The uptake and degradation of polychlorinated biphenyls in constructed wetlands planted with Myriophyllum aquaticum

The unregulated dismantling and improper disposal of electronic waste lead to severe soil contamination by polychlorinated biphenyls (PCBs). Constructed wetlands (CWs) play an important role in PCBs removal as a result of the co-existence of anaerobic and aerobic conditions. However, the effects and mechanisms of different PCBs concentrations in soils on plant uptake and PCBs degradation within CWs are unclear. We evaluated the uptake and degradation of PCBs at different concentrations by Myriophyllum aquaticum (Vell.) Verdc. Planting significantly increased PCBs removal by 8.70% (p < 0.05) in soils with 1500 and 2500 μg/kg PCBs, whereas no significant effect was observed at 500 and 1000 μg/kg. PCBs levels did not significantly affect plant growth and PCBs accumulation. The contribution of plant uptake to PCBs removal was only 0.10-0.12%, indicating that microbial degradation was the dominant pathway for PCBs removal after planting with M. aquaticum. In the treatments with PCBs ≥ 1500 μg/kg, M. aquaticum increased the microbial population, altered the microbial community structure and enriched PCB-degrading bacteria. Functional prediction revealed that microbes in M. aquaticum rhizosphere secreted more peroxidase and glycosyltransferase than non-plant control, which were likely involved in PCBs metabolism.

A novel mechanism of enhanced PCBs degradation associated with nitrogen in the rhizosphere of the wetland plant Myriophyllum aquaticum

Co-contamination of polychlorinated biphenyls (PCBs) and nitrogen (N) is widespread. Here, N removal and PCBs degradation were investigated in constructed wetlands populated with Myriophyllum aquaticum, and the role of N in PCBs degradation was explored as well. Nearly 97% of N was removed in the planted system, whereas less than 40% was removed in the plant-free system. Compared to the treatment with plants and no N amendment, N addition enhanced plant growth by 31.9% and PCBs removal by 9.90%. PCBs attenuation was mainly attributed to microbial degradation rather than plant uptake. Using DNA stable-isotope probing, 26 operational taxonomic units were identified across all treatments, of which 25 were linked to PCBs degradation for the first time. Some PCB-degraders were associated with nitrification/denitrification and were significantly enriched in the treatment that included both plants and N application, indicating that PCBs degradation was promoted by recruiting ammonia-oxidising and denitrifying microbes with PCBs metabolic ability. This was confirmed by the higher A13/A12 ratios for the bphC, amoA, and nirK genes and their significant positive correlations. Overall, the findings clarify the novel mechanism by which N promotes PCBs degradation in constructed wetlands and offers a theoretical basis for efficiently removing inorganic elements and persistent organic pollutants.

Effect of plant-self debris on nitrogen removal, transformation and microbial community in mesocosm constructed wetlands planted with Myriophyllum aquaticum

Aquatic macrophytes debris decomposition could release pollutants and nutrients into the water of constructed wetlands (CWs), but their role in nitrogen removal and transformation remains poorly understood. The present study investigated the effects of plant-self debris on nitrogen removal and microbial communities in mesocosm CWs planted with Myriophyllum aquaticum. During the 68-day operation, the plant debris addition did not change the mean removal efficiency of ammonium (NH₄⁺-N) and total nitrogen (TN) of CWs but showed significant differences over the operation time. The NH₄⁺-N and organic nitrogen released from the plant debris decomposition affected the nitrogen removal. The plant debris decreased the effluent nitrate concentration and N₂O emission fluxes of the CWs with the increased abundance of denitrifying bacterial genera, indicating that plant debris decomposition increased the denitrification activities via dissolved organic carbon release. High-throughput sequencing indicated that the plant debris altered the distribution and composition of the microbial community in the sediments. Proteobacteria was the dominant phylum (28-52%), and the relative abundance of denitrifying bacteria genera was significantly higher in the sediments with debris addition (37-40%) than in the non-addition (6.6-7.7%). The present study provided new insights into the role of macrophytes in pollutant removal and the plant management strategy of CWs.

Uptake, accumulation, and translocation of organophosphate esters by watermifoil (Myriophyllum aquaticum) in an aquatic ecosystem: effects of chemical structure and concentrations

In order to explore the environmental behavior of organophosphate esters (OPEs) in aquatic environment, the accumulation and distribution of OPEs in water, sediment, and plant were investigated. In this study, watermifoil (Myriophyllum aquaticum) were exposed with ten OPEs for concentrations of 200 ng/g, 500 ng/g, 1000 ng/g, and 2000 ng/g, respectively. The concentrations of Σ₁₀OPEs in rhizosphere sediment were higher than those in non-rhizosphere sediment, demonstrating that rhizosphere processes tend to transport OPEs into the rhizosphere sediment. Most of the selected OPEs were not in equilibrium between water and sediment, and trend to retain in sediment. In addition, OPEs with relatively higher hydrophobicity had trend to retained in Myriophyllum aquaticum roots, whereas OPEs with lower hydrophobicity were more likely transported to shoots. In this study, octanol-water partition coefficient (K_OW) had significantly positive correlations with organic carbon-normalized soil-water partition coefficients (K_OC) and root-water concentration factors (RWCFs), but K_OW was negatively correlated with translocation factors (TFs). Moreover, the substituent types and initial levels of OPEs also have impacts on the plant uptake and accumulation. These observations will improve our understanding of the distribution and translocation of OPEs in aquatic environment.

