A Novel Inorganic Sulfur Compound Metabolizing Ferroplasma-Like Population Is Suggested to Mediate Extracellular Electron Transfer

Efficient degradation of aqueous dichloromethane by an enhanced microbial electrolysis cell: Degradation kinetics, microbial community and metabolic mechanisms

Dichloromethane (DCM) has been listed as a toxic and harmful water pollutant, and its removal needs attention. Microbial electrolysis cells (MECs) are viewed as a promising alternative for pollutant removal, which can be strengthened from two aspects: microbial inoculation and acclimation. In this study, the MEC for DCM degradation was inoculated with the active sludge enhanced by Methylobacterium rhodesianum H13 (strain H13) and then acclimated in the form of a microbial fuel cell (MFC). Both the introduction of strain H13 and the initiation in MFC form significantly promoted DCM degradation. The degradation kinetics were fitted by the Haldane model, with Vmax, Kh, Ki and vmax values of 103.2 mg/L/hr, 97.8 mg/L, 268.3 mg/L and 44.7 mg/L/hr/cm2, respectively. The cyclic voltammogram implies that DCM redox reactions became easier with the setup of MEC, and the electrochemical impedance spectrogram shows that the acclimated and enriched microbes reduced the charge transfer resistance from the electrode to the electrolyte. In the biofilm, the dominant genera shifted from Geobacter to Hyphomicrobium in acclimation stages. Moreover, Methylobacterium played an increasingly important role. DCM metabolism mainly occurred through the hydrolytic glutathione S-transferase pathway, given that the gene dcmA was identified rather than the dhlA and P450/MO. The exogenous electrons facilitated the reduction of GSSG, directly or indirectly accelerating the GSH-catalyzed dehalogenation. This study provides support for the construction of an efficient and stable MEC for DCM removal in water environment.

Comparison of bioleaching of a sulfidic copper ore (chalcopyrite) in column percolators and in stirred-tank bioreactors including microbial community analysis

Chalcopyrite is the most abundant Cu-sulfide and economically the most important copper mineral in the world. It is known to be recalcitrant in hydrometallurgical processing and therefore chalcopyrite bioleaching has been thoroughly studied for improvement of processing. In this study, the microbial diversity in 22 samples from the Sarcheshmeh copper mine in Iran was investigated via 16S rRNA gene sequencing. In total, 1063 species were recognized after metagenomic analysis including the ferrous iron- and sulfur-oxidizing acidophilic genera Acidithiobacillus, Leptospirillum, Sulfobacillus and Ferroplasma. Mesophilic as well as moderately thermophilic acidophilic ferrous iron- and sulfur-oxidizing microorganisms were enriched from these samples and bioleaching was studied in shake flask experiments using a chalcopyrite-containing ore sample from the same mine. These enrichment cultures were further used as inoculum for bioleaching experiments in percolation columns for simulating heap bioleaching. Addition of 100 mM NaCl to the bioleaching medium was assessed to improve the dissolution rate of chalcopyrite. For comparison, bioleaching in stirred tank reactors with a defined microbial consortium was carried out as well. While just maximal 32% copper could be extracted in the flask bioleaching experiments, 73% and 76% of copper recovery was recorded after 30 and 10 days bioleaching in columns and bioreactors, respectively. Based on the results, both, the application of moderately thermophilic acidophilic bacteria in stirred tank bioreactors, and natural enrichment cultures of mesoacidophiles, with addition of 100 mM NaCl in column percolators with agglomerated ore allowed for a robust chalcopyrite dissolution and copper recovery from Sarcheshmeh copper ore via bioleaching.

Pore-Matched Sponge for Microorganisms Pushes Electron Extraction Limit in Microbial Fuel Cells

Microbial fuel cells (MFCs) are of great potential for wastewater remediation and chemical energy recovery. Nevertheless, limited by inefficient electron transfer between microorganisms and electrode, the remediation capacity and output power density of MFCs are still far away from the demand of practical application. Herein, a pore-matching strategy is reported to develop uniform electroactive biofilms by inoculating microorganisms inside a pore-matched sponge, which is assembled of core-shell polyaniline@carbon nanotube (PANI@CNT). The maximum power density achieved by the PANI@CNT bioanode is 7549.4 ± 27.6 mW m-2 , which is higher than the excellent MFCs with proton exchange membrane reported to date, while the coulombic efficiency also attains a considerable 91.7 ± 1.2%. The PANI@CNT sponge enriches the exoelectrogen Geobacter significantly, and is proved to play the role of conductive pili in direct electron transfer as it down-regulates the gene encoding pilA. This work exemplifies a practicable strategy to develop excellent bioanode to boost electron extraction in MFCs and provides in-depth insights into the enhancement mechanism.

