Humin: No longer inactive natural organic matter

The evolution of structural characteristics and redox properties of humin during the composting of sludge and corn straw

Humins (HMs), the insoluble faction of humic substances (HSs), play a pivotal role in the bioremediation of pollutants by acting as electron shuttles that modulate the interactions between microorganisms and pollutants. This crucial function is intricately linked to their structural composition and electron transfer capabilities. However, the dynamics of the electron transfer capacity (ETC) of HM extracted during the composting process and its determinants have yet to be fully elucidated. This study undertakes a comprehensive analysis of the ETC of HM derived from composting, employing electrochemical techniques alongside spectroscopic methods and elemental analysis to explore the influencing factors, including the electron accepting capacity (EAC), electron donating capacity (EDC), and electron reversible rate (ERR). Our findings reveal substantial variations in the EAC and EDC of HM throughout the composting process, with EAC values ranging from 133.03-220.98 μmol e- gC-1 and EDC values from 111.17-229.33 μmol e- gC-1. Notably, the composting process enhances the ERR and EDC of HM while diminishing their EAC. This shift is accompanied by an augmented presence of aromatic structures, polar functional groups, quinones, and nitrogen - and sulfur-containing moieties, thereby boosting the HM's EDC. Conversely, the reduction in EAC is associated with a decline in lignin carbon content and the abundance of oxygen-containing moieties, as well as the diminishment of visible fulvic-like and protein-like substances within HM. Importantly, humic-like substances and nitrogen-containing moieties within HM demonstrated the capacity for repeated electron transfer, underscoring their significance in the context of environmental remediation.

Humin accelerates interspecies electron transfer between Shewanella oneidensis MR-1 and Methanosarcina barkeri

Existing researches involving accelerated interspecies electron transfer (IET) by solid redox mediators focus mainly on the conductive nature of these materials. Although non-conductive solid redox mediator-humin has been reported to promote methanogenic performance of anaerobic granular sludge, likely through accelerating IET of microorganisms, this phenomenon has not been validly proven. In this study, a wetland sediment sourced HM (HMWS) was added into a co-culture of a syntrophic bacteria Shewanella oneidensis MR-1 and an archaeal Methanosarcina barkeri with ethanol as sole electron donor to examine whether HMWS can accelerate the IET between these two species. Results showed that the average ethanol consumption rate and methanogenic rate in the HMWS-containing co-culture were 1.6-fold and 2.9-fold higher than that without HMWS, indicating that HMWS enhanced the cooperation of S. oneidensis MR-1 and M. barkeri. Further experiment suggested that except for interspecies acetate transfer, an alternative HMWS-mediated IET pathway occurred between M. barkeri and S. oneidensis MR-1. The estimated total IET rate in the HMWS-containing co-culture was 0.53 ± 0.1 meq/(d-1·g cell-dry weight-1), which was 2.2-fold higher than that without HMWS. CO/CO and NO/NH2 were the functional groups of HMWS contributing to HMWS-mediated IET, and flavin and cytochrome c of microorganisms participated in the reduction and oxidation of HMWS. The findings are of significance for understanding the diverse IET occurred in natural environments and providing a novel strategy for renewable bioenergy processes with high efficiency.

Upcycling Humins via Esterification Reactions of Hydroxyl Groups: From Functional Powders to PLA Foams and Compatibilized Blends

The valorization of humins side streams from bio-refineries holds significant economic and sustainability potential. One plausible strategy involves using them as building blocks to create new materials. However, humins pose conceptual challenges in their natural state due to their high viscosity, processing difficulties, and temperature sensitivity. This article presents a synthetic strategy for modifying humins properties to make them thermally stable and processable. Employing a sequence of esterification reactions and varying the reagent steric length, we showcase the selective transformation of humins into thermally-stable fine powders and low-viscosity liquids. We extend this approach by reacting humins with polyesters such as polylactic acids and polycaprolactone. In particular, we detail a one-pot single-step synthesis of micro-phase separated compatibilized blends of polylactic acid and humins capped with the polylactic acid arms. Processed via solution-casting, the obtained materials behave as high-strength thermoplastic elastomers having uniform foam morphologies and material characteristics superior to the pure polylactic acid. By varying the content of D-enantiomers, we demonstrate an additional possibility of manipulating the cellular structures of the foams. Finally, we provide a solution to product circularity by reporting a dissolution recycling method.