Transcriptomics Insights into Phosphorus Stress Response of Myriophyllum aquaticum

Through excellent absorption and transformation, the macrophyte Myriophyllum (M.) aquaticum can considerably remove phosphorus from wastewater. The results of changes in growth rate, chlorophyll content, and roots number and length showed that M. aquaticum could cope better with high phosphorus stress compared with low phosphorus stress. Transcriptome and differentially expressed genes (DEGs) analyses revealed that, when exposed to phosphorus stresses at various concentrations, the roots were more active than the leaves, with more DEGs regulated. M. aquaticum also showed different gene expression and pathway regulatory patterns when exposed to low phosphorus and high phosphorus stresses. M. aquaticum's capacity to cope with phosphorus stress was maybe due to its improved ability to regulate metabolic pathways such as photosynthesis, oxidative stress reduction, phosphorus metabolism, signal transduction, secondary metabolites biosynthesis, and energy metabolism. In general, M. aquaticum has a complex and interconnected regulatory network that deals efficiently with phosphorus stress to varying degrees. This is the first time that the mechanisms of M. aquaticum in sustaining phosphorus stress have been fully examined at the transcriptome level using high-throughput sequencing analysis, which may indicate the direction of follow-up research and have some guiding value for its future applications.

Glutamine synthetase plays an important role in ammonium tolerance of Myriophyllum aquaticum

High-strength ammonium (NH₄⁺), the main characteristic of swine wastewater, poses a significant threat to the rural ecological environment. As a novel phytoremediation technology, Myriophyllum aquaticum wetlands have high tolerance and removal rate of NH₄⁺. Glutamine synthetase (GS), a pivotal enzyme in nitrogen (N) metabolism, is hypothesized to play an important role in the tolerance of M. aquaticum to high NH₄⁺. Herein, the responses of M. aquaticum to GS inhibition by 0.1 mM methionine sulfoximine (MSX) under 15 mM NH₄⁺ were investigated. After 5 days, visible NH₄⁺ toxicity symptoms were observed in MSX-treated plants. Compared with the control, the NH₄⁺ accumulation in the leaves increased by 20.99 times, while that of stems and roots increased by 3.27 times and 47.76 %, suggesting that GS inhibition had a greater impact on the leaves. GS inhibition decreased pigments in the leaves by 8.64 %-41.06 %, triggered oxidative stress, and affected ions concentrations in M. aquaticum. The concentrations of glutamine (Gln) and asparagine decreased by 63.46 %-97.43 % and 12.37 %-76.41 %, respectively, while the concentrations of most other amino acids increased after 5 days of MSX treatment, showing that GS inhibition reprogrammed the amino acids synthesis. A decrease in Gln explains the regulations of N-related genes, including increased expression of AMT in roots and decreased expression of GS, GOGAT, GDH, and AS, which would cause further NH₄⁺ accumulation via promoting NH₄⁺ uptake and decreasing NH₄⁺ assimilation in M. aquaticum. This study revealed for the first time that GS inhibition under high NH₄⁺ condition can lead to phytotoxicity in M. aquaticum due to NH₄⁺ accumulation. The physiological and molecular responses of the leaves, stems, and roots confirmed the importance of GS in the high NH₄⁺ tolerance of M. aquaticum. These findings provide new insights into NH₄⁺ tolerance mechanisms in M. aquaticum and a theoretical foundation for the phytoremediation of high NH₄⁺-loaded swine wastewater.

Partial nitrification in free nitrous acid-treated sediment planting Myriophyllum aquaticum constructed wetland strengthens the treatment of black-odor water

Black-odor water pollution in rural areas, especially swine wastewater, can lead to the deterioration of water quality and thus seriously affect the daily life of people in the area. However, there is a lack of effective treatment measures with simultaneous attention to carbon, nitrogen and sulfur pollution in rural black-odor water bodies. This study evaluated the feasibility of an in-situ pilot-scale constructed wetland (CW) for the synchronous removal of COD, ammonium, and sulfur compounds in the swine wastewater. In this study, the operation strategy of CW sediment pretreated with free nitrous acid (FNA) and Myriophyllum aquaticum plantation was established. Throughout the 114-day operation, the total removal efficiencies of COD and ammonium nitrogen in experimental groups were 81.2 ± 4.2 % and 72.8 ± 1.8 %, respectively, which were significantly higher than CW without any treatment. Removal efficiencies of Sulfur compounds, i.e. sulfide, sulfate, thiosulfate, and sulfite, were 92.3 ± 1.9 % (61.2 % higher than the no-treatment group), 42.1 ± 3.8 %, 97.9 ± 1.7 %, and 42.7 ± 4.5 % respectively. High-throughput sequencing and qPCR revealed that experimental group significantly increased denitrification genes (nirK, nosZ) and sulfur oxidation genes (soxB, fccAB) and enriched the corresponding microbial taxa (Bacillus, Conexibacter and Clostridium sensu stricto). Moreover, metabolic pathways related to nitrogen and sulfur and the degradation of organic matter were up-regulated. These results indicated that partial nitrification in CW planted with M. aquaticum promoted sulfur oxidation denitrification and heterotrophic denitrification. Overall, the in-situ pilot-scale study revealed that the cultivation of M. aquaticum in FNA-treated CW can be a sustainable approach to treat black-odor water bodies.