Profiling microbial communities in an extremely acidic environment influenced by a cold natural carbon dioxide spring: A study of the Mefite in Ansanto Valley, Southern Italy

The Ansanto Valley's Mefite, one of the Earth's largest non-volcanic CO2 gas emissions, is distinguished by its cold natural carbon dioxide springs. These emissions originate from the intricate tectonics and geodynamics of the southern Apennines in Italy. Known for over two millennia for its lethal concentration of CO2 and other harmful gases, the Mefite has a reputation for being toxic and dangerous. Despite its historical significance and unique geological features, there is a lack of information on the microbial diversity associated with the Mefite's gas emissions. This study presents an integrated exploration of the microbial diversity in the mud soil, using high-throughput sequencing of 16S rRNA (Prokaryotes) and ITS2 (Fungi), alongside a geochemical site characterisation. Our findings reveal that the Mefite's unique environment imposes a significant bottleneck on microbial diversity, favouring a select few microbial groups such as Actinobacteria and Firmicutes for Prokaryotes, and Basidiomycota for Fungi.

Metals recovery from polymetallic sulfide tailings by bioleaching functional bacteria isolated with the improved 9K agar: Comparison between one-step and two-step processes

Due to the characteristics of simple process, environmental friendliness and low operating costs, biometallurgy has become a popular technology for metals recovering from low-grade ores and tailings. In order to enhance the efficiency of bioleaching functional bacteria acquisition, the 9K agar was optimized by adjusting the ratio of two solutions to achieve better and faster solidification for the functional bacteria growth and isolation. By using the improved 9K agar, six functional stains within genera of Acidithiobacillus ferriphilus, A. ferrooxidans and Leptospirillum ferrooxidans were isolated from the enrichment of acid mine drainage. After the Fe2+ oxidation ability evaluation, three strains of WT1-1, XT2-2, and YT3-1 within the three genera were selected and employed as the individual inoculum for the bioleaching from polymetallic sulfide tailings. Eventually, a maximum leaching efficiency of 58.37% Cu, 53.14% Al, 80.09% Mg, and 76.95% Zn were observed by A. ferriphilus WT1-1 after 28 d. To further improve the bioleaching efficiency, the three strains were mixed proportionally as the inoculum in both one-step and two-step bioleaching processes. Comparing to the pure cultures, the leaching efficiencies of Cu and Mg were significantly enhanced in both one- and two-steps, while no significant change in Zn. By comparing the one- and two-step processes, leaching efficiencies of Al, Mg, and Zn were higher in the one-step process, whereas Cu was observed to be higher in the two-step process. Therefore, the selection on leaching process of one or two steps should be determined based on tailings composition and target metals.

Mechanisms of bioleaching: iron and sulfur oxidation by acidophilic microorganisms

Bioleaching offers a low-input method of extracting valuable metals from sulfide minerals, which works by exploiting the sulfur and iron metabolisms of microorganisms to break down the ore. Bioleaching microbes generate energy by oxidising iron and/or sulfur, consequently generating oxidants that attack sulfide mineral surfaces, releasing target metals. As sulfuric acid is generated during the process, bioleaching organisms are typically acidophiles, and indeed the technique is based on natural processes that occur at acid mine drainage sites. While the overall concept of bioleaching appears straightforward, a series of enzymes is required to mediate the complex sulfur oxidation process. This review explores the mechanisms underlying bioleaching, summarising current knowledge on the enzymes driving microbial sulfur and iron oxidation in acidophiles. Up-to-date models are provided of the two mineral-defined pathways of sulfide mineral bioleaching: the thiosulfate and the polysulfide pathway.

Electroactive Microorganisms in Advanced Energy Technologies

Large-scale production of green and pollution-free materials is crucial for deploying sustainable clean energy. Currently, the fabrication of traditional energy materials involves complex technological conditions and high costs, which significantly limits their broad application in the industry. Microorganisms involved in energy production have the advantages of inexpensive production and safe process and can minimize the problem of chemical reagents in environmental pollution. This paper reviews the mechanisms of electron transport, redox, metabolism, structure, and composition of electroactive microorganisms in synthesizing energy materials. It then discusses and summarizes the applications of microbial energy materials in electrocatalytic systems, sensors, and power generation devices. Lastly, the research progress and existing challenges for electroactive microorganisms in the energy and environment sectors described herein provide a theoretical basis for exploring the future application of electroactive microorganisms in energy materials.

