Nitrate Removal Research Articles

Tuning Nitrogen Configurations in Nitrogen-Doped Graphene Encapsulating Fe3C Nanoparticles for Enhanced Nitrate Electroreduction.

Electrochemical nitrate reduction reaction (NO3RR) offers a promising technology for the synthesis of ammonia (NH3) and removal of nitrate in wastewater. Herin, we fabricate a series of Fe3C nanoparticles in controllable pyridinic-N doped graphene (Fe3C@NG-X) by a self-sacrificing template method for the NO3RR. Fe3C@NG-10 exhibits high catalytic performance with a Faradaic efficiency (FE) of 94.03% toward NH3 production at -0.5 V vs. Reversible hydrogen electrode (RHE) and an NH3 yield rate of 477.73 mmol h-1 gcat-1. Additionally, the catalyst also has a FE above 90% across a broad potential range and NO3- concentration range (12.5-500 mM). During the electrocatalytic process, the material experienced structural reconstruction, forming Fe/Fe3C@NG-X heterojunction. Experimental investigations demonstrate that the remarkable electrocatalytic activity is attributed to the high proportion of pyridinic-N content, and the reconstruction further enhances the overall reduction process.

Interfacial Water Regulation for Nitrate Electroreduction to Ammonia at Ultralow Overpotentials.

Nitrate electroreduction is promising for achieving effluent waste-water treatment and ammonia production with respect to the global nitrogen balance. However, due to the impeded hydrogenation process, high overpotentials need to be surmounted during nitrate electroreduction, causing intensive energy consumption. Herein, a hydroxide regulation strategy is developed to optimize the interfacial H2O behavior for accelerating the hydrogenation conversion of nitrate to ammonia at ultralow overpotentials. The well-designed Ru─Ni(OH)2 electrocatalyst shows a remarkable energy efficiency of 44.6% at +0.1V versus RHE and a nearly 100% Faradaic efficiency for NH3 synthesis at 0V versus RHE. In situ characterizations and theoretical calculations indicate that Ni(OH)2 can regulate the interfacial H2O structure with a promoted H2O dissociation process and contribute to the spontaneous hydrogen spillover process for boosting NO3 - electroreduction to NH3 at Ru sites. Furthermore, the assembled rechargeable Zn-NO3 -/ethanol battery system exhibits an outstanding long-term cycling stability during the charge-discharge tests with the production of high-value-added ammonium acetate, showing great potential for simultaneously achieving nitrate removal, energy conversion, and chemical synthesis. This work can not only provide a guidance for interfacial H2O regulation in extensive hydrogenation reactions but also inspire the design of a novel hybrid flow battery with multiple functions.

Effective organic matter removal via bio-adsorption prior to anammox process and utilization of carbon-rich sludge.

Excessive organic matter in the anaerobic ammonia oxidation (Anammox) leads to the growth of a large number of heterotrophic bacteria, which disrupts the anaerobic ammonia oxidation. The adsorption-anaerobic ammonia oxidation process can effectively reduce excessive organic matter, capturing it instead of consuming it, which is a sustainable development technology. In this study, utilizing the excellent adsorption performance of aerobic granular sludge (AGS), an adsorption-regeneration process was employed to remove organic matter at the front end of the Anammox process through bio-adsorption in an artificial simulated domestic sewage environment, and it was successfully used for denitrification. Stirring rate is a key factor affecting sludge granulation. As a parallel experiment of sludge granulation, two Sequencing Batch Reactors (SBRs) (R1 and R2) were operated simultaneously at different stirring rates. After 153 days, the particle size of the two reactors was analyzed, revealing that the proportion of particles larger than 200μm was over 50%, and granular sludge was successfully formed in both reactors. Long-term operational results indicate that at a temperature of 16.5±1°C, varying initial pH levels (6.5, 6.7, 7.2, and 8.5) significantly affect the removal efficiency of chemical oxygen demand (COD). COD is rapidly adsorbed and removed within a short period. Among the tested initial pH values, a pH of 6.7 yielded the best total chemical oxygen demand (tCOD) removal efficiency, achieving up to 95%. Additionally, the study examined the effects of different carbon sources on denitrification, revealing that under carbon-rich conditions, the denitrification rate was highest, reaching 1.44mgN/(g VSS·h). Compared to endogenous denitrification, the denitrification rate increased by 40%, and the nitrate (NO₃⁻-N) removal efficiency reached 100%.