Physiological response and tolerance of Myriophyllum aquaticum to a wide range of ammonium concentrations

Myriophyllum aquaticum (M. aquaticum) can be used in constructed wetlands (CWs) to effectively purify swine wastewater with high-ammonia nitrogen (NH₃-N and NH₄⁺-N) concentrations. However, the understanding of its tolerance mechanism to ammonia nitrogen is limited. The physiological response and tolerance mechanism of M. aquaticum to a wide range of NH₄⁺ concentrations (0-35 mM) were investigated in the present study. The results indicated that M. aquaticum can tolerate NH₄⁺ concentrations of up to 30 mM for 21 days and grow well with high nutrient (N, P) uptake. A suitable concentration of NH₄⁺ for a better growth of M. aquaticum was 0.5-20 mM. The free NH₄⁺ content was no obviously increase at NH₄⁺ concentration below 15 mM, indicated there was no obviously ammonium accumulation. Exogenous NH₄⁺ inhibited K⁺ absorption and improved Ca²⁺ absorption, indicating mineral cation could mediate NH₄⁺ homeostasis under NH₄⁺ stress. Moreover, comparison with those in the control group, the activities of glutamine synthetase (GS), glutamate synthetase (GOGAT) in M. aquaticum increased by 52.7%-115% at 1-20 mM NH₄⁺, and superoxide dismutase (SOD) increased by 29.2-143% at 1-35 mM NH₄⁺. This indicated that the high NH₄⁺ tolerance of M. aquaticum was mainly due to the balance of free NH₄⁺ content in tissues, as well as improved nitrogen metabolism and antioxidant system. This could be attributed to the role of the GS-GOGAT cycle and SOD. In conclusion, M. aquaticum, which tolerates high NH₄⁺ concentration and has a high N uptake ability, can be used as a good candidate specie to help develop more efficient management strategies for treating high-NH₄⁺ wastewater in CW systems.

Enhancing nitrogen removal from anaerobically-digested swine wastewater through integration of Myriophyllum aquaticum and free nitrous acid-based technology in a constructed wetland

Despite of low operation costs and convenient maintenance, the application of natural systems for swine wastewater treatment has been limited by large construction area and unsatisfactory effluent quality. Introducing ammonium high uptake aquatic plants and shifting nitrogen removal pathway from nitrate to nitrite in constructed wetlands (CWs) has been regarded as promising approach to promote their performances. This study aimed to establish nitrite pathway and enhance N removal via free nitrous acid (FNA)-sediment treatment and Myriophyllum aquaticum vegetation in the CWs treating anaerobically digested swine wastewater. Nitrite pathway was successfully and stably achieved in the M. aquaticum CW with FNA-treated sediment. The overall removal efficiencies of ammonium nitrogen and total nitrogen were 42.3 ± 10.2% and 37.7 ± 9.3% in the planted CWs with FNA-treated sediment, which were 76.3% and 65.4% higher than those in the conventional oxidation pond system, respectively. Microbial community analysis (qPCR and metagenomics) suggested that the nitrite pathway established through FNA-sediment treatment was based on the inactivation of nitrite oxidizing bacteria (lower nxrA gene abundance) and the reduction of relative abundances of NOB (especially Nitrobacter and Nitrospira). During the denitrification processes, the integration of M. aquaticum vegetation with FNA-sediment treatment can lower the nitrate reduction by decreasing narG gene abundances and decreasing the relative abundances of napA affiliated bacteria (especially Bradyrhizobium), while strengthening reduction of nitrite and nitrous oxide by increasing nirK and nosZ gene abundances and enriching the corresponding affiliated microbial taxa, Mycobacterium and Bacillus, respectively. Our findings suggest that applying FNA-based technology in CW systems is technically and economically feasible, which holds promise for upgrading current CW systems treating swine wastewater to meet future water quality requirements.

Removal effects of Myriophyllum aquaticum on combined pollutants of nutrients and heavy metals in simulated swine wastewater in summer

