and 3 times those of AMs (NH₃, IC₅₀=123 mg N/L; NH₄⁺, IC₅₀=2133 mg N/L), respectively. Consistent with general belief, the AMs were more impacted by FAN. However, the HMs were more adversely affected by NH₄⁺ when the pH was ≤8.0. A low TAN (1.0-4.0 g N/L) could cause irreversible inhibition of the AMs due to significant cell death, whereas the activity of HMs could be fully or even over recovered from severe ammonia stress (FAN≤ 0.9 g N/L or TAN≤10 g N/L; pH ≤8.0). The different tolerance responses of AMs and HMs might be associated with the cell morphology, multiple energy-converting systems, and Gibbs free energy from substrate-level phosphorylation.