Water Table Fluctuations Control Nitrate and Ammonium Fate in Coastal Aquifers

AbstractCoastal aquifers experience water table fluctuations that push and pull water and air through organic‐rich soils. This exchange affects the supply of oxygen, dissolved organic carbon (DOC), and nitrogen (N) to shallow aquifers and influences groundwater quality. To investigate the fate of N species, we used a meter‐long column containing a sequence of natural organic topsoil and aquifer sediments. A fluctuating head was imposed at the column bottom with local, nitrate‐rich groundwater (16.5 mg/L NO3‐N). We monitored in‐situ redox potential and collected pore water samples for analysis of inorganic N species and DOC over 16 days. Reactive processes were more complex than anticipated. The organic‐rich topsoil remained anaerobic, while mineral sediments beneath alternated between aerobic, when the water table dropped and sucked air across preferential flow paths, and anaerobic conditions, when the water table was high. A fluid flow and reactive transport model shows that when the water table rises into organic‐rich soils, it limits the flow of oxygen, while the soils release DOC, which stimulates the removal of nitrate from groundwater by denitrification. At the end of the experiment, we introduced seawater to the column to mimic a storm surge. Seawater mobilized N and DOC from shallow soil horizons, which could reach the aquifer if the surge is long enough. These processes are relevant for groundwater quality in developed coastal areas with anthropogenic N sources, as climate change and rising seas will drive changes in water table and flood dynamics.

Highly selective and efficient electrochemical nitrate removal via Ni-doped Cu(OH)2 nanowires promoting H* reaction

Highly selective and efficient electrochemical nitrate removal via Ni-doped Cu(OH)2 nanowires promoting H* reaction

Unlocking the potential of simultaneous organics oxidation and nitrate reduction over an S-scheme magnetic CoFe2O4@g-C3N4 heterojunction under visible light irradiation: Performance and mechanism

In this study, a novel S-scheme CoFe2O4@g-C3N4 (CF@GCN) heterojunction was prepared via a simple hydrothermal method and its visible-light-driven catalytic potential was evaluated in terms of tetracycline (TC) oxidation and nitrate reduction simultaneously. A series of characterizations validated the effective fabrication of CF@GCN, exhibiting remarkable photocatalytic potentials. In the separated system of TC and nitrate, a nearly 100% removal of TC was observed in 60 min under the optimal conditions (pH = 5.0, Catalyst dosage = 0.4 g/L, TC concentration = 5.0 mg/L). The quenching experiments emphasized the concurrent participation of both non-radical species (h+) and radical species (O2•− and HO•) in TC degradation. Meanwhile, more than 96% of nitrate removal was observed in 90 min under the optimal conditions (pH = 3.0, Catalyst dosage = 0.4 g/L, nitrate concentration = 100 mg/L, in the presence of formic acid). In the combined system, both TC oxidation and nitrate reduction occurred simultaneously and the efficiency of TC oxidation was lower compared to the individual system. The novel S-scheme composite has demonstrated exceptional potential in enhancing the reusability and stability of the catalyst, even after five cycles of trials. A detailed mechanistic pathway for TC oxidation and nitrate reduction was proposed, relying on the identification of reactive species and intermediates. In general, our findings propose a promising method with synergistic properties for the simultaneous oxidation of organics and reduction of nitrate in wastewater over an S-scheme heterojunction.

Carbon sources derived from the invasive plant Spartina alterniflora improved the nitrogen removal in seawater constructed wetland.

Invasive alien plants pose a great threat to local plants and ecosystems. How to effectively alleviate this hazard is an unresolved issue. This study explored the carbon release characteristics of an invasive plant Spartina alterniflora and evaluated the ability of nitrogen removal from shrimp culture wastewater through constructing seawater wetland. The results showed that fresh S. alterniflora had a significantly higher carbon release potential and bioavailability than that of withered S. alterniflora, and alkali-heat treatment could increase the carbon release with an average COD release rate of 33.39 mg/g from fresh S. alterniflora. The removal rate of total nitrogen was improved by about 22% in seawater constructed wetlands by adding fresh S. alterniflora biomass. Moreover, the addition of fresh S. alterniflora biomass was beneficial to the increase in the abundance of denitrification-related genera Vibrio, which might be the key to the improvement of nitrate removal efficiency in seawater constructed wetland systems. These findings indicated that invasive plants S. alterniflora as carbon sources of seawater wetland was a feasible and effective resource utilization strategy.