Geobatteries in environmental biogeochemistry: Electron transfer and utilization

The efficiency of direct electron flow from electron donors to electron acceptors in redox reactions is significantly influenced by the spatial separation of these components. Geobatteries, a class of redox-active substances naturally present in soil-water systems, act as electron reservoirs, reversibly donating, storing, and accepting electrons. This capability allows the temporal and spatial decoupling of redox half-reactions, providing a flexible electron transfer mechanism. In this review, we systematically examine the critical role of geobatteries in influencing electron transfer and utilization in environmental biogeochemical processes. Typical redox-active centers within geobatteries, such as quinone-like moieties, nitrogen- and sulfur-containing groups, and variable-valent metals, possess the potential to repeatedly charge and discharge. Various characterization techniques, ranging from qualitative methods like elemental analysis, imaging, and spectroscopy, to quantitative techniques such as chemical, spectroscopic, and electrochemical methods, have been developed to evaluate this reversible electron transfer capacity. Additionally, current research on the ecological and environmental significance of geobatteries extends beyond natural soil-water systems (e.g., soil carbon cycle) to engineered systems such as water treatment (e.g., nitrogen removal) and waste management (e.g., anaerobic digestion). Despite these advancements, challenges such as the complexity of environmental systems, difficulties in accurately quantifying electron exchange capacity, and scaling-up issues must be addressed to fully unlock their potential. This review underscores both the promise and challenges associated with geobatteries in responding to environmental issues, such as climate change and pollutant transformation.

Changes in Soil Humin Macromolecular Structure Resulting from Long-Term Catch Cropping

The aim of this study was to assess the effect of long-term catch crop application on the structural properties of humin, which is considered the most recalcitrant fraction of soil organic matter. Soil samples from a 30-year field experiment on triticale cultivated with and without catch crops were analysed to determine the total organic carbon content and fractional composition of humic substances. Meanwhile, humin isolated from bulk soil was analysed to determine its elemental composition and spectroscopic properties measured with UV-Vis, fluorescence, and 13C-CPMAS-NMR. It was found that catch crop farming enhanced the formation of highly reactive humus substances, like low-molecular-weight fractions and humic acids, while decreasing the humin fraction. The higher H/C and O/C atomic ratios of humin and the UV-Vis, fluorescence, and 13C-CPMAS-NMR results confirmed a higher share of oxygen-containing functional groups in humin isolated from the soil with catch crop rotation, also corroborating its greater aliphatic nature. Under the conditions of our field experiment, the results indicated that organic residues from catch crops quickly undergo the decay process and are transformed mainly into highly reactive humus substances, which can potentially improve soil health, while mineral fertilisation alone without catch crops favours the stabilisation and sequestration of carbon.

Effects of humic electron mediators on reductive dechlorination of polychlorinated biphenyl by consortia enriched from terrestrial and marine environments

Humic electron mediators can facilitate the reductive dehalogenation of organohalogenated compounds by accelerating electron transfer. To investigate the effect of humic electron mediators on the microbial anaerobic reductive dechlorination of Polychlorinated biphenyls (PCBs), three types of humic electron mediators, humin (HM), humic acid (HA), and anthraquinone-2,6-disulfonic acid (AQDS, HA analogs), were added to PCB dechlorination cultures enriched from different sources in terrestrial and marine environments (T and M cultures). The results showed that meta- and para-site dechlorination occurred in the M culture, while only meta-site dechlorination occurred in the T culture. The dechlorination process N and the dechlorination process H or H' are presented in both cultures. HM enhanced PCB dechlorination metabolic activity in both cultures mainly by promoting meta-site dechlorination. HA showed a weak promoting effect on the M culture by promoting para-chlorine removal but inhibited the dechlorination metabolism of the terrestrial-origin culture, inhibiting meta-chlorine removal. AQDS showed inhibitory effects on both cultures by inhibiting the microbial removal of meta-chlorine. High-throughput sequencing and qPCR results suggest that HM is not a carbon source for the potential dechlorinating metabolism of Dehalococcoides but may promote reductive dechlorination by changing the community structure, and AQDS may inhibit anaerobic reductive dechlorination of PCBs by inhibiting the growth of Dehalococcoides. This study provides insights into the mechanism of enhancing PCB microbial dechlorination mediated by humic substances and plays a significant role in extending the application prospects of PCBs bioremediation technology.