Swine wastewater (SW) treatment by Myriophyllum aquaticum is an important biotechnology for its resource utilization. However, some knowledge gaps remain in compound-pollutant removal in SW, especially in practical applications. To clarify the responses of M. aquaticum to the compound pollutants as well as the related operational parameters in SW treatment, three initial doses (0.5, 1.0, and 1.5 kg per pond in 150 L simulated SW) of M. aquaticum and a control (no plant; CK) were allocated to 12 ponds under a plastic roof in Nanjing city of Eastern China during 75 days in the summer of 2019. Results showed that M. aquaticum could be used as a pioneer plant to efficiently remove compounded pollutants of nitrogen (N), phosphorus (P), and especially for heavy metals in simulated SW. Compared with CK, M. aquaticum assisted in improving the total N, NH₄⁺-N, NO₃^--N, NO₂^--N, and dissolved organic N by 30.1%, 100%, 100%, 97.6%, 20.2%, 39.8% whereas Cu, Zn, and Cd by 50.4%, 36.4% and 47.9% on average during the 75-day experiment in summer, respectively. Moreover, concentrations of Cu and Cd at day 75 were in the ranges of 1.92-2.82 and 0.64-1.47 g kg^-1 DW, respectively, exceeding the corresponding limits of the heavy-metal hyperaccumulator. For the operational parameters, the optimized initial dose was 1.0 kg per pond with M. aquaticum harvested after 45 summer days, respectively. Given that M. aquaticum has been widely used as animal feed in recent years and limit values for Cu and Zn in animal feed are not set in China, the toxicities of Cu and Zn should be assessed and the guideline of their limit values needs to be established for safe feed production. Interestingly, NH₄⁺-N could dominate the removal of heavy metals especially Cd in the simulated SW, however, related mechanisms are needed for further study.

Microbial communities responded to tetracyclines and Cu(II) in constructed wetlands microcosms with Myriophyllum aquaticum

Combined antibiotic and heavy metal pollution has generated considerable concern. Constructed wetlands (CWs) have been shown to efficiently remove pollutants; however, the microbial community responses to combined pollutants remain enigmatic. In this study, seven microcosm CWs were planted with Myriophyllum aquaticum, spiked with tetracyclines (TCs) (300-30,000 μg/L), alone or with Cu(II), to investigate the response of plant-associated microbial communities. TCs and the Cu/TC ratio greatly affected the performance of CWs. Tetracyclines led to higher microbial diversity, evenness and richness, while UniFrac distances and principal coordinate (PC_O) and redundancy analyses revealed that the co-presence of TCs and Cu(II) led to variations in bacterial communities. Proteobacteria, Cyanobacteria and Bacteroidetes were the dominant microbial phyla and Cloacibacterium, Hydrogenophaga, Rheinheimera and Denitratisoma accounted for 6.2-21.0% of all genera. Therefore, the co-occurrence of heavy metals should be considered when judging the removal potential of TCs in phytoremediation.

Cadmium tolerance and detoxification in Myriophyllum aquaticum: physiological responses, chemical forms, and subcellular distribution

Submerged macrophytes have been found to be promising in removing cadmium (Cd) from aquatic ecosystems; however, the mechanism of Cd detoxification in these plants is still poorly understood. In the present study, Cd chemical forms and subcellular distributing behaviors in Myriophyllum aquaticum and the physiological mechanism underlying M. aquaticum in response to Cd stress were explored. During the study, M. aquaticum was grown in a hydroponic system and was treated under different concentrations of Cd (0, 0.01, 0.05, 0.25, and 1.25 mg/L) for 14 days. The differential centrifugation suggested that most Cd was split in the soluble fraction (57.40-66.25%) and bound to the cell wall (24.92-38.57%). Furthermore, Cd in M. aquaticum was primarily present in NaCl-extractable Cd (51.76-91.15% in leaves and 58.71-84.76% in stems), followed by acetic acid-extractable Cd (5.17-22.42% in leaves and 9.54-16.56% in stems) and HCl-extractable Cd (0.80-12.23% in leaves and 3.56-18.87% in stems). The malondialdehyde (MDA) and hydrogen peroxide (H₂O₂) concentrations in M. aquaticum were noticeably increased under each Cd concentration. The activities of catalase (CAT), guaiacol peroxidase (POD), and superoxide dismutase (SOD) in leaves were initially increased under relatively low concentrations of Cd but were decreased further with the increasing concentrations of Cd. The ascorbate (AsA), glutathione (GSH), and nitric oxide (NO) concentrations in stems increased with increasing Cd concentrations. Taken together, our results indicate that M. aquaticum can be used successfully for phytoremediation of Cd-contaminated water, and the detoxification mechanisms in M. aquaticum include enzymatic and non-enzymatic antioxidants, subcellular partitioning, and the formation of different chemical forms of Cd.

Potential of Myriophyllum aquaticum for phytoremediation of water contaminated with tetracycline antibiotics and copper

Water pollution caused by antibiotics and heavy metals has attracted considerable concern, and efficient approaches are urgently needed for their removal. The objective of this study was to investigate the potential of Myriophyllum aquaticum for long-term phytoremediation of wastewater containing tetracycline (TC) antibiotics and copper. Seven hydroponic microcosms were constructed, spiked with tetracycline, oxytetracycline (OTC) and chlortetracycline (CTC) (300-30,000 μg/L), alone or simultaneously with Cu (II), and operated for 12 weeks. The TC removal efficiencies using the hydroponic microcosms here were commensurate or higher than those in previous studies. However, the Cu/TC ratio greatly affected the removal, accumulation of TCs by M. aquaticum, and plant growth. Low levels of Cu (II) (<1000 μg/L) promoted TC removal, but excessive Cu (II) (>10,000 μg/L) impeded it. Mass balance analysis showed that most TCs (45%-64% on average) accumulated in the roots of M. aquaticum. Plant biomass was correlated with the removal of COD, TN, TP, and NH₄⁺-N (p ≤ 0.05) but not with removal of the TCs. Proteobacteria, Bacteroidetes, and Fusobacteria were dominant in the microbial communities, but they showed little correlation with the TC removal. M. aquaticum can be employed as an effective means of TC removal from water. The co-existence of heavy metals should be considered when evaluating the removal potential of TCs in phytoremediation.