Optimization and kinetic study of nitrate removal from aqueous solution using tea waste

AbstractBACKGROUNDNitrate contamination in groundwater poses significant risks to public health and the environment worldwide. This study explores the optimized use of tea waste, a byproduct of tea extraction and consumption, as an adsorbent for the removal of nitrate from aqueous solutions.RESULTSScanning electron microscopy and Brunauer–Emmett–Teller (BET) analysis revealed that tea waste has irregular cluster cavities and visible pore formations, indicating numerous active sites for nitrate adsorption. The specific surface area, total pore volume, and pore diameter were found to be 2.4914 m2 g−1, 0.0036 cm3 g−1, and 9.8077 nm, respectively. A total of 20 adsorption experiments were conducted based on conditions recommended by a statistical tool using Design‐Expert 13.0 software. The data showed a strong fit to a quadratic model, with a significant P‐value of 0.0109 and a high determination coefficient (R2 = 0.9327). Using response surface methodology, the study achieved a maximum nitrate removal efficiency of 84.49% under optimal conditions, which included an initial nitrate concentration of 50 mg L−1, an adsorbent weight of 0.706 g, and a contact time of 15 min. Further analysis of adsorption isotherms and kinetics revealed that the Freundlich isotherm (R2 = 0.9556) and the pseudo‐second‐order kinetic model (R2 = 0.9454) best described the experimental data.CONCLUSIONThese findings indicate that tea waste is an effective and sustainable adsorbent for mitigating nitrate contamination in aqueous solutions, offering a promising approach to address water quality challenges globally. © 2024 Society of Chemical Industry (SCI).

Simultaneous removal of heavy metals and inorganic nitrogen by using the biofilm of Marichromatium gracile YL28.

Heavy metal and nitrogen contaminations are serious concerns in aquatic environments. Marichromatium gracile YL28, a marine purple sulfur bacterium, has shown great potential as a bioremediation agent for removing inorganic nitrogen from marine water. This study further investigated its ability to simultaneously absorb heavy metals, including Pb(II), Cu(II), Cd(II) and Cr(VI), and remove inorganic nitrogen. The contributions of photopigment and extracellular polymeric substances (EPS) in the YL28 biofilm to heavy metal adsorption and tolerance were also evaluated. The YL28 biofilm demonstrated higher adsorption efficiencies for heavy metal ions than planktonic cells. A high level of EPS was detected in the biofilm. The effects of four heavy metal on the inhibition of photopigment synthesis showed that high concentrations of Cu(II) greatly inhibited the production of BChl a and Car. The adsorption efficiencies of Pb(II), Cu(II), Cd(II), and Cr(VI) in the YL28 biofilm reactor reached 86.59%, 72.94%, 80.06%, and 95.95%, respectively. Elevated concentrations of heavy metal ions only marginally impeded ammonia nitrogen removal; they impacted neither nitrite and nitrate removals nor hindered the simultaneous elimination of three inorganic nitrogen compounds. Coupled with their ability to remove inorganic nitrogen, the high adsorption capacity and tolerance of YL28 biofilms toward heavy metal suggest a promising solution for mitigating metal pollutants.

Optimized Mn cycle enhanced synchronous removal of nitrate and antibiotics driven by manganese oxides/solid carbon composites: Microbiota assembly patterns and electron transport.

The reactive substance consisting manganese oxides (MnOx) and solid carbon have been reported to be effective in polishing secondary wastewater; however, the treatment characteristics and mechanism remains limited. In this study, MnOx/carbon (Mn-C) composites were applied in biofilters to evaluate simultaneous removal of nitrate and sulfamethoxazole (SMX), with the single carbon composites as control. Results showed that the effluent concentrations of NO3--N and SMX were below 2.87 mg L-1 and 7.97 μg L-1 under hydraulic retention time (HRT) of 6 h. The intermittent aeration optimized Mn cycle with treatment performance improved under lower HRT and Mn(II) accumulation decreased. Mn-C composites could reduce the emission of N2O, CO2 and CH4. The dominant genera gradually evolved from fermentation to glycogen aggregation, and from heterotrophic/sulfur autotrophic to heterotrophic denitrifiers by intracellular substance and manganese autotrophic/heterotrophic bacteria. Microbial network analysis indicated higher antagonism, lower modularity and shorter average path among microbes in Mn-C biofilters, which highlighted microbial differentiation and faster electron transfer. Improved functions of denitrification and Mn respiration, and the increasing genes encoding electron transfer chain, including NADH dehydrogenase, Cytc and ubiquinone, further elucidated the superiority of Mn-C composites. These results improved our understanding of Mn-C composites application in low-carbon wastewater treatment.