Formation/characterization of humin-mediated anaerobic granular sludge and enhanced methanogenic performance

This study presents a novel method for accelerating the granulation of methanogenic anaerobic granular sludge (AnGS) in an upflow anaerobic sludge blanket (UASB) reactor using solid-phase humin (HM). The results demonstrated that HM-mediated AnGS (HM-AnGS) formed rapidly within 50 days. The increase in particle size, settling velocity and mechanical strength was attributed to the rapid granulation of the HM-AnGS. The maximum methane yield of the HM-AnGS was 5-fold higher than that of the control group. This is consistent with the findings, which showed that HM-AnGS had 3.2-3.4 times more methyl-coenzyme M reductase (Mcr) activity and 2.4-2.9 times more adenosine triphosphate (ATP) than control groups. Molecular analyses indicate that HM most likely accelerated interspecies electron transfer (IET) in HM-AnGS (e.g., from Enterococcus to Methanosaeta). Furthermore, the HM-AnGS was effective in recovering energy from actual slaughterhouse wastewater.

Co-inoculation of fungi and desert cyanobacteria facilitates biological soil crust formation and soil fertility

The inoculation of cyanobacteria for enriching soil nutrients and forming biological soil crusts (BSCs) is considered an effective means to restore degraded soil. However, there are limited studies on the application of co-inoculation of fungi and cyanobacteria for degraded soil remediation. In this study, a high exopolysaccharide-secreting fungi Zh2 was isolated from lichen BSCs in Hobq Desert, and co-inoculated with a cyanobacterial strain identified as Phormidium tenue in different proportions to form BSCs on sand during a 35 days incubation period. Results revealed significant differences in crust biomass and soil properties among crusts with different cyanobacterial/fungal inoculation ratios. Microbial biomass, soil nutrient content and enzyme activities in crusts co-inoculated with cyanobacteria and fungi were higher than those inoculated with cyanobacteria and fungi alone. The inoculation of cyanobacteria contributed to the fulvic-like accumulation, and the inoculated fungi significantly increased the humic-like content and soil humification. Redundancy analysis showed that the inoculation of cyanobacteria was positively correlated with the activities of urease and phosphatase, and the content of fulvic-like. Meanwhile, the inoculation of fungi was positively correlated with the contents of total carbon, total nitrogen and humic-like, the activities of catalase and sucrase. Cyanobacteria and fungi play distinct roles in improving soil fertility and accumulating dissolved organic matter. This study provides new insights into the effects of cyanobacteria and fungi inoculations on the formation and development of cyanobacterial-fungus complex crusts, offering a novel method for accelerating induced crust formation on the surface of sand.

Plant Biostimulant as an Environmentally Friendly Alternative to Modern Agriculture

Ensuring the safety of crop production presents a significant challenge to humanity. Pesticides and fertilizers are commonly used to eliminate external interference and provide nutrients, enabling crops to sustain growth and defense. However, the addition of chemical substances does not meet the environmental standards required for agricultural production. Recently, natural sources such as biostimulants have been found to help plants with growth and defense. The development of biostimulants provides new solutions for agricultural product safety and has become a widely utilized tool in agricultural. The review summarizes the classification of biostimulants, including humic-based biostimulant, protein-based biostimulant, oligosaccharide-based biostimulant, metabolites-based biostimulants, inorganic substance, and microbial inoculant. This review attempts to summarize suitable alternative technology that can address the problems and analyze the current state of biostimulants, summarizes the research mechanisms, and anticipates future technological developments and market trends, which provides comprehensive information for researchers to develop biostimulants.