Effect of ammonia stress on carbon metabolism in tolerant aquatic plant-Myriophyllum aquaticum

In this study, the tips of Myriophyllum aquaticum (M. aquaticum) plants were planted in open-top plastic bins and treated by simulated wastewater with various ammonium-N concentrations for three weeks. The contents of related carbohydrates and key enzyme activities of carbon metabolism were measured, and the mechanisms of carbon metabolism regulation of the ammonia tolerant plant M. aquaticum under different ammonium-N levels were investigated. The decrease in total nonstructural carbohydrates, soluble sugars, sucrose, fructose, reducing sugar and starch content of M. aquaticum were induced after treatment with ammonium-N during the entire stress process. This finding showed that M. aquaticum consumed a lot of carbohydrates to provide energy during the detoxification process of ammonia nitrogen. Moreover, ammonia-N treatment led to the increase in the activitives of invertase (INV) and sucrose synthase (SS), which contributed to breaking down more sucrose to provide substance and energy for plant cells. Meanwhile, the sucrose phosphate synthase (SPS) activity was also enhanced under stress of high concentrations of ammonium-N, especially on day 21. The result indicated that under high-concentration ammonium-N stress, SPS activity can be significantly stimulated by regulating carbon metabolism of M. aquaticum, thereby accumulating sucrose in the plant body. Taken together, M. aquaticum can regulate the transformation of related carbohydrates in vivo by highly efficient expression of INV, SPS and SS, and effectively regulate the osmotic potential, thereby delaying the toxicity of ammonia nitrogen and improving the resistance to stress. It is very important to study carbon metabolism under ammonia stress to understand the ammonia nitrogen tolerance mechanism of M. aquaticum.

Evaluation of cadmium hyperaccumulation and tolerance potential of Myriophyllum aquaticum

Enrichment of the hyperaccumulator bank is important for phytoremediation, and studying new hyperaccumulators has become a research hotspot. In this study, cadmium (Cd), the main representative factor of heavy-metal-polluted water, was the research object, and the Cd bioenrichment ability and tolerance of Myriophyllum aquaticum were studied for the first time. The experiment was conducted for 28 days by establishing experimental groups with different Cd concentrations (0, 10, 20, 40, 80, and 160 mg/L). The results show that M. aquaticum is a new Cd hyperaccumulator. There was no notable damage in the 40 mg/L Cd treatment group, and the Cd enrichment ability of M. aquaticum reached 17,970 ± 1020.01 mg/kg, while the bioconcentration factor (BCF) reached 449.25. At the same time, the antioxidant system (superoxide dismutase (SOD) and peroxidase (POD)) and proline (Pro) levels of M. aquaticum maintained normal plant physiology, but there were physiological anomalies in M. aquaticum at high concentrations and under long-term treatment. The results show that M. aquaticum has a high Cd bioenrichment ability and tolerance in water and can be used for phytoremediation of river water polluted by Cd.

Responses of the growth and physiological characteristics of Myriophyllum aquaticum to coexisting tetracyclines and copper in constructed wetland microcosms

Antibiotic and heavy metal pollution of aquatic environments are issues of serious concern, and the macrophyte Myriophyllum aquaticum may provide a viable solution for the removal of these contaminants. However, the toxic effects of coexisting tetracyclines (TCs) and Cu(II) on this plant species are currently unclear. In the present study, we constructed wetland microcosms planted with M. aquaticum and spiked these with three TCs (tetracycline, oxytetracycline, and chlortetracycline) and Cu(II) at concentrations ranging from 100 to 10,000 μg/L to investigate how Cu(II) influences the growth and tolerance responses of plants to TCs. After 12 weeks, we found that TCs had accumulated in the plants, and that plant growth and characteristics were significantly affected by the levels of both TCs and Cu(II). While low Cu(II) levels had a synergistic effect on the accumulation of TCs, high levels were observed to reduce accumulation. However, low levels of TCs and Cu(II) had a hormesis effect on plant growth, with plant biomass and leaf chlorophyll content decreasing and the malondialdehyde content and activities of antioxidant enzymes gradually increasing with an increase in TC dosage. The coexistence of low levels of Cu(II) was, however, found to alleviate these adverse effects. Principal component analysis revealed a close relationship among plant biomass, chlorophyll content, malondialdehyde content, and antioxidant enzyme activities. Considering that the Cu/TC ratio was shown to markedly affect M. aquaticum growth, the respective proportions of these pollutants should be taken into consideration in the future design of constructed wetlands.