Sustainable management of riverine N2O emission baselines

Abstract The riverine N2O fluxes are assumed to linearly increase with nitrate loading. However, this linear relationship with a uniform EF5r is poorly constrained, which impedes the N2O estimation and mitigation. Our meta-analysis discovered a universal N2O emission baseline (EF5r = k/[NO3−], k = 0.02) for natural rivers. Anthropogenic impacts caused an overall increase in baselines and the emergence of hotspots, which constitute two typical patterns of anthropogenic sources. The k values of agricultural and urban rivers increased to 0.09 and 0.05, respectively, with 11% and 14% of points becoming N2O hotspots. Priority control of organic and NH4+ pollution could eliminate hotspots and reduce emissions by 51.6% and 63.7%, respectively. Further restoration of baseline emissions on nitrate removal is a long-term challenge considering population growth and declining unit benefits (ΔN-N2O/N-NO3−). The discovery of EF-lines emphasized the importance of targeting hotspots and managing baseline emissions sustainably to balance social and environmental benefits.

Hybrid nanofiltration–electrodialysis system efficiency in nitrate removal: optimization by RSM and validation

Hybrid nanofiltration–electrodialysis system efficiency in nitrate removal: optimization by RSM and validation

Bacterial bioaugmentation of woodchip bioreactors to increase nitrate removal in cold agricultural drainage water

ABSTRACT Woodchip bioreactors (WBRs) are biological systems designed to prevent excess nitrate (NO3 −) leaching from agricultural fields to aquatic ecosystems. Nitrate is removed by microbial denitrification, but the enzyme-mediated process slows down at cold temperatures (<10°C), where NO3 − removal in WBRs can be less than 20%. We studied the use of bacterial bioaugmentation in replicated test-scale WBRs (∼0.1 m3) as an environmental technology to increase NO3 − removal at cold temperatures. Nitrate removal rates increased following injection of a nitrate-reducing inoculum (Pseudomonas proteolytica and Klebsiella sp.), but the effect disappeared within a week and was reproduced in control WBRs by injection of sterile medium (phosphate buffer saline). Metagenome analyses showed a shift in the bacterial community composition after bioaugmentation in the planktonic phase of the woodchip reactors, but not in the solid phase (woodchip matrix). Only in the planktonic phase, Pseudomonas and Klebsiella increased their relative abundance as monitored by 16S rRNA gene sequences. In addition, an increased abundance of genes related to NO3 − transformation after bacterial inoculation was observed in the metagenomic sequences. After one week, bacterial community composition became similar to its initial state, indicating resilience of the WBR microbial communities. We conclude that improved inoculation methods are needed to unlock the potential of bioaugmentation to increase NO3 − removal at cold temperatures and make it a relevant technology for practical use at field-scale.

Insights into the influence of organic and salinity on the two-stage partial nitritation/anammox process in treating food waste digestate

