Identification of the autotrophic denitrifying community in nitrate removal reactors by DNA-stable isotope probing

Cooperation between autotrophic and heterotrophic denitrifiers under low C/N ratios revealed by individual-based modelling

Denitrifying biofilms, in which autotrophic denitrifiers (AD) and heterotrophic denitrifiers (HD) coexist, play a crucial role in removing nitrate from water or wastewater. However, it is difficult to elucidate the interactions between HD and AD through sequencing-based experimental methods. Here, we developed an individual-based model to describe the interspecies dynamics and priority effects between sulfur-based AD (Thiobacillus denitrificans) and HD (Thauera phenylcarboxya) under different C/N ratios. In test I (coexistence simulation), AD and HD were initially inoculated at a ratio of 1:1. The simulation results showed excellent denitrification performance and a coaggregation pattern of denitrifiers, indicating that cooperation was the predominant interaction at a C/N ratio of 0.25 to 1.5. In test II (invasion simulation), in which only one type of denitrifier was initially inoculated and the other was added at the invasion time, denitrifiers exhibited a stratification pattern in biofilms. When HD invaded AD, the final HD abundance decreased with increasing invasion time, indicating an enhanced priority effect. When AD invaded HD, insufficient organic carbon sources weakened the priority effect by limiting the growth of HD populations. This study reveals the interaction between autotrophic and heterotrophic denitrifiers, providing guidance for optimizing wastewater treatment process.

Controlling carbon dioxide-to-hydrogen ratio to improve hydrogen utilization and denitrification rates of hydrogenotrophic autotrophic denitrification through homoacetogenesis-heterotrophic denitrification pathway

Hydrogenotrophic denitrification, an environment-friendly process for organic-free influents, is limited due to poor hydrogen mass transfer efficiency and significant pH fluctuations. In this study, we manipulated the carbon dioxide-to-hydrogen ratio to improve hydrogenotrophic denitrification. When carbon dioxide-to-hydrogen ratio was 1:1 (carbon dioxide, 200 ml: hydrogen, 200 ml), the hydrogen utilization and denitrification rates were 2.4 times and 3.0 times that when carbon dioxide-to-hydrogen ratio was 0:1 (carbon dioxide, 0 ml: hydrogen, 200 ml), respectively. The pH fluctuation decreased from 3.1±0.3 to 0.2±0.1. Furthermore, the hydrogenotrophic denitrification, acetoclastic denitrification, homoacetogenic, and electron transfer activities of the sludge were improved. A high carbon dioxide-to-hydrogen ratio augmented the acid-producing and heterotrophic denitrifying microorganism populations. By maintaining a high carbon dioxide-to-hydrogen ratio, the dominant hydrogenotrophic autotrophic denitrification pathway was transformed into a homoacetogenesis-heterotrophic denitrification pathway, thereby achieving higher hydrogen utilization and denitrification rates.

Partial S(0)-driven autotrophic denitrification process facilitated the quick natural enrichment of anammox bacteria at room temperature

Anaerobic ammonium oxidation (anammox) is well-known to be an environmental and promising biotechnology. However, the natural enrichment of anammox bacteria is still a challenging topic. In this study, partial S(0)-driven autotrophic denitrification (PSAD) was developed to stably supply nitrite, and natural enrichment of anammox bacteria was rapidly realized in a single sequencing moving bed biofilm reactor at room temperature. With the initiation of PSAD, anammox bacteria spontaneously emerged within 12 days, and PSAD-anammox coupling system was realized successfully. And then, the influent concentration of ammonium continuously increased to the same concentration as the nitrate, and the mean total nitrogen removal efficiency reached 92.77 %, which was mainly contributed by anammox. Moreover, the coupling of PSAD and anammox reduced the risk of sulfate emissions. cDNA high throughput sequencing revealed that the relative abundance of Candidatus Brocadia and Thiobacillus reached 39.03 % and 13.48 % at the 88th day. Oligotyping analysis illustrated that GATTTAAT and GTCCCA were the dominant Ca. Brocadia and Thiobacillus oligotypes in PSAD-anammox coupling system, respectively. DNA-based stable isotope probing further deciphered that Thiobacillus was the actual performer of PSAD and supported the nitrite for anammox bacteria in PSAD-anammox coupling system. Overall, this work provided a new strategy to naturally enrich anammox bacteria.

