Humic substances as electron acceptor for anaerobic oxidation of methane (AOM) and electron shuttle in Mn (IV)-dependent AOM

Effluent organic matter facilitates anaerobic methane oxidation coupled with nitrous oxide reduction in river sediments

Effluent organic matter (EfOM) from wastewater treatment plants (WWTPs) contains humic-like substances that function as electron shuttles, thereby facilitating microbially-mediated redox reactions. However, the mechanisms governing the coupled processes of anaerobic oxidation of methane (CH4) (AOM) and nitrous oxide (N2O) reduction in river sediments, which receive WWTPs effluents, remain poorly understood. In this study, an incubation experiment with anoxic river sediments was conducted to assess the impacts of EfOM on AOM and nitrous oxide reduction using different effluent dilution ratios. The results showed that EfOM significantly enhanced both processes. Specifically, the AOM rate increased from 8.1 to 14.3 μg gdw-1 d-1, while the N2O reduction rate increased from 29.2 to 56.5 μg gdw-1 d-1. The results of batch tests demonstrated that AOM process enhanced N2O reduction in the presence of EfOM, highlighting the critical role of EfOM in linking these processes. Nitrate-dependent anaerobic methane oxidation (n-DAMO) archaea and denitrifying bacteria dominated the sediment incubated with EfOM. Metagenomic and metatranscriptomic analyses revealed that the denitrifying bacteria exclusively reduce N2O, confirming the role of EfOM in facilitating electron transfer between n-DAMO archaea and N2O reducers. This indicates that effluent discharge could be a potential factor driving the concurrent sinks of methane and nitrous oxide, offering a perspective for investigating the impacts of WWTPs effluent on greenhouse gas sinks in freshwater ecosystems.

Efficacy of nitrate and biochar@birnessite composite microspheres for simultaneous suppression of As(III) mobilization and greenhouse gas emissions in flooded paddy soils

Elevated As(III) pollution and greenhouse gas (GHG) emissions are two primary environmental concerns associated with flooded paddy soils. Herein, a novel biochar@birnessite composite microsphere was engineered using a biochar, birnessite and sodium alginate formulation. The microspheres were applied along with nitrate to examine their efficacy in suppressing As(III) mobilization and GHG emissions in an As-contaminated flooded paddy soil. After a 10-day incubation period, the combined nitrate + microsphere treatment achieved desirable remediation effects versus a nitrate-alone treatment, with mobile As(III) (initially 0.1 mM in flooded layer) completely immobilized and N2O, CH4 and CO2 emissions declining by 89 %, 73 % and 31 %, respectively. As(III) immobilization was ascribed to oxidation/adsorption/coprecipitation by FeOx/MnOx regenerated from successive cycles of Feammox/Mnammox and nitrate-reduction coupled with Fe(II) oxidation (NRFO)/nitrate-reduction coupled with Mn(II) oxidation (NRMO). Moreover, NRFO/NRMO-derived full denitrification displayed high thermodynamic feasibility, leading to full denitrification with the generation of N2 rather than N2O. The co-occurrence of anaerobic oxidation of methane (AOM) driven by biochar-shuttling and coupled reduction of nitrate/FeOx/MnOx fostered anaerobic oxidation of CH4 to CO2. A portion of the resulting CO2 was incorporated into poorly-soluble carbonate minerals leading to lower CO2 emission and soil carbon sequestration. Metagenomic sequencing revealed that the nitrate + microsphere treatment enriched the abundances of key microorganisms linked to As/Fe/Mn oxidation and GHG mitigation (e.g., Geobacter, Streptomyces, Cupriavidus and Chloroflexus). Our findings document the efficacy of nitrate + biochar@birnessite microsphere treatment as an effective remediation strategy to simultaneously mitigate As(III) pollution and GHG emissions in flooded paddy soils.