Low biological phosphorus removal from effluents treated by slow sand filters

The legislation for environment protection requires strict controls of the wastewater releasing in water bodies. The wastewater treatment plants (WWTP) have been used for organic matter degradation; however, the residual total phosphorus (TP) removal has not been efficient. TP and nitrogen present in wastewater are associated to eutrophication of water bodies and algae growth. Therefore, this study discusses the efficiency of phosphorus removal by a slow filter (SF), complementary to a WWTP and the microbial community involved. The results showed that the use of SF, with or without macrophytes, is not suitable to remove TP. Spatial variation in microbial communities distributed in three distinct zones was identified in the SF. Proteobacteria, Bacteroidetes, Chloroflexi and Firmicutes covered the hydrolytic and fermentative bacteria. The acetogenesis, nitrification, and denitrification, as well as the removal of phosphorus from the effluent, were performed by representatives affiliated to different groups. Alphaproteobacteria, Betaproteobacteria, and Gammaproteobacteria among these, Dokdonella sp., Frateuria sp., Comamonas sp., Diaphorobacter sp., Nitrosospira sp., Ferruginibacter sp., Flavobacterium sp., and the uncultured OD1 were the most abundant bacteria in the SF. The low efficiency for TP removing from SF effluents can be explained by the low abundance of phosphorus accumulating organisms (PAOs), with the association of the low concentration of biodegradable organic matter in the inlet effluent. Therefore, the alternative to using SF as a complement to WWTPs, as recommended by some Brazilian environmental agencies, did not prove to be viable and new approaches must be evaluated. KEY POINTS: • The phosphorus removal was performed by a slow filter system in a WWTP but obtained a low efficiency. • Microbial spatial variation was distributed into distinct zones from slow filter. • Low abundance of PAOs was observed due to the low availability of organic matter.

Sustainability of the rice-crayfish co-culture aquaculture model: microbiome profiles based on multi-kingdom analyses

While the rice-crayfish culture (RCFP) model, an important aquaculture model in Asia, is generally considered a sustainable model, its sustainability in terms of microbial community profiles has not been evaluated. In this study, multi-kingdom analyses of microbiome profiles (i.e., bacteria, archaea, viruses, and eukaryotes) were performed using environmental (i.e., water and sediment) and animal gut (i.e., crayfish and crab gut) microbial samples from the RCFP and other aquaculture models, including the crab-crayfish co-culture, crayfish culture, and crab culture models, to evaluate the sustainability of the RCFP systematically. Results showed that RCFP samples are enriched with a distinct set of microbes, including Shewanella, Ferroplasma, Leishmania, and Siphoviridae, when compared with other aquaculture models. Additionally, most microbes in the RCFP samples, especially microbes from different kingdoms, were densely and positively connected, which indicates their robustness against environmental stress. Whereas microbes in different aquaculture models demonstrated moderate levels of horizontal gene transfer (HGT) across kingdoms, the RCFP showed relatively lower frequencies of HGT events, especially those involving antibiotic resistance genes. Finally, environmental factors, including pH, oxidation-reduction potential, temperature, and total nitrogen, contributed profoundly to shaping the microbial communities in these aquaculture models. Interestingly, compared with other models, the microbial communities of the RCFP model were less influenced by these environmental factors, which suggests that microbes in the latter have stronger ability to resist environmental stress. The findings collectively reflect the unique multi-kingdom microbial patterns of the RCFP model and suggest that this model is a sustainable model from the perspective of microbiome profiles.

Corrosion mitigation by nitrite spray on corroded concrete in a real sewer system