Three-dimensional biofilm electrode reactors (3D-BERs) for wastewater treatment

Three-dimensional biofilm electrode reactors (3D-BERs) are highly efficient in refractory wastewater treatment. In comparison to conventional bio-electrochemical systems, the filled particle electrodes act as both electrodes and microbial carriers in 3D-BERs. This article reviews the conception and basic mechanisms of 3D-BERs, as well as their current development. The advantages of 3D-BERs are illustrated with an emphasis on the synergy of electricity and microorganisms. Electrode materials utilized in 3D-BERs are systematically summarized, especially the critical particle electrodes. The configurations of 3D-BERs and their integration with wastewater treatment reactors are introduced. Operational parameters and the adaptation of 3D-BERs to varieties of wastewater are discussed. The prospects and challenges of 3D-BERs for wastewater treatment are then presented, and the future research directions are proposed. We believe that this timely review will help to attract more attentions on 3D-BERs investigation, thus promoting the potential application of 3D-BERs in wastewater treatment.

Coupling transformation of carbon, nitrogen and sulfur in a long-term operated full-scale constructed wetland

The coupling transformation of carbon, nitrogen and sulfur compounds has been studied in lab-scale and pilot-scale constructed wetlands (CWs), but few studies investigated full-scale CW. In this study, we used batch experiments to investigate the potentials of carbon, nitrogen and sulfur transformation in a long-term operated, full-scale horizontal subsurface flow wetland. The sediments collected from the HSFW were incubated for 48 h in the laboratory with supplying various dosages of carbon, nitrogen and sulfur compounds. The results showed that heterotrophic denitrification was the main pathway. At the same time, the sulfide (S^2-)-based autotrophic denitrification was also present. Increasing TOC concentration or NO₃^- concentration could promote heterotrophic denitrification but did not inhibit the sulfide-based autotrophic denitrification. In our experiment, the highest NO₃^- removal via autotrophic denitrification was 25.23% while that via heterotrophic denitrification was 73.66%, leading to the total NO₃^- removal of 98.89%. The results also demonstrated that NO₃^- rather than NO₂^- was the preferable electron acceptor for both heterotrophic and sulfide-based autotrophic denitrification in the CW. Increasing S^2- concentrations promote NO₃^- removal from 12.99% to 25.23% without organic carbon, but varying NO₃^- or NO₂^- has no effects. These results indicated that concentrations of S^2-, instead of NO₃^- or NO₂^-, was the limiting factor for sulfide-based autotrophic denitrification in the studied CW. The microbial community analysis and correlation analysis between the transformation of carbon, nitrogen and sulfur compounds and relative abundance of bacteria further confirmed that in the CW, the key pathways coupling transformation were heterotrophic denitrification and sulfide-based autotrophic denitrification. Overall, the current study will enhance understanding of carbon, nitrogen, and sulfur transformation in CW and support better design and treatment efficiency.

Role of porous polymer carriers and iron-carbon bioreactor combined micro-electrolysis and biological denitrification in efficient removal of nitrate from wastewater under low carbon to nitrogen ratio

In the current research, a novel bioreactor composed of porous polymer carriers and iron-carbon (PPC@FeC) was established through bacterial immobilized technology. The influence of key factors was studied on the nitrate removal performance of the PPC@FeC bioreactor. The experimental results showed that the highest removal rate of nitrate (7.33 mg L^-1 h^-1) can be obtained with short hydraulic retention times (HRT = 2.0 h) and low carbon-to-nitrogen ratio (C/N = 2.0). The results of high-throughput sequencing revealed that Zoogloea sp. L2 was the dominant strain in bioreactor responsible for nitrate removal. Moreover, the SEM and XRD analyses elucidated that Fe₂O₃ was the final product produced by the interaction of FeC and strain L2. These findings showed that the PPC@FeC bioreactor successfully combined micro-electrolysis and biological denitrification, which exhibited great potential in removing nitrate effectively from wastewater under low C/N ratio and short HRT conditions.