Antimony mobility in soils: current understanding and future research directions

Antimony (Sb) has gained increased attention over the past few decades due to its possible detrimental effects on biota and its potential to leach and disperse from contaminated soils. The fate of Sb in the environment is largely controlled by its chemical speciation, as well as the speciation of solid phases (e.g. Mn/Fe-oxyhydroxides) that interact with Sb in soils. Microbes have the capacity to facilitate a multitude of oxidation and reduction reactions in soils. Therefore, they exert control over the reactivity of Sb in the environment, either directly and/or indirectly, by changing Sb speciation and/or affecting the redox state of soil solid phases. Here, we outline processes that determine the behaviour of Sb in soils. We conclude that based on laboratory studies there is a good theoretical understanding of pure soil components interacting with Sb species. However, comparatively little is known concerning the contribution of these interactions in complex natural systems that are dynamic in terms of biogeochemical conditions and that can hardly be simulated using laboratory incubations. We note that important biochemical foundations of microbially driven Sb conversions (i.e. molecular constraints on organisms, genes and enzymes involved) have emerged recently. Again, these are based on laboratory incubations and investigations in environments high in Sb. In this regard, an important remaining question is which microorganisms actively impact Sb speciation under real-world conditions, in particular where Sb concentrations are low. Multiple dissolved Sb species have been described in the literature. We note that more analytical development is needed to identify and quantify possible key Sb species in natural systems, as well as anthropogenically impacted environments with only moderate Sb concentrations. With these research needs addressed, we believe that the Sb fate in the environment can be more accurately assessed, and remediation options can be developed.

Role of Fe and Mn in organo-mineral-microbe interactions: evidence of carbon stabilization and transformation of organic matter leading to carbon greenhouse gas emissions

Up to 90% of organic matter (OM) in soils and sediments are stabilized and protected against microbial decomposition through organo-mineral interactions, formation of soil aggregates, pH, and oxygen availability. In soils and sediment systems, OM is associated with mineral constituents promoting carbon persistence and sequestration of which iron (Fe) and manganese (Mn) are crucial components. Under anoxic condition, microbes couple the decomposition of OM to the oxidative/reductive transformation of Fe/Mn minerals leading to carbon greenhouse gas (C-GHG) emissions (i.e. CH4 and CO2). Although these organo-mineral-microbe interactions have been observed for decades, the bio-geochemical mechanisms governing the switch from OM stability toward OM degradation are not fully understood. Interest in this field have been growing steadily given the interest in global warming caused by OM decomposition leading to C-GHG emissions. This review emphasizes the dual role of Fe/Mn minerals in both OM stability and decomposition. Additionally, we synthesize the conceptual understanding of how Fe/Mn minerals govern OM dynamics and the resultant C-GHG emissions via microbial-mediated carbon transformation. We highlight the need for interdisciplinary research to better understand organo-Fe/Mn mineral-microbial interactions to develop management handles for climate mitigation strategies.

Current trends in electromicrobiology of methane oxidation

With many methane oxidation processes now recognized as being electrochemically driven, microbial methane oxidation is becoming an emerging focus in electromicrobiology. This review examines the current trends in the electromicrobiology of methane oxidation. We begin by reviewing recent advances in the understanding of the microbial and physiological diversity involved in microbial methane oxidation. We highlight the versatile role of aerobic methane-oxidizing bacteria in electrochemically driven methane oxidation, and the non-syntrophic lifestyle of anaerobic methanotrophic archaea (ANME) enabled by their extracellular electron transfer (EET) pathways. These aspects are followed by a review of recent findings on the potential reversibility of methanogen metabolism, with a focus on the proposed EET pathways that may facilitate their shift to a methane-oxidizing phenotype, a topic that remains under active investigation and debate. Finally, we examine the biogeochemical cycles and the application potential involving electrochemically driven methane oxidation.