Nutrients release and greenhouse gas emission during decomposition of Myriophyllum aquaticum in a sediment-water system

Aquatic macrophytes play a significant role in nutrients removal in constructed wetlands, yet nutrients could be re-released due to plant debris decomposition. In this study, Myriophyllum aquaticum was used as a model plant debris and three debris biomass levels of 3 g, 9 g dry biomass, and 20 g fresh biomass (D3, D9, and F20, respectively) were used to simulate 120-d plant debris decomposition in a sediment-water system. The biomass first-order decomposition rate constants of D3, D9, and F20 treatments were 0.0058, 0.0117, and 0.0201 d^-1, respectively with no significant difference of decomposition rate among three mass groups (p > 0.05). Plant debris decomposition decreased nitrate and total nitrogen concentrations but increased ammonium, organic nitrogen, and dissolved organic carbon (DOC) concentrations in overlying water. The parallel factor analysis confirms that three components of DOC in overlying water changed over decomposition time. Emission fluxes of methane and nitrous oxide in the plant debris treatments were several to thousands of times higher than the control group within the initial 0-45 d, which was mainly attributed to DOC released from the plant debris. Plant debris decomposition can affect the gas emission fluxes for relatively shorter time (30-60 d) than water quality (>120 d). The 16S rRNA, nirK, nirS and hazA gene abundance increased in the early stage for plant debris treatments, and then decreased to the end of 120-d incubation time while ammonia monooxygenase α-subunit A gene abundance of ammonia-oxidizing archaea and bacteria had no large variations during the entire decay time compared with no plant debris treatment. The results demonstrate that decomposition of M. aquaticum debris could affect greenhouse gas emission fluxes and microbial gene abundance in the sediment-water system besides overlying water quality.

Are vegetated drainage ditches effective for nitrogen removal under cold temperatures?

Vegetated ditches are widely used to treat agricultural wastewater, but effective nitrogen removal at low temperatures remains a challenge because plants wilt in the winter. In this study, three simulated drainage ditches vegetated with Myriophyllum aquaticum were operated with low, medium, and high water levels to study ammonium nitrogen (NH₄⁺-N) removal under cold temperatures. The M. aquaticum ditches had a mean NH₄⁺-N removal efficiency of 75.8-86.8% throughout cold period. Based on nitrogen mass balance, plant uptake, sediment adsorption, and microbial removal accounted for 12.4-21.5%, 0.0-8.1%, and 38.9-54.6% of the influent total nitrogen loading, respectively. The accumulation of nitrate confirmed that intense microbial nitrification occurred in M. aquaticum ditches even at low temperature. These results suggest that M. aquaticum is appropriate as a cold-tolerant plant for NH₄⁺-N removal in drainage ditches.

Stimulation of optimized influent C:N ratios on nitrogen removal in surface flow constructed wetlands: Performance and microbial mechanisms

Exploring optimal C:N ratio is necessary to ensure balanced microbial nitrification and denitrification in constructed wetlands (CWs), which has become an important management practice for more efficient nitrogen removal and sustainability of CWs. Surface flow constructed wetlands (SFCWs) vegetated with Myriophyllum aquaticum were designed to investigate the effects of five different influent C:N ratios (0:1, 2.5:1, 5:1, 10:1, and 15:1) on nitrogen removal performance and microbial communities over a 175-day experimental period. Compared to the influent C:N ratios of 0:1, higher NH₄⁺-N, NO₃^--N, and total nitrogen (TN) removal efficiencies and lower NO₃^--N accumulation were observed at influent C:N ratios higher than 5:1. In addition, the highest TN removal efficiency (70.4%) and the lowest nitrous oxide emission flux (4.12 mg m^-2 d^-1) were obtained at the influent C:N ratio of 5:1. High-throughput sequencing revealed that influent C:N ratios altered the distribution and composition of microbial communities in the sediment, which resulted in a dynamic interplay between N-transforming functional microbes and NH₄⁺-N and NO₃^--N removal. In particular, the dominant denitrifiers, including Desulfovibrio, Zoogloea, and Dechloromonas, were more abundant in the sediment with an influent C:N ratio of 5:1, which contributed to the high N removal rate. These findings may be used to screen for an optimum influent C:N ratio to maintain the sustainability of SFCWs with higher N removal efficiency.

Rapid removal of tetracycline by Myriophyllum aquaticum: Evaluation of the role and mechanisms of adsorption

As a floating plant, Myriophyllum aquaticum provides a large surface area under water, and thus has high potential for the removal of pollutants through adsorption. The aim of this study was to evaluate the potential adsorption of tetracycline (TC) by M. aquaticum, and examine the underlying mechanisms. M. aquaticum exhibited a high potential for TC removal from water. Adsorption was the main mechanism for rapid TC removal by live M. aquaticum plants, due to its large contact area and ion exchange, accounting for about 99% and 54% of the total amount of TC removed within 2 h and 5 d, respectively. Further, the roots of M. aquaticum exhibited a higher adsorption capacity than the stems or leaves, as the roots had the largest specific surface area. Fourier transform infrared spectroscopy analysis and identification of functional groups showed that -OH, -COOH, and -NH₂ groups are involved in the adsorption process. The use of M. aquaticum may be a promising approach for TC removal from aquatic environments, especially in terms of shortening reaction times.