Electroconductive moving bed membrane bioreactor (EcMB-MBR) for single-step decentralized wastewater treatment: Performance, mechanisms, and cost

Membrane bioreactor (MBR) is an advantageous technology for wastewater treatment. However, efficient nutrient removal and membrane fouling mitigation remain major challenges in its applications. In this study, an electroconductive moving bed membrane bioreactor (EcMB-MBR) was proposed for simultaneous removal of organics and nutrients from domestic wastewater. The EcMB-MBR was composed of a submerged MBR, filled with electrodes and free-floating conductive media. Conductive media were introduced to reduce energy consumption in an electrochemical MBR, to improve nitrogen removal, and to mitigate membrane fouling. The results showed that COD, total nitrogen, and total phosphorus removal was up to 97.1 ± 1.4%, 88.8 ± 4.2%, and 99.0 ± 0.9%, respectively, in comparison with those of 93.4 ± 1.5%, 65.2 ± 5.3%, and 20.4 ± 11.3% in a conventional submerged MBR. Meanwhile, a total membrane resistance reduction of 26.7% was obtained in the EcMB-MBR. The optimized operating condition was determined at an intermittent electricity exposure time of 10 min-ON/10 min-OFF, and a direct current density of 15 A/m². The interactions between electric current and conductive media were explored to understand the working mechanism in this proposed system. The conductive media reduced 21% of the electrical resistivity in the mixed liquor at a selected packing density of 0.20 (v/v). The combination of electrochemical process and conductive media specially enhanced the reduction of nitrate-nitrogen (NO₃^--N) through hybrid bio-electrochemical denitrification processes. These mechanisms involved with electrochemically assisted autotrophic denitrification by autotrophic denitrifying bacteria. As a result, 5.2% of NO₃^--N remained in the effluent of EcMB-MBR in comparison with that of 29.5% in the MBR. Membrane fouling was minimized via both mechanical scouring and electrochemical decomposition/precipitation of organic/particulate foulants. Furthermore, a preliminary cost analysis indicated that an additional operating cost of 0.081 USD/m³, accounting for 10 - 30% increment of the operating cost of a conventional MBR, was needed to enhance the nitrogen and phosphorus removal correspondingly in the EcMB-MBR.

Degradation pathway and microbial mechanism of high-concentration thiocyanate in gold mine tailings wastewater

As one of the inorganic pollutants with the highest concentration in the waste water of gold tailings using biohydrometallurgy, thiocyanate (SCN-) was effectively degraded in this research adopting a two-stage activated sludge biological treatment, and the corresponding degradation pathway and microbial community characteristics in this process were also studied. The results showed that SCN- at 1818.00 mg L-1 in the influent decreased to 0.68 mg L-1 after flowing through the two-stage activated sludge units. Raman spectroscopy was used to study the changes of relevant functional groups, finding that SCN- was degraded in the COS pathway. Based on 16S rRNA high-throughput sequencing technology, the microbial diversity in this system was analyzed, and the results indicated that Thiobacillus played a major role in degrading SCN-, of which the abundance in these two activated sludge units was 32.05% and 20.37%, respectively. The results further revealed the biological removal mechanism of SCN- in gold mine tailings wastewater.