Immobilization or mobilization of heavy metal(loid)s in lake sediment-water interface: Roles of coupled transformation between iron (oxyhydr)oxides and natural organic matter

Iron (Fe) (oxyhydr)oxides and natural organic matter (NOM) are active substances ubiquitously found in sediments. Their coupled transformation plays a crucial role in the fate and release risk of heavy metal(loid)s (HMs) in lake sediments. Therefore, it is essential to systematically obtain relevant knowledge to elucidate their potential mechanism, and whether HMs provide immobilization or mobilization effect in this ternary system. In this review, we summarized (1) the bidirectional effect between Fe (oxyhydr)oxides and NOM, including preservation, decomposition, electron transfer, adsorption, reactive oxygen species production, and crystal transformation; (2) the potential roles of coupled transformation between Fe and NOM in the environmental behavior of HMs from kinetic and thermodynamic processes; (3) the primary factors affecting the remediation of sediments HMs; (4) the challenges and future development of sediment HM control based on the coupled effect between Fe and NOM from theoretical and practical perspectives. Overall, this review focused on the biogeochemical coupling cycle of Fe, NOM, and HMs, with the goal of providing guidance for HMs contamination and risk control in lake sediment.

Denitrification performance of the nitrate-dependent manganese redox strain Dechloromonas sp. YZ8 under copper ion (Cu(Ⅱ)) stress: Promotion mechanism and immobilization efficacy

A novel nitrate-dependent manganese (Mn) redox strain was isolated and identified as Dechloromonas sp.YZ8 in this study. The growth conditions of strain YZ8 were optimized by kinetic experiments. The nitrate (NO3--N) removal efficiency was 100.0 % at 16 h at C/N of 2.0, pH of 7.0, and Mn(II) or Mn(IV) addition of 10.0 or 500.0 mg L-1, along with an excellent Mn redox capacity. Transmission electron microscopy supported the Mn redox process inside and outside the cells of strain YZ8. When strain YZ8 was exposed to different concentrations of copper ion (Cu(II)), it turned out that moderate amounts of Cu(II) increased microbial activity and metabolic activities. Moreover, it was discovered that the appropriate amount of Cu(II) promoted the conversion of Mn(IV) and Mn(II) to Mn(III) and improved electron transfer capacity in the Mn redox system, especially the Mn redox process dominated by Mn(IV) reduction. Then, δ-MnO2 and bio-manganese oxides (BMO) produced during the reaction process have strong adsorption of Cu(II). The surface valence changes of δ-MnO2 before and after the reaction and the production of BMO, Mn(III)-rich intermediate black manganese ore (Mn3O4), and Mn secondary minerals together confirmed the Mn redox pathway. The study provided new insights into the promotion mechanism and immobilization effects of redox-coupled denitrification of Mn in groundwater under Cu(II) stress.

Overload of dissolved organic matter (DOM) in riparian infiltration zone increasing the pollution risk of naphthalene, insight from the competitive inhibition of naphthalene biodegradation by DOM

Riparian infiltration zones are crucial for maintaining water quality by reducing the aqueous concentrations of polycyclic aromatic hydrocarbons (PAHs) through adsorption and biodegradation within the aquatic ecosystem. Dissolved organic matter (DOM) are ubiquitous in riparian infiltration zones where they extensively engage in the adsorption and biodegradation of PAHs, thereby influencing PAHs natural attenuation potential within riparian infiltration zones. Few studies have explored the natural attenuation mechanisms of PAHs influenced by DOM in riparian infiltration zones. In this study, the natural attenuation mechanisms of naphthalene (a typical PAHs component), under the influence of DOM, were explored, based on a case riverside source area. Analysis of microbial community structures, and the electron acceptor (e.g., Fe(III), DO/NO3-, SO42-)/electron donor (naphthalene and DOM) concentration changes within the riparian infiltration zone revealed a competitive inhibition relationship between DOM and naphthalene during microbial metabolism. Biodegradation experiments showed that when the concentration of DOM is higher than 4.0 mg·L-1, it inhibits the biodegradation of naphthalene. DOM competitively inhibits the biodegradation of naphthalene through the following mechanisms: (i) triggering microbial antioxidative defense mechanisms, diminishing the available resources for microbial participation in naphthalene degradation; (ii) altering microbial community structure; (iii) modulating microbial EPS composition, reducing the efficiency of microorganisms in utilizing carbon sources; and (iv) inhibiting the expression levels of downstream genes involved in naphthalene degradation. The competitive inhibition constants of DOM with concentrations of 1.0, 2.0, 4.0, 8.0, and 16.0 mg·L-1 on naphthalene biodegradation are -2.0 × 10-3, -5.0 × 10-3,1.0 × 10-3, 4.0 × 10-4, and 1.0 × 10-4, respectively. These findings enhance understanding of PAHs attenuation in riparian infiltration zone, providing a basis for assessing and managing PAHs pollution risks during riparian extraction.