Change of tetracycline speciation and its impacts on tetracycline removal efficiency in vermicomposting with epigeic and endogeic earthworms

Tetracycline pollution is common in Chinese arable soils, and vermicomposting is an effective approach to accelerate tetracycline bioremediation. However, current studies mainly focus on the impacts of soil physicochemical properties, microbial degraders and responsive degradation/resistance genes on tetracycline degradation efficiencies, and limited information is known about tetracycline speciation in vermicomposting. This study explored the roles of epigeic E. fetida and endogeic A. robustus in altering tetracycline speciation and accelerating tetracycline degradation in a laterite soil. Both earthworms significantly affected tetracycline profiles in soils by decreasing exchangeable and bound tetracycline but increasing water soluble tetracycline, thereby facilitating tetracycline degradation efficiencies. Although earthworms increased soil cation exchange capacity and enhanced tetracycline adsorption on soil particles, the significantly elevated soil pH and dissolved organic carbon benefited faster tetracycline degradation, attributing to the consumption of soil organic matter and humus by earthworms. Different from endogeic A. robustus which promoted both abiotic and biotic degradation of tetracycline, epigeic E. foetida preferently accelerated abiotic tetracyline degradation. Our findings described the change of tetracycline speciation during vermicompsiting process, unraveled the mechanisms of different earthworm types in tetracycline speciation and metabolisms, and offered clues for effective vermiremediation application at tetracycline contaminated sites.

A new strategy for treating Pb2+ and Zn2+ pollution with industrial waste derivatives Humin

Metal pollution caused by industrial waste accumulation is a long-term and far-reaching problem. Humin (HM), as a highly condensed organic component insoluble in alkaline or water solution, is often discarded as humic acid industrial waste. However, the abundant active functional groups in HM reported by some researches make it possible for HM to remove metals. In this study, a waste reuse strategy was proposed to reduce the pressure of industrial metal pollution on the environment. HM was obtained from lignite waste residue. Scanning electron microscopy, energy spectrum and Fourier infrared spectroscopy, combined with the adsorption models were employed to reveal the mechanism of HM adsorption. The results showed that HM had multiple adsorption mechanism and high biological stability. The adsorption capacity of HM to Zn²⁺ and Pb²⁺ were 194.88 mg/g and 289.59 mg/g respectively. HM adsorbed Zn²⁺ mainly by physical multilayer adsorption. And the adsorption of Pb²⁺ by HM was mainly a monolayer chemical reaction, which depended on its active functional groups and the exchange of valence electrons. Notably, HM could simultaneously remove Pb²⁺ and Zn²⁺ and almost did not affect its original adsorption capacity to single ions. These results will provide a new strategy for the treatment of metal pollution in the future and alleviate the pressure of multiple metal pollution of the environment.

Humic Substances as a Versatile Intermediary

Humic substances are organic ubiquitous components arising in the process of chemical and microbiological oxidation, generally called humification, the second largest process of the carbon cycle. The beneficial properties of these various substances can be observed in many fields of life and health, whether it is the impact on the human organism, as prophylactic as well as the therapeutic effects; animal physiology and welfare, which is widely used in livestock farming; or the impact of humic substances on the environment and ecosystem in the context of renewal, fertilization and detoxification. Since animal health, human health and environmental health are interconnected and mutually influencing, this work brings insight into the excellence of the use of humic substances as a versatile mediator contributing to the promotion of One Health.