Characterization of Nitrate-Dependent As(III)-Oxidizing Communities in Arsenic-Contaminated Soil and Investigation of Their Metabolic Potentials by the Combination of DNA-Stable Isotope Probing and Metagenomics

Arsenite (As(III)) oxidation has important environmental implications by decreasing both the mobility and toxicity of As in the environment. Microbe-mediated nitrate-dependent As(III) oxidation (NDAO) may be an important process for As(III) oxidation in anoxic environments. Our current knowledge of nitrate-dependent As(III)-oxidizing bacteria (NDAB), however, is largely based on isolates, and thus, the diversity of NDAB may be underestimated. In this study, DNA-stable isotope probing (SIP) with ¹³C-labeled NaHCO₃ as the sole carbon source, amplicon sequencing, and shotgun metagenomics were combined to identify NDAB and investigate their NDAO metabolism. As(III) oxidation was observed in the treatment amended with nitrate, while no obvious As(III) oxidation was observed without nitrate addition. The increase in the gene copies of aioA in the nitrate-amended treatment suggested that As(III) oxidation was mediated by microorganisms containing the aioA genes. Furthermore, diverse putative NDAB were identified in the As-contaminated soil cultures, such as Azoarcus, Rhodanobacter, Pseudomonas, and Burkholderiales-related bacteria. Metagenomic analysis further indicated that most of these putative NDAB contained genes for As(III) oxidation and nitrate reduction, confirming their roles in NDAO. The identification of novel putative NDAB expands current knowledge regarding the diversity of NDAB. The current study also suggests the proof of concept of using DNA-SIP to identify the slow-growing NDAB.

Poised potential is not an effective strategy to enhance bio-electrochemical denitrification under cyclic substrate limitations

Bio-electrochemical denitrification (BED) is a promising organic carbon-free nitrate remediation technology. However, the relationship between engineering conditions, biofilm community composition, and resultant functions in BED remains under-explored. This study used deep sequencing and variation partitioning analysis to investigate the compositional shifts in biofilm communities under varied poised potentials in the batch mode, and correlated these shifts to reactor-level functional differences. Interestingly, the results suggest that the proliferation of a key species, Thiobacillus denitrificans, and community diversity (the Shannon index), were almost equally important in explaining the reactor-to-reactor functional variability (e.g. variability in denitrification rates was 51% and 38% attributable to key species and community diversity respectively, with a 30% overlap), but neither was heavily impacted by the poised potential. The findings suggest that while enriching the key species may be critical in improving the functional efficiency of BED, poised potentials may not be an effective strategy to achieve the desired level of enrichment in substrate-limited real-world conditions.

Metagenomic analysis reveals enhanced nutrients removal from low C/N municipal wastewater in a pilot-scale modified AAO system coupling electrolysis

The conventional biological nutrients removal process is challenged by insufficient organic carbon in influent. To cross such an organic-dependent barrier, a pilot-scale electrolysis-integrated anaerobic/anoxic/oxic (AAO) process was developed for enhanced removal of nitrogen (N) and phosphorus (P) from low carbon/nitrogen (C/N) municipal wastewater. Average removal efficiencies of total nitrogen (TN) and total phosphorus (TP) in the electrolysis-AAO reached to 77.24% and 95.08% respectively, showing increases of 13.88% and 21.87%, as compared to the control reactor. Spatial variations of N and P showed that NH₄⁺-N removal rate was promoted in aerobic zone of electrolysis-AAO. The intensified TN elimination, which was mostly reflected by abatement of NO₃^--N with the concomitant slight accumulation of NH₄⁺-N and NO₂^--N, mainly occurred in anoxic2 compartment as the electrons supplied by electrolysis. Furthermore, minor P contents were measured and remained almost unchanged along the reaction units, indicating that chemical precipitation should be the dominant mechanism of P-removal in electrolysis-AAO. From the metagenomic-based taxonomy, phylum Actinobacteria was dramatically inhibited, and phylum Proteobacteria dominated the electrolysis-AAO. Particularly, nitrifying bacteria and multifarious autotrophic denitrifiers were enriched, meanwhile, a significant evolution of heterotrophic denitrifiers was found in electrolysis-AAO compared to control, which was mostly reflected by the inhibition of genus Candidatus Microthrix. Batch tests further confirmed that autotrophic denitrifiers using H₂ and Fe²⁺ as essential electron sinks were mainly responsible for the electrolysis-induced denitrification. Differential metabolic capacities were revealed from the perspectives of functional enzymes and genes, and network analysis allowed insight of microbial taxa-functional genes associations and shed light on stronger relevance between autotrophic denitrifiers and denitrification-associated genes in the electrolysis-AAO system.