Effect of copper on the removal of tetracycline from water by Myriophyllum aquaticum: Performance and mechanisms

Pollution with antibiotics and heavy metals necessitates efficient approaches for their removal. This study was conducted to investigate the role of Cu in the tetracycline (TC) removal potential of the floating plant Myriophyllum aquaticum and determine the underlying mechanisms. Myriophyllum aquaticum exhibited high TC removal potential from water (60% at 50 mg·L^-1 TC and 10 g·L^-1M. aquaticum). Adsorption was the main mechanism of TC removal within 2 h, accounting for over 75% and 90% of total TC removal with and without Cu(II), respectively. Fourier-transform infrared spectroscopy and functional group identification showed that OH, COOH, and NH₂ were involved in TC adsorption through ion exchange. Cu(II) may act as a bridge during TC adsorption with M. aquaticum, but competitive adsorption of Cu(II) and TC on M. aquaticum occurs in case of excessive Cu(II). Myriophyllum aquaticum can serve as an important bioresource for effectively removing TC and Cu(II) from aquatic environments.

[Decomposition of Myriophyllum aquaticum and the Associated Release of Nitrogen and Phosphorus]

Decomposition of wetland plants could release pollutants, which may affect the removal efficiency and effluent quality of constructed wetlands. The experimental decomposition test of Myriophyllum aquaticum was carried out for 60 d using nylon bags, and release characteristics of nitrogen and phosphorus during the decomposition process were studied. The results showed that the decomposition rate of M. aquaticum was fastest during the first 0-4 d, with a weight loss of 30%, while the degradation rate slowed gradually during the period 4-60 d, with weight loss of 31%. The fitting first-order kinetic decomposition rate constant was 0.0142 d^-1, and the calculated time to degrade 50% of dry matter was 48.8 d. The water pH decreased rapidly from 7.60 to 5.63 during 0-4 d, stabilized during 4-32 d, and finally increased to 7.03 (which was close to the control sample without M. aquaticum). The dissolved oxygen concentration decreased rapidly from 6.30 mg·L^-1 to 0.61 mg·L^-1 during 0-4 d, and remained in an anaerobic state. The total nitrogen concentration in the water increased rapidly to 12.7 mg·L^-1 within 2 h, gradually decreased to 5.80 mg·L^-1 during 2 h-32 d, and then finally increased slightly. The phosphorus concentration increased rapidly to 18.4 mg·L^-1 at the beginning of the experiment, and then gradually stabilized. The main forms of nitrogen and phosphorus released by M. aquaticum were organic nitrogen (accounting for 65.7%-94.7% of total nitrogen) and inorganic phosphorus (accounting for 61%-89% of total phosphorus), respectively. The total nitrogen content of M. aquaticum increased from 24.3 mg·g^-1 to 60.5 mg·g^-1 with increasing degradation time; the total phosphorus decreased initially from 6.09 mg·g^-1 to 2.94 mg·g^-1 and then remained constant. These trends may have been related to the fixation of nitrogen by attached microorganisms. Therefore, suitable harvesting and management strategies should be adopted for wetland plants to reduce secondary pollution.

Allelopathic Effects of Native Versus Invasive Plants on One Major Invader

Allelopathy is defined as the effects (stimulatory and inhibitory) of a plant on the development of neighboring plants through the release of secondary compounds. Autoallelophaty is the beneficial or harmful effect of a plant species on itself. The allelopathic potential belonging to a native species could induce a biotic resistance against invasive plants, whereas allelochemicals released by exotic species could favor the establishment of invasive species (invasional meltdown). The aim of our study was to examine the potential allelopathic effect of four plant species on the target species Ludwigia hexapetala using two experiments. In the first experiment, we tested the allelopathic effect of root and leaf leachates of the two congeneric exotic species Ludwigia hexapetala and Ludwigia peploides on L. hexapetala, while in the second experiment, we studied the allelopathic effect of root and leaf leachates of a sympatric exotic species Myriophyllum aquaticum and of one native species Mentha aquatica on L. hexapetala. We measured the stem length to calculate the relative growth rate and four physiological traits (nitrogen balance index and flavonol, chorophyll, anthocyanin indices) of the target plants on a weekly basis. At the end of the experiment, we determined the aboveground and belowground biomass. We also counted the number of lateral branches and measured their lengths. We found that the root leachates of L. peploides and of Myriophyllum aquaticum had stimulated the synthesis of flavonols of L. hexapetala. Leaf leachate of L. hexapetala also stimulated its own flavonol synthesis. Also, the root leachate of L. peploides had stimulated the total biomass and length of lateral branches of L. hexapetala, whereas the production of lateral branches had been stimulated by root leachates of both Ludwigia species and by leaf leachate of Myriophyllum aquaticum. The autoallelopathy of L. hexapetala could explain its invasiveness. Both leachates produced by Mentha aquatica had no effect on the physiological and morphological traits of the invasive L. hexapetala and indicated no biotic resistance in the recipient community. The two invasive plant species Myriophyllum aquaticum and L. peploides could favor the establishment of L. hexapetala. These results suggested an "invasional meltdown."

Does Soil Nutrient Heterogeneity Improve the Growth Performance and Intraspecific Competition of the Invasive Plant Myriophyllum aquaticum?