Wide distribution of extracellular electron transfer functionality in natural proteinaceous organic materials for microbial reductive dehalogenation

Extracellular electron transfer materials (EETMs) in the environment, such as humic substances and biochar, are formed from the humification/heating of natural organic materials. However, the distribution of extracellular electron transfer (EET) functionality in fresh natural organic materials has not yet been explored. In the present study, we reveal the wide distribution of EET functionality in proteinaceous materials for the first time using an anaerobic pentachlorophenol dechlorinating consortium, whose activity depends on EETM. Out of 11 natural organic materials and 13 reference compounds, seven proteinaceous organic materials (albumin, beef, milk, pork, soybean, yolk, and bovine serum albumin) functioned as EETMs. Carbohydrates and lipids did not function as EETMs. Comparative spectroscopic analyses suggested that a β-sheet secondary structure was essential for proteins to function as EETMs, regardless of water solubility. A high content of reduced sulfur was potentially involved in EET functionality. Although proteinaceous materials have thus far been considered simply as nutrients, the wide distribution of EET functionality in these materials provides new insights into their impact on biogeochemical cycles. In addition, structural information on EET functionality can provide a scientific basis for the development of eco-friendly EETMs.

Differences in the composition, source, and stability of suspended particulate matter and sediment organic matter in Hulun Lake, China

The occurrence, migration, transformation, and stability of sediment (SOM) and suspended particulate (SPOM) organic matters have important effects on the environmental behaviors of carbon, nitrogen, phosphorus, and other pollutants in a water environment. The content, composition, fluorescence characteristics, source, and stability of SOM and SPOM in Hulun Lake, a typical lake in cold and arid region of China, were compared by sequential extraction, three-dimensional fluorescence spectroscopy, parallel factor technique, carbon-nitrogen ratio, and stable carbon isotope. Contents of SOM and SPOM in north and west were higher than those in east and south. The average content of SPOM (24.70 ± 4.63 g/kg) was slightly higher than that of SOM (23.04 ± 10.27 g/kg), but the difference was not significant. Humin was the dominant component in SOM and SPOM, accounting for 73.7% and 61.2%, respectively. Humus was the main fluorescence component of water-extractable organic matter in SOM and SPOM, accounting for 79.9% and 70.4%, respectively, of the total fluorescence intensity. SOM and SPOM were derived from terrestrial sources with a relative contribution rate of about 70%. SPOM was more influenced by autochthonous sources and had a significantly lower humification degree and stability than SOM. Effects of climate changes on migration, transformation, stability, and bioavailability of organic matters and endogenous pollutants closely related to organic matters in lakes of cold and arid regions should be paid attention in the future.

Anaerobic mineralization of toluene by enriched soil-free consortia with solid-phase humin as a terminal electron acceptor

The anaerobic biodegradation of toluene proceeds very slowly owing to limited electron acceptors in contaminated aquifer. The liquid reagents traditionally used to enhance this process readily migrate away from the contaminated site, and continuous addition would cause secondary pollution. In our previous study, the reduced solid-phase humic substances (humin), which are redox active, were found to act as electron donors to promote the microbial reactions. Here, we provide new evidence that humin can promote the anaerobic biodegradation of toluene as a terminal electron acceptor. When inoculating nitrate-reducing (NR) and iron-reducing (IR) consortia with toluene degradation activities, the average toluene degradation rates reached 21.20 ± 1.18 μmol/(L·d) and 15.43 ± 0.41 μmol/(L·d) in the presence of a sediment humin (HMcj), and 94.69% ± 4.26% and 93.20% ± 3.73% of the electrons released from toluene oxidation to CO₂ could be recovered by the reduction of HMcj, respectively. Spectroscopy analyses revealed that quinone moieties and nitrogen-containing moieties may be the electron-accepting groups of HMcj. Based on 16S rRNA sequencing, Cellulomonas spp. were the possible functional bacteria in the culture with NR consortium as the inoculum, while Azospira spp., Cellulomonas spp. and Bacillus spp. were the possible functional bacteria in the culture with IR consortium as the inoculum. Further Phylogenetic Investigation of Communities by Reconstruction of Unobserved States (PICRUSt) analyses indicated that toluene oxidation and extracellular electron transfer functions were more abundant in HMcj amended cultures, suggesting a possible enhancement mechanism by HMcj. Additionally, experiments using natural groundwater illustrated that toluene degradation was highly dependent on its concentration, HMcj dosage, pH, and salinity. The study of a column filled with HMcj-coated quartz sand demonstrated a desirable level of toluene degradation in a continuous-flow mode without the presence of other electron acceptors. This study provided an effective and green approach for the remediation of the toluene-contaminated groundwater.