Feasibility of Adjusting the S2O32-/NO3- Ratio to Adapt to Dynamic Influents in Coupled Anammox and Denitrification Systems

In this study, anammox, sulfur-based autotrophic denitrification, and heterotrophic denitrification (A/SAD/HD) were coupled in an expanded granular sludge bed (EGSB) reactor to explore the feasibility of enhancing denitrification performance by adjusting the S₂O₃^2-/NO₃^- (S/N) ratio to accommodate dynamic influents. The results indicated that the optimal influent conditions occurred when the conversion efficiency of ammonium (CEA) was 55%, the S/N ratio was 1.24, and the chemical oxygen demand (COD) was 50 mg/L, which resulted in a total nitrogen removal efficiency (NRE) of 95.0% ± 0.5%. The S/N ratio regulation strategy was feasible when the influent COD concentration was less than 100 mg/L and the CEA was between 57% and 63%. Characterization by 16S rRNA sequencing showed that Candidatus Jettenia might have contributed the most to anammox, while Thiobacillus and Denitratisoma were the dominant taxa related to denitrification. The findings of this study provide insights into the effects of CEA and COD on the performance of the A/SAD/HD system and the feasibility of the S/N ratio regulation strategy.

Denitrification with non-organic electron donor for treating low C/N ratio wastewaters

Denitrification with non-organic electron donors for treating low C/N ratio wastewater has attracted growing interests. Hydrogen, reduced sulfur compounds and ferrous ions are mainly used in autotrophic denitrification, holding promise for achieving practical applications. Recently, the development of autotrophic denitrification-based processes, such as bioelectrochemically-supported hydrogenotrophic denitrification and sulfur-/iron-based denitrification assisted multi-contaminant removal, provide opportunities for applying these processes in wastewater treatment. Exploration of the autotrophic denitrification process in terms of contaminant removal mechanism, interaction among functional microorganisms, and potential full-scale applications is thus of great importance. Here, an overview of the commonly used non-organic electron donors, e.g., hydrogen, reduced sulfur compounds and ferrous ions, in denitrification for treating low C/N ratio wastewater is provided. Also, the feasibility of applying the combined processes based on autotrophic denitrification with the compounds is discussed. Furthermore, challenges and future possibilities as well as concerns about the practical applications are envisaged in this review.

A novel start-up strategy for mixotrophic denitrification biofilters by rhamnolipid and its performance on denitrification of low C/N wastewater

A novel start-up strategy for sulfur-based mixotrophic denitrification biofilters (mDNBFs) by rhamnolipid was investigated for the first time. Rhamnolipid with gradient concentrations (0-120 mg/L) was added into five lab-scale mDNBFs. Results showed that rhamnolipid could promote biomass yield and nitrogen removal rate (NRR) by 71.7% and 68.7%, respectively, while its effect on EPS and adhesion force was concentration-dependent. The spatial distribution characteristics of microbial communities demonstrated the enrichment of main heterotrophic denitrifying bacteria outcompeted that of the autotrophs, with a more pronounced difference in high concentration rhamnolipid-treated mDNBFs. Furthermore, highest abundance of napA, narG, nirK and nosZ genes was observed in 80 mg/L rhamnolipid-treated mDNBF. Interfacial processes including solubilizing effect and hydration repulse and variations of organics were discussed to explicate the underlying mechanism. The study enlightened that an appropriate concentration (∼80 mg/L) of rhamnolipid may be a good solution for accelerating biofilm formation and enriching denitrifying bacteria to promote denitrification performance of mDNBFs treating low C/N wastewater.