Microbially influenced concrete corrosion (MICC) in sewers is caused by the activity of sulfide-oxidizing microorganisms (SOMs) on concrete surfaces, which greatly deteriorates the integrity of sewers. Surface treatment of corroded concrete by spraying chemicals is a low-cost and non-intrusive strategy. This study systematically evaluated the spray of nitrite solution in corrosion mitigation and re-establishment in a real sewer manhole. Two types of concrete were exposed at three heights within the sewer manhole for 21 months. Nitrite spray was applied at the 6th month for half of the coupons which had developed active corrosion. The corrosion development was monitored by measuring the surface pH, corrosion product composition, sulfide uptake rate, concrete corrosion loss, and the microbial community on the corrosion layer. Free nitrous acid (FNA, i.e. HNO₂), formed by spraying a nitrite solution on acidic corrosion surfaces, was shown to inhibit the activity of SOMs. The nitrite spray reduced the corrosion loss of concrete at all heights by 40-90% for six months. The sulfide uptake rate of sprayed coupons was also reduced by about 35%, leading to 1-2 units higher surface pH, comparing to the control coupons. The microbial community analysis revealed a reduced abundance of SOMs on nitrite sprayed coupons. The long-term monitoring also showed that the corrosion mitigation effect became negligible in 15 months after the spray. The results consistently demonstrated the effectiveness of nitrite spray on the MICC mitigation and identified the re-application frequencies for full scale applications.

A novel granular sludge-based and highly corrosion-resistant bio-concrete in sewers

Bio-concrete is known for its self-healing capacity although the corrosion resistance was not investigated previously. This study presents an innovative bio-concrete by mixing anaerobic granular sludge into concrete to mitigate sewer corrosion. The control concrete and bio-concrete (with granular sludge at 1% and 2% of the cement weight) were partially submerged in a corrosion chamber for 6 months, simulating the tidal-region corrosion in sewers. The corrosion rates of 1% and 2% bio-concrete were about 17.2% and 42.8% less than that of the control concrete, together with 14.6% and 35.0% less sulfide uptake rates, 15.3% and 55.6% less sulfate concentrations, and higher surface pH (up to 1.8 units). Gypsum and ettringite were major corrosion products but in smaller sizes on bio-concrete than that of control concrete. The total relative abundance of corrosion-causing microorganisms, i.e. sulfide-oxidizing bacteria, was significantly reduced on bio-concrete, while more sulfate-reducing bacteria (SRB) was detected. The corrosion-resistance of bio-concrete was mainly attributed to activities of SRB derived from the granular sludge, which supported the sulfur cycle between the aerobic and anaerobic corrosion sub-layers. This significantly reduced the net production of biogenic sulfuric acid and thus corrosion. The results suggested that the novel granular sludge-based bio-concrete provides a highly potential solution to reduce sewer corrosion.

Organic carbon coupling with sulfur reducer boosts sulfur based denitrification by Thiobacillus denitrificans

Sulfur autotrophic denitrification utilizes elemental sulfur as the electron donor for nitrate removal from aquatic environments. Organic carbon could stimulate the conversion of sulfur and facilitates the S⁰-based denitrification process in the mix-trophic. In this study, the co-cultured system of sulfur reducer (Geobacter sulfurreducens) and Thiobacillus denitrificans was used to investigate that how organic carbon could boost the S⁰-based denitrification. The results showed that the rate of S⁰-based denitrification was improved with C/N ratio of 0.13 and this improvement continued even after the acetate was exhausted. Sulfur probe test and Raman analysis suggested that reduced sulfur species (S_x^2-) were formed with the addition of organic carbon. The S_x^2- could recombine with element sulfur and the bioavailability of S⁰ would be improved, as a result, the rate of S⁰-based denitrification increased as well. Nitrate reduction rate could further increase with the C/N ratio of 0.88, but it would decrease significantly when the C/N ratio increased to 1.50 as the high concentration of generated S^2-. Our results provided explanations that why organic carbon addition would improve the bioavailability of S⁰ which could further promote the S⁰-dominant denitrification process.

Effective bioleaching of low-grade copper ores: Insights from microbial cross experiments

The interaction between microorganisms and minerals was a hot topic to reveal the transformation of key elements that affecting bioleaching efficiency. Three typical low-grade copper ores, the main copper-bearing components of which were primary sulfide, secondary sulfide and high-oxidative sulfide copper, were obtained from Dexing, Zijinshan and Luanshya copper mine, respectively. Meanwhile, six typical microorganisms were isolated from each of the three habitats, and assembled as communities based on their origins. Cross bioleaching was carried out under identical conditions. The leaching parameters showed that native strains played excellent roles in their corresponding ore bioleaching process, and community structure was greatly determined by mineral composition, indicating that domestication for longitudinal adaption was an effective way to improve microbial leaching performance. Leptospirillum ferriphilum and Acidithiobacillus ferrooxidans promoted copper release by shifting redox potential and pH of the leachate, respectively, indicating that microbial population regulation was another effective way to improve bioleaching efficiency.