Biogenic ferrihydrite-humin coprecipitate as an electron donor for the enhancement of microbial denitrification by Pseudomonas stutzeri

Nitrate pollution of groundwater has become an increasingly serious environmental problem that poses a great threat to aquatic ecosystems and to human health. Previous studies have shown that solid-phase humin (HM) can act as an additional electron donor to support microbial denitrification in the bioremediation of nitrate-contaminated groundwater where electron donor is deficient. However, the electron-donating capacities of HMs vary widely. In this study, we introduced ferrihydrite and prepared ferrihydrite-humin (Fh-HM) coprecipitates via biotic means to strengthen their electron-donating capacities. The spectroscopic results showed that the crystal phase of Fh did not change after coprecipitation with HM in the presence of Shewanella oneidensis MR-1, and iron may have complexed with the organic groups of HM. The Fh-HM coprecipitate prepared with an optimal initial Fh-HM mass ratio of 14:1 enhanced the microbial denitrification of Pseudomonas stutzeri with an electron-donating capacity 2.4-fold higher than that of HM alone, and the enhancement was not caused by greater bacterial growth. The alginate bead embedding assay indicated that the oxidation pathway of Fh-HM coprecipitate was mainly through direct contact between P. stutzeri and the coprecipitate. Further analyses suggested that quinone and organic-complexed Fe were the main electron-donating fractions of the coprecipitate. The results of the column experiments demonstrated that the column filled with Fh-HM-coated quartz sand exhibited a higher denitrification rate than the one filled with quartz sand, indicating its potential for practical applications.

The Emergence of Extracellular Electron Mediating Functionality in Rice Straw-Artificial Soil Mixture during Humification

This study aimed to elucidate the origin of extracellular electron mediating (EEM) functionality and redox-active center(s) in humic substances, where they are ubiquitously distributed. Here, we show the emergence of EEM functionality during the humification of rice straw in artificial soil (kaolin and sand) with a matric potential of -100 cm at 20 °C for one year. We used the dechlorination activity of an EEM material-dependent pentachlorophenol-dechlorinating anaerobic microbial consortium as an index of the EEM functionality. Although rice straw and its mixture with artificial soil did not initially have EEM functionality, it emerged after one month of humification and increased until six months after which the functionality was maintained for one year. Chemical and electrochemical characterizations demonstrated that the emergence and increase in EEM functionality were correlated with the degradation of rice straw, formation of quinone structures, a decrease in aromatic structures, an increase in nitrogenous and aliphatic structures, and specific electric capacitance during humification. The newly formed quinone structure was suggested as a potential redox-active center for the EEM functionality. These findings provide novel insights into the dynamic changes in EEM functionality during the humification of organic materials.

Humin and biochar accelerated microbial reductive dechlorination of 2,4,6-trichlorophenol under weak electrical stimulation

The extracellular electron transfer (EET) is regarded as one of the crucial factors that limit the application of the bioelectrochemical system (BES). In this study, two different solid-phase redox mediators (RMs), biochar (1.2 g/L, T-B) and humin (1.2 g/L, T-H) were used for boosting the microorganisms accessing the electrons required for 2,4,6-TCP dechlorination under weak electrical stimulation (-0.278 V vs. Standard hydrogen electrode). BES with dissolved RM anthraquinone-2,6-disulfonate (AQDS 0.5 mmol/L, T-A) was used as a comparison. The results showed that dechlorination of 2,4,6-TCP could be greatly accelerated by biochar (1.78 d^-1) and humin (1.50 d^-1) than AQDS (0.24 d^-1) and no RM control (T-M, 0.27 d^-1). Moreover, phenol became the predominant dechlorination product in T-H (78.5 %) and T-B (63.0 %) instead of 4-CP in T-M (67.1 %) and T-A (89.8 %). Pseudomonas, Sulfurospirillum, Desulfuromonas, Dehalobacter, Anaeromyxobacter, and Dechloromonas belonging to Proteobacteria or Firmicutes rather than Chloroflexi might be responsible for the dechlorination activity. Notably, different RMs tended to stimulate distinct electroactive bacteria. Pseudomonas was the most abundant microorganism in T-M (41.92 %) and T-A (17.24 %), while Rhodobacter was most prevalent in T-H (20.04 %) and Azonexus was predominant in T-B (48.48 %). This study is essential in advancing the understanding of EET in BES for microbial degradation of organohalide contaminants under weak electrical stimulation.