pH control and microbial community analysis with HCl or CO2 addition in H2-based autotrophic denitrification

H₂-based autotrophic denitrification is promising to remove nitrate from water or wastewater lacking organic carbon sources, and pH is one of its most important process parameters. HCl and CO₂ addition are known as adequate pH control methods for practical purposes. However, because of H₂, added CO₂ may participate in microbial metabolisms and affect denitrification mechanisms. Here, a combined micro-electrolysis and autotrophic denitrification (CEAD) reactor, in which H₂ is generated based on galvanic-cell reactions between zero-valent iron and carbon, was optimized and continuously operated for 233 days by adding HCl or CO₂ to control pH in the range of 7.2-8.2. Microbial communities were compared between the two pH-control methods through high-throughput sequencing of 16S rRNA, nirS, and nirK genes. Under a low COD/N ratio of 0.5 in the influent (with ∼36 mgNO₃^--N/L), when adding HCl, the total nitrogen (TN) removal efficiency reached 91.4% ± 0.9% with a 28-h hydraulic retention time (HRT). When adding CO₂, the TN removal efficiency was improved to 96.5% ± 1.7% with 24-h HRT. Significant differences of 16S rRNA and nirS genes between the two pH-control stages indicated the variation of microbial communities and nirS-type denitrifiers. With HCl addition, Thiobacillus, unclassified Comamonadaceae, Arenimonas, Limnobacter, and Thermomonas, which were reported previously as likely autotrophic or heterotrophic denitrifiers, were most dominant in the biofilms. With CO₂ addition, the biofilms became dominated by Anaerolineaceae and Methylocystaceae (related to organic carbon metabolism), Denitratisoma (likely heterotrophic denitrifier), and uncultured bacteria TK10 and AKYG587. The results suggest that the added CO₂ not only contributed to pH control but also participated in microbial metabolisms. This study provides useful insights into microbial mechanisms and further optimization of H₂-based autotrophic denitrification in water and wastewater treatment.

Microbial distribution and turnover in Antarctic microbial mats highlight the relevance of heterotrophic bacteria in low-nutrient environments

Maritime Antarctica has shown the highest increase in temperature in the Southern Hemisphere. Under this scenario, biogeochemical cycles may be altered, resulting in rapid environmental change for Antarctic biota. Microbes that drive biogeochemical cycles often form biofilms or microbial mats in continental meltwater environments. Limnetic microbial mats from the Fildes Peninsula were studied using high-throughput 16S rRNA gene sequencing. Mat samples were collected from 15 meltwater stream sites, comprising a natural gradient from ultraoligotrophic glacier flows to meltwater streams exposed to anthropogenic activities. Our analyses show that microbial community structure differences between mats are explained by environmental NH4+, NO3-, DIN, soluble reactive silicon and conductivity. Microbial mats living under ultraoligotrophic meltwater conditions did not exhibit a dominance of cyanobacterial photoautotrophs, as has been documented for other Antarctic limnetic microbial mats. Instead, ultraoligotrophic mat communities were characterized by the presence of microbes recognized as heterotrophs and photoheterotrophs. This suggests that microbial capabilities for recycling organic matter may be a key factor to dwell in ultra-low nutrient conditions. Our analyses show that phylotype level assemblages exhibit coupled distribution patterns in environmental oligotrophic inland waters. The evaluation of these microbes suggests the relevance of reproductive and structural strategies to pioneer these psychrophilic ultraoligotrophic environments.