Enhanced Extracellular Electron Transfer in Magnetite-Mediated Anaerobic Oxidation of Methane Coupled to Humic Substances Reduction: The Pivotal Role of Membrane-Bound Electron Transfer Proteins

Humic substances are organic substances prevalent in various natural environments, such as wetlands, which are globally important sources of methane (CH4) emissions. Extracellular electron transfer (EET)-mediated anaerobic oxidation of methane (AOM)-coupled with humic substances reduction plays an important role in the reduction of methane emissions from wetlands, where magnetite is prevalent. However, little is known about the magnetite-mediated EET mechanisms in AOM-coupled humic substances reduction. This study shows that magnetite promotes the reduction of the AOM-coupled humic substances model compound, anthraquinone-2,6-disulfonate (AQDS). 13CH4 labeling experiments further indicated that AOM-coupled AQDS reduction occurred, and acetate was an intermediate product of AOM. Moreover, 13CH313COONa labeling experiments showed that AOM-generated acetate can be continuously reduced to methane in a state of dynamic equilibrium. In the presence of magnetite, the EET capacity of the microbial community increased, and Methanosarcina played a key role in the AOM-coupled AQDS reduction. Pure culture experiments showed that Methanosarcina barkeri can independently perform AOM-coupled AQDS reduction and that magnetite increased its surface protein redox activity. The metatranscriptomic results indicated that magnetite increased the expression of membrane-bound proteins involved in energy metabolism and electron transfer in M. barkeri, thereby increasing the EET capacity. This phenomenon potentially elucidates the rationale as to why magnetite promoted AOM-coupled AQDS reduction.

Low-molecular-weight organic acids inhibit the methane-dependent arsenate reduction process in paddy soils

Anaerobic methane oxidation (AOM) can drive soil arsenate reduction, a process known as methane-dependent arsenate reduction (M-AsR), which is a critical driver of arsenic (As) release in soil. Low molecular weight organic acids (LMWOAs), an important component of rice root exudates, have an unclear influence and mechanism on the M-AsR process. To narrow this knowledge gap, three typical LMWOAs-citric acid, oxalic acid, and acetic acid-were selected and added to As-contaminated paddy soils, followed by the injection of 13CH4 and incubation under anaerobic conditions. The results showed that LMWOAs inhibited the M-AsR process and reduced the As(III) concentration in soil porewater by 35.1-65.7 % after 14 days of incubation. Among the LMWOAs, acetic acid exhibited the strongest inhibition, followed by oxalic and citric acid. Moreover, LMWOAs significantly altered the concentrations of ferrous iron and dissolved organic carbon in the soil porewater, consequently impacting the release of As in the soil. The results of qPCR and sequencing analysis indicated that LMWOAs inhibited the M-AsR process by simultaneously suppressing microbes associated with ANME-2d and arrA. Our findings provide a theoretical basis for modulating the M-AsR process and enhance our understanding of the biogeochemical cycling of As in paddy soils under rhizosphere conditions.