Promotion of Nitrogen Fixation of Diverse Heterotrophs by Solid-Phase Humin

Although biological nitrogen fixation (BNF) proceeds under mild conditions compared to the energy-intensive Haber–Bosch process, the slow kinetics of BNF necessitate the promotion of BNF activity in its practical application. The BNF promotion using purified nitrogenases and using genetically modified microorganisms has been studied, but these enzymes are unstable and expensive; moreover, designing genetically modified microorganisms is also a difficult task. Alternatively, the BNF promotion in non-modified (wild-type) microorganisms (enriched consortia) with humin has been shown, which is a humic substance insoluble at any pH and functions as an extracellular electron mediator. However, the taxonomic distribution of the diazotrophs promoted by humin, the levels of BNF promotion, and the underlying mechanism in BNF promotion with humin remain unknown. In this study, we show that taxonomically diverse heterotrophic diazotrophs, harboring nifH clusters I, II, and III, promoted their BNF by accepting extracellular electrons from humin, based on the characterization of the individual responses of isolated diazotrophs to humin. The reduced humin increased the acetylene reduction activity of the diazotrophs by 194–916% compared to the level achieved by the organic carbon source, causing adenosine triphosphate (ATP) synthesis in the diazotroph cells without increase in the CO₂ production and direct electron donation to the MoFe protein of the nitrogenase in the cells without relying on the biological electron transfer system. These would result in BNF promotion in the wild-type diazotroph cells beyond their biochemical capacity. This significant promotion of BNF with humin would serve as a potential basis for sustainable technology for greener nitrogen fixation.

Promotion of biological H2 (Bio-H2) production by the nitrogen-fixing anaerobic microbial consortia using humin, a solid-phase humic substance

Dark fermentative biological hydrogen (Bio-H₂) production is expected to be a clean and sustainable H₂ production technology, and the technologies have been studied to increase in the product yield as index. This study achieved high product yields of Bio-H₂ using nitrogen-fixing consortia under nitrogen-deficient conditions with glucose or mannitol as substrate and humin as the extracellular electron mediator: 4.12 mol-H₂/mol-glucose and 3.12 mol-H₂/mol-mannitol. The high Bio-H₂ production was observed under the conditions where both nitrogenase and hydrogenase were active in the presence of humin. Nitrogenase activity was confirmed by acetylene reduction activity and hydrogenase activity by Bio-H₂ production under nitrogenase-inhibiting conditions with NH₄NO₃. [Fe-Fe] hydrogenase detected by a specific PCR and acetate, butyrate, formate, lactate, and pyruvate produced as by-products suggested the involvement of both pyruvate-ferredoxin-oxidoreductase and pyruvate formate lyase pathways in Bio-H₂ production. Humin promoted the Bio-H₂ production beyond the capacity of the consortium, which had reached saturation with the optimum concentrations of glucose and mannitol. Carbon balance suggested the concurrent H₂ consumption by hydrogenotrophic methanogenesis and acetogenesis. Bio-H₂ production of the washed and starved consortium with reduced humin under conditions with or without NH₄NO₃ suggests that humin promoted hydrogenase and nitrogenase activity by donating extracellular electrons. Clostridium and Ruminococcus in the consortia were considered major hydrogen producers. Thus, this study demonstrated the outstanding potential of nitrogen-fixing consortia under nitrogen-deficient conditions with humin as an extracellular electron mediator for dark fermentative Bio-H₂ production with high yields.