Biophysiological and factorial analyses in the treatment of rural domestic wastewater using multi-soil-layering systems

Multi-soil-layering (MSL) system was developed as an attractive alternative to traditional land-based treatment techniques. Within MSL system, the environmental cleanup capability of soil is maximized, while the soil microbial communities may also change during operation. This study aimed to reveal the nature of biophysiological changes in MSL systems during operation. The species diversity in soil mixture blocks was analyzed using Illumina HiSeq sequencing of the 16S rRNA gene. The interactive effects of operating factors on species richness, community diversity and bacteria abundance correlated with COD, N and P removal were revealed through factorial analysis. The results indicated the main factors, aeration, bottom submersion and microbial amendment, had different significant effects on microbial responses. The surface area and porosity of zeolites in permeable layers decreased due to the absorption of extracellular polymeric substances. The findings were applied for the design and building of a full-size MSL system in field and satisfied removal efficiency was achieved. The results of this study can help better understand the mechanisms of pollutant reduction within MSL systems from microbial insights. It will have important implications for developing appropriate strategies for operating MSL systems with high efficiency and less risks.

DNA-SIP Reveals the Diversity of Chemolithoautotrophic Bacteria Inhabiting Three Different Soil Types in Typical Karst Rocky Desertification Ecosystems in Southwest China

Autotrophs that inhabit soils receive less attention than their counterparts in other ecosystems, such as deep-sea and subsurface sediments, due to the low abundance of autotrophs in soils with high organic contents. However, the karst rocky desertification region is a unique ecosystem that may have a low level of organic compounds. Therefore, we propose that karst rocky desertification ecosystems may harbor diverse autotrophic microbial communities. In this study, DNA-SIP was employed to identify the chemolithoautotrophic bacteria inhabiting three soil types (i.e., grass, forest, and agriculture) of the karst rocky desertification ecosystems. The results indicated that potential chemolithoautotrophic population was observed in each soil type, even at different time points after amending ¹³C-NaHCO₃, confirming our hypothesis that diverse autotrophs contribute to the carbon cycle in karst soils. Bacteria, such as Ralstonia, Ochrobactrum, Brevibacterium, Acinetobacter, and Corynebacterium, demonstrated their potential to assimilate inorganic carbon and reduce nitrate or thiosulfate as electron acceptors. Putative mixotrophs were identified by DNA-SIP as well, suggesting the metabolic versatility of soil microbiota. A co-occurrence network further indicated that autotrophs and heterotrophs may form associated communities to sustain the ecosystem function. Our current study revealed the metabolic diversity of autotrophic bacteria in soil habitats and demonstrated the potentially important role of chemoautotrophs in karst rocky desertification ecosystems.

Simultaneous removal of thiocyanate and nitrogen from wastewater by autotrophic denitritation process

Pollutants containing sulfur as electron donors will play an important role in the energy-saving denitritation process when organic carbon source was insufficient in wastewater. However, thiocyanate (SCN^-), a hazardous pollutant, has not been characterized in denitritation. In this study, the effects of key environmental factors on removal of thiocyanate and nitrogen were investigated in denitritation. The results showed that the maximum removal efficiency of nitrogen was observed in complete removal of thiocyanate and nitrite. The elemental sulfur was observed prior to complete depletion of thiocyanate. The efficiency of denitritation was promoted by NaHCO₃ and weakly-alkaline environment. In the sludge containing dominant Thiobacillus genus, nitrite was reduced in the conversion of thiocyanate into elemental sulfur and further into sulfate. The stoichiometric ratio of NO₂^--N to SCN^--N was close to 2.0 when thiocyanate was converted completely into sulfate, which verified complete removal of thiocyanate and nitrite at the NO₂^--N/SCN^--N ratio of 2.0.