ABSTRACT Food waste digestate (FWD), which contains significant levels of ammonium, organic matter, and salinity, can interfere with treatment performance of the anammox process. In this study, a two-stage partial nitritation/anammox (PN/A) process was established to investigate nitrogen removal and microbial response in treating FWD at a nitrogen loading rate (NLR) of 0.27 ± 0.02 gN/L/d. High concentrations of free ammonia (58 mg/L) and free nitrous acid (0.3 mg/L) facilitated the initiation of the partial nitritation (PN) process, achieving an average NO2 −/NH4 + ratio of 1.28 ± 0.08. For the anammox process, a nitrogen removal rate (NRR) of 0.72 ± 0.13 gN/L/d was achieved. Free ammonia (NH3) stripping, Anammox pathway, and denitrification pathway contributed 4.1 ± 0.3%, 5.1 ± 0.2%, and 84.0 ± 1.5% of the total nitrogen removal, respectively. Nitrosomonas, a salt-tolerant ammonia-oxidizing bacteria (AOB), was enriched to 1.0%, while Nitrospira, a nitrite-oxidizing bacteria (NOB), was effectively suppressed to 0.003%. The salt-tolerant anammox genera unclassified_f__Brocadiaceae (13.9%) and Candidatus_Kuenenia (4.8%) dominated the nitrogen removal pathway. The high enrichment of unclassified_f__Brocadiaceae ensured stable operation of the anammox process at 0.62 ± 0.11% salinity, even with a high initial FA inhibition concentration of 40 mg/L. Additionally, norank_f_A4b (1.34%) and norank_f_norank_o_SBR1031 (52.1%) facilitated the hydrolysis of refractory organic matter. Denitrifying bacteria, including Hyphomicrobium, Truepera, and unclassified_c__Alphaproteobacteria, played significant roles in nitrate removal, with a CODconsumed/NO3 − removed ratio of 2.7 ± 0.2. This study highlights the application of a two-stage PN/A process for rapid startup and effective nitrogen removal from FWD.

Simultaneous devulcanization and denitrification: a novel approach for valorization of both ground tire rubber and nitrate-containing wastewater.

This study focused on a new approach for valorization of both ground tire rubber (GTR) and nitrate-containing wastewater via simultaneous devulcanization and denitrification. Initially, sulfur-based autotrophic denitrifiers were successfully enriched from three different seed sludge sources, biological nutrient removal (BNR) sludge, anaerobic digester sludge and BNR sludge of a leather organized industrial zone WWTP. Average nitrate removal efficiencies were 96-98%. Biological devulcanization (biodevulcanization) of GTR was later investigated with these enriched cultures. Results revealed that biodevulcanization was only achieved with the culture enriched from BNR sludge of the leather organized industrial zone WWTP, as 3.9% sulfur removal (desulfurization efficiency). Metal sulfate precipitation was speculated to cause an underestimation of the desulfurization ratio. Fourier-transform infrared spectroscopy (FTIR) results demonstrated a decrease in the intensity of the C-S bonds and an increase in intensity of S-O bonds in treated GTR samples. This was attributed to the oxidation of sulfidic crosslinks, i.e. verification of biodevulcanization. This study indicated that simultaneous biodevulcanization and denitrification could be a promising process for valorization of both GTR and nitrate-containing wastewater which in turn would support circular economy and sustainable development.

Methanol and thiosulphate from pulp mill waste streams support denitrification under salinity stress

Methanol and thiosulphate are two common compounds found in pulp mill environment. In this study, for the first time, methanol driven, and thiosulphate driven denitrification under salinity stress were studied in fluidized bed bioreactors (FBBR) in a comparative manner. Concentration of salinity, due to Cl-, Na+, SO42- and K+ that are present in saline industrial wastewaters, and nitrate loading rate were gradually increased up to 8.1 % and 4.67 ± 0.30 gNO3--N/L/d to determine the reactors’ resistance and denitrification rate capacity. Moreover, the suitability of real pulp mill methanol-rich foul condensate as alternative electron donor was revealed in the heterotrophic reactor. With foul condensate, heterotrophic denitrification maintained > 97 % nitrate removal at a rate of 2.40 ± 0.10 gNO3--N/L/d under 6.8 % salinity, also decreasing nitrite accumulation as compared to pure methanol. Thiosulphate driven autotrophic denitrification showed lower salinity and nitrate loading tolerance than the methanol driven, removing nitrate > 97 % at salinity up to 5.6 % and rate of 1.27 ± 0.10 gNO3--N/L/d. Microbial community composition in the heterotrophic reactor remained stable at different salinities, with denitrifying genera Methylovorus, Paracoccus and Pseudomonas playing the major roles. On the other hand, in the thiosulphate driven reactor, inoculated with a pure culture of Thiobacillus denitrificans, many other microorganisms, including Moheibacter, became enriched. However, nitrate removal was mainly linked with T. denitrificans abundance as indicated by decreasing share of the community co-occurring with the decrease of the reactors’ performance. In summary, this study showed that thiosulphate and foul condensate supported denitrification under salinity stress, indicating potential for high-rate fluidized bed pre-treatment for nitrate removal from pulp mill NOx-SO2 scrubber wastewater prior to direction to subsequent activated sludge process.