Redox reaction between solid-phase humins and Fe(III) compounds: Toward a further understanding of the redox properties of humin and its possible environmental effects

Redox reactions between humic substances and Fe(III) compounds play a critical role in the biogeochemical cycle of pollutants. Most humic substances in soils and sediments are in a solid form (i.e. humin (HM)). In order to assess the contribution of electron shuttling via HM within the electron transfer network in natural environments and to predict environmental fate of pollutants associated with iron oxides, it is necessary to understand the electron transfer processes from HM to the environmentally relevant Fe(III) minerals, and to examine the redox reversibility of HM. The results of this study demonstrated that non-reduced HMs could only donate electrons to dissolved ferric citrate and poorly crystalline ferrihydrite, but reduced HMs could also reduce hematite and magnetite that had high crystallinity. The degree of reduction depended on the difference in redox potential and the crystallinity of the Fe(III) compounds. The electron-accepting capacities of different HMs correlated well with their organic carbon content, and quinones and Fe-bound organic component were important electron-accepting groups in HMs. Furthermore, the redox reversibility experiments demonstrated that HMs could maintain stable electron transfer capacities over three reduction-oxidation cycles, indicating that the HM could be an environmentally sustainable electron shuttle. Our results suggest that (1) HM may play an unrecognized and important role in biogeochemical cycles of pollutants in Fe(III) mineral-rich environments; (2) electron shuttling through HM to ferric citrate and ferrihydrite can occur even in the presence of O₂; and (3) HM would be a promising material for environmental remediation.

Effect of Humin and Chemical Factors on CO2-Fixing Acetogenesis and Methanogenesis

Acetogenesis and methanogenesis have attracted attention as CO₂-fixing reactions. Humin, a humic substance insoluble at any pH, has been found to assist CO₂-fixing acetogenesis as the sole electron donor. Here, using two CO₂-fixing consortia with acetogenic and methanogenic activities, the effect of various parameters on these activities was examined. One consortium utilized humin and hydrogen (H₂) as electron donors for acetogenesis, either separately or simultaneously, but with a preference for the electron use from humin. The acetogenic activity was accelerated 14 times by FeS at 0.2 g/L as the optimal concentration, while being inhibited by MgSO₄ at concentration above 0.02 g/L and by NaCl at concentrations higher than 6 g/L. Another consortium did not utilize humin but H₂ as electron donor, suggesting that humin was not a universal electron donor for acetogenesis. For methanogenesis, both consortia did not utilize extracellular electrons from humin unless H₂ was present. The methanogenesis was promoted by FeS at 0.2 g/L or higher concentrations, especially without humin, and with NaCl at 2 g/L or higher concentrations regardless of the presence of humin, while no significant effect was observed with MgSO₄. Comparative sequence analysis of partial 16S rRNA genes suggested that minor groups were the humin-utilizing acetogens in the consortium dominated by Clostridia, while Methanobacterium was the methanogen utilizing humin with H₂.

Promotion of biological nitrogen fixation activity of an anaerobic consortium using humin as an extracellular electron mediator

Nitrogen fertiliser is manufactured using the industrial Haber–Bosch process, although it is extremely energy-consuming. One sustainable alternative technology is the electrochemical promotion of biological nitrogen fixation (BNF). This study reports the promotion of BNF activity of anaerobic microbial consortia by humin, a solid-phase humic substance, at any pH, functioning as an extracellular electron mediator, to levels of 5.7–11.8 times under nitrogen-deficient conditions. This was evidenced by increased acetylene reduction activity and total nitrogen content of the consortia. Various humins from different origins promoted anaerobic BNF activity, although the degree of promotion differed. The promotion effected by humin differed from the effects of chemical reducing agents and the effects of supplemental micronutrients and vitamins. The promotion of anaerobic BNF activity by only reduced humin without any other electron donor suggested that humin did not serve as organic carbon source but as extracellular electron mediator, for electron donation to the nitrogen-fixing microorganisms. The next generation sequencing (NGS) of partial 16S rRNA genes showed the predominance of Clostridiales (Firmicutes) in the consortia. These findings suggest the effectiveness of humin as a solid-phase extracellular electron mediator for the promotion of anaerobic BNF activity, potentially to serve for the basis for a sustainable technology.