Genome-Centered Metagenomics Analysis Reveals the Symbiotic Organisms Possessing Ability to Cross-Feed with Anammox Bacteria in Anammox Consortia

Although using anammox communities for efficient wastewater treatment has attracted much attention, the pure anammox bacteria are difficult to obtain, and the potential roles of symbiotic bacteria in anammox performance are still elusive. Here, we combined long-term reactor operation, genome-centered metagenomics, community functional structure, and metabolic pathway reconstruction to reveal multiple potential cross-feedings during anammox reactor start-up according to the 37 recovered metagenome-assembled genomes (MAGs). We found Armatimonadetes and Proteobacteria could contribute the secondary metabolites molybdopterin cofactor and folate for anammox bacteria to benefit their activity and growth. Chloroflexi-affiliated bacteria encoded the function of biosynthesizing exopolysaccharides for anammox consortium aggregation, based on the partial nucleotide sugars produced by anammox bacteria. Chlorobi-affiliated bacteria had the ability to degrade extracellular proteins produced by anammox bacteria to amino acids to affect consortium aggregation. Additionally, the Chloroflexi-affiliated bacteria harbored genes for a nitrite loop and could have a dual role in anammox performance during reactor start-up. Cross-feeding in anammox community adds a different dimension for understanding microbial interactions and emphasizes the importance of symbiotic bacteria in the anammox process for wastewater treatment.

Stable-Isotope Probing Reveals the Activity and Function of Autotrophic and Heterotrophic Denitrifiers in Nitrate Removal from Organic-Limited Wastewater

Combined heterotrophic and autotrophic denitrification (HAD) is a sustainable and practical method for removing nitrate from organic-limited wastewater. However, the active microorganisms responsible for denitrification in wastewater treatment have not been clearly identified. In this study, a combined microelectrolysis, heterotrophic, and autotrophic denitrification (CEHAD) process was established. DNA-based stable isotope probing was employed to identify the active denitrifiers in reactors fed with either ¹³C-labeled inorganic or organic carbon sources. The total nitrogen removal efficiencies reached 87.2-92.8% at a low organic carbon concentration (20 mg/L COD). Real-time polymerase chain reaction of the  nirS gene as a function of the DNA buoyant density following the ultracentrifugation of the total DNA indicated marked ¹³C-labeling of active denitrifiers. High-throughput sequencing of the fractionated DNA in H¹³CO₃^-/¹²CH₃¹²COO^--fed and H¹²CO₃^-/¹³CH₃¹³COO^--fed reactors revealed that Thermomonas-like phylotypes were labeled by ¹³C-bicarbonate, while Thauera-like and Comamonas-like phylotypes were labeled by ¹³C-acetate. Meanwhile, Arenimonas-like and Rubellimicrobium-like phylotypes were recovered in the "heavy" DNA fractions from both reactors. These results suggest that nitrate removal in CEHAD is catalyzed by various active microorganisms, including autotrophs, heterotrophs, and mixotrophs. Our findings provide a better understanding of the mechanism of nitrogen removal from organic-limited water and wastewater and can be applied to further optimize such processes.

Simultaneous nitrite and ammonium production in an autotrophic partial denitrification and ammonification of wastewaters containing thiocyanate

Various products are observed in biological oxidation and reduction of molecules containing elements of variable valence. The variability is caused by the diversity of microorganisms and their metabolic enzymes, which may develop into novel processes in wastewater treatment. The study aimed to develop a novel denitrification process forming nitrite and ammonium in wastewaters containing thiocyanate. High-efficiency nitrite and ammonium production was observed due to autotrophic partial denitrification and ammonification as a result of nitrate and thiocyanate removal. Nitrite, ammonium and sulfate were observed as the ultimate products. The increased NO₃^--N/SCN^--N ratio in the treated wastewater resulted in the decreased removal efficiency of nitrate, and the increased nitrate-to-nitrite transformation ratio and the ratio of NO₂^--N to NH₄⁺-N. Thiocyanate sulfur was oxidized to sulfate via intermediate elementary sulfur providing electron to nitrate or nitrite. The Thiobacillus genus dominated in the sludge providing ammonium and nitrite as substrate for the potentially anammox process.