Efficient nitrate removal via microorganism-iron oxide co-evolution on biocathode surface.

Sediment microbial fuel cell (SMFC) is a device for biological denitrification, in which electrons produced by sediment microorganisms can be transferred to the upper layer of the water column lacking electron donors. However, the low efficiency of denitrifying bacteria in acquiring electrons and enriching at the cathode greatly hinders the application of SMFC for nitrogen removal. In this study, we report a novel method of constructing a high-performance biocathode by modifying electrodes with zero-valent iron to enhance the enrichment and electron transfer of electroactive bacteria. The surface chemical and biological analysis of the biocathode revealed that the ZVI gradually oxidized to form magnetite and goethite, and finally stabilized into better crystallized lepidocrocite. On the other hand, the microbial community of the biocathode gradually evolved into a community dominated by denitrifying bacteria, specifically Clostridium. The co-evolved "Clostridium-lepidocrocite" composite endows the sediment microbial fuel cell with a 99% nitrate removal capacity. These results indicate that the cathode constructed by using ZVI modified electrode achieves efficient nitrate reduction by denitrifying bacteria. Furthermore, the construction method of biocathode may also have the potential application in water remediation and the geochemical cycling of elements.

Effective removal of nitrate and phosphate ions from water using nickel-doped calcium alginate beads

Effective removal of nitrate and phosphate ions from water using nickel-doped calcium alginate beads

A mixotrophic denitrification composite for treatment of nitrate-contaminated water: Performance, microbial community and functional genes

A mixotrophic denitrification composite was prepared with sulfur, pyrite, sawdust and calcium carbonate (SPSC) for the treatment of nitrate-contaminated wastewater. As a homogeneous integrated composite, fast mass transfer, convenient operation and transportation and easy attachment for microorganisms were obtained in the SPSC-based denitrification system. The long-term operation result showed that SPSC bioreactor exhibited strong resistance to the fluctuations of influent nitrate load, and 98.3% of the nitrate in the influent was reduced even under low hydraulic retention time (HRT). The SPSC system achieved an average nitrate removal rate of 4.59 mg-N/L/h, which was nearly triple the rate of sulfur autotrophic denitrification (SAD) system that utilized sulfur as its sole electron donor. The sulfate yield of the SPSC system (4.52mg SO42-/ mg NO3--N) was significantly lower than those of the SAD system (9.36mg SO42-/ mg NO3--N). Heterotrophic and autotrophic denitrifiers coexisted in the SPSC system, which was beneficial for the denitrification process. Metagenomic analysis indicated that the SPSC composite enriched the functional genes, which encoding key enzymes (nitrate reduction, sulfur oxidation and energy generation) involved in nitrogen, sulfur and carbon metabolism. Collectively, the SPSC composite synthesized in this study has provided a high efficiency, low-cost and promising technology for the treatment of nitrate-contaminated water.

Biological nitrogen fixation driven by methane anaerobic oxidation supports the complex biological communities in cold-seep habitat

Cold seeps represent a class of chemosynthetic ecosystems that are prevalent in deep-sea environments. Despite extensive research on the functional microbiology of cold seeps, it remains unclear how methane oxidation is coupled with the nitrogen and sulfur cycles in the reduced sediment habitats across various spatial scales. Here, we employed metagenomic sequencing to investigate the microbial communities within the Haima cold seep sediments, with particular attention to microorganisms involved in biogeochemical cycles at varying spatial scales. In the surface layer of regular sediment areas, the sulfur-oxidizing bacteria such as Chromatiates, PS1, SZUA-229, and GCF-002020875 were the most numerically abundant groups. These bacteria recycle sulfide and generate sulfate while facilitating nitrate removal to support the methane anaerobic oxidation in the subsurface layer. However, in the biogenous sediments of cold seep ecosystems, methane anaerobic oxidation and nitrogen fixation emerge as the predominant processes. The microbial coupling groups, including sulfate reducing bacteria C00003060 and ANME-1, utilize energy from methane anaerobic oxidation to complete the nitrogen fixation. These findings suggest that cold-seep diazotrophs can support complex biological communities by supplying fixed nitrogen. This study enhances our understanding of the functional microbial structure and metabolic capabilities within cold seep ecosystems.