Spatial heterogeneity in soil nutrient availability is considered to play an important role in promoting plant invasion success and can affect interspecific competition. Although some clonal plants have been demonstrated to be correlated with resource heterogeneity in terrestrial systems, little is known about how soil nutrient heterogeneity affects the growth of invasive aquatic plants or their population structure. A greenhouse experiment was therefore conducted to study the response of the invasive aquatic plant Myriophyllum aquaticum to the spatial heterogeneity of soil nutrients under three plant densities (one, four, or twelve plants 0.28 m2) with a constant amount of soil nutrients. The results showed that soil nutrient heterogeneity significantly increased the number of shoots in the single-plant density treatment. However, heterogeneous soil nutrient treatment significantly increased the number of shoots at the expense of total biomass and aboveground biomass in the twelve-plant density treatment. The heterogeneous soil nutrient treatment had low effects on other growth traits and intraspecific competition under different plant density treatments. These results indicate that spatial heterogeneity in soil nutrient availability may facilitate the spread of M. aquaticum.

A wastewater treatment system combining Myriophyllum aquaticum and activated sludge: Optimization of construction conditions and evaluation of wastewater treatment performance

Although Myriophyllum aquaticum exhibits efficient nitrogen and phosphorus removal from wastewater, it has poor performance on organic matter removal. Here, a wastewater treatment system combining M. aquaticum and activated sludge was developed to improve its removal of organic matter. The Box-Behnken response surface methodology was used to optimize the construction conditions of the system, and the effects of time, temperature, illumination intensity, pollutant load, and dissolved oxygen (DO) on plant mass increment (PMI) and microbial biomass (MB) of the system were investigated. The wastewater remediation potential of the system was then evaluated. The results show that temperature and illumination intensity significantly affected PMI (p < 0.01), and that time, pollutant load, and DO were the most significant factors affecting MB (p < 0.01). The optimal construction conditions were 18.77 days in length, a temperature of 20.42 °C, an illumination intensity of 5827.61 Lx, a pollutant load of 120.61 mg/g plant, and a DO of 3.21 mg/L. The inoculation of activated sludge caused MB of the system to increase by four times relative to the non-inoculated system, suggesting successful formation of biofilms on M. aquaticum. Additionally, the removal of chemical oxygen demand (COD) from wastewater was significantly enhanced by the combined approach compared with a system relying solely on M. aquaticum. This study provides a new method for improving the remediation efficiency of M. aquaticum by combining the use of this species and activated sludge.

Evaluating organics removal performance from lagoon-pretreated swine wastewater in pilot-scale three-stage surface flow constructed wetlands

Pilot-scale three-stage surface flow constructed wetlands (SFCWs) planted with Myriophyllum aquaticum were constructed to study the organics removal performance from lagoon-pretreated swine wastewater. The removal performance of organics in the SFCWs was evaluated using deterministic and probabilistic methods and the results were consistent. The SFCWs achieved a relatively high removal efficiency (79.0-82.7%) for a wide influent COD concentration range (456-1010 mg L^-1). No significant difference (p > 0.05) of COD removal efficiency and first-order removal rate constant among the various strengths of influent suggested that the present loading rates (2.74-6.06 g m^-2 d^-1) have not yet reached the maximum removal capacity of the SFCWs. The mean emission fluxes of methane from the SFCW units fed with different strengths of wastewater were 25-1210 mg m^-2 d^-1. A significantly positive correlation (p < 0.01) between methane emission fluxes and COD loading rates indicated that the anaerobic digestion of organics was an important process for organics removal in the SFCWs. No significant organics accumulation in the sediment over time suggested that plant harvest could be in favor of reducing the organics accumulation in the substrate and should be considered important during management of constructed wetlands.

Flow Management to Control Excessive Growth of Macrophytes - An Assessment Based on Habitat Suitability Modeling

Mediterranean rivers in intensive agricultural watersheds usually display outgrowths of macrophytes - notably alien species - due to a combination of high concentrations of nutrients in the water runoff and low flows resulting from water abstraction for irrigation. Standard mechanical and chemical control is used to mitigate the problems associated with excessive growth of plant biomass: mainly less drainage capacity and higher flood risk. However, such control measures are cost and labor-intensive and do not present long-term efficiency. Although the high sensitivity of aquatic vegetation to instream hydraulic conditions is well known, management approaches based on flow management remain relatively unexplored. The aim of our study was therefore to apply physical habitat simulation techniques promoted by the Instream Flow Incremental Method (IFIM) to aquatic macrophytes - the first time it has been applied in this context - in order to model shifts in habitat suitability under different flow scenarios in the Sorraia river in central Portugal. We used this approach to test whether the risk of invasion and channel encroachment by nuisance species can be controlled by setting minimum annual flows. We used 960 randomly distributed survey points to analyze the habitat suitability for the most important aquatic species (including the invasive Brazilian milfoil Myriophyllum aquaticum, Sparganium erectum, and Potamogeton crispus) in regard to the physical parameters 'flow velocity,' 'water depth,' and 'substrate size'. We chose the lowest discharge period of the year in order to assess the hydraulic conditions while disturbances were at a low-point, thus allowing aquatic vegetation establishment and subsistence. We then used the two-dimensional hydraulic River2D software to model the potential habitat availability for different flow conditions based on the site-specific habitat suitability index for each physical parameter and species. Our results show that the growth and distribution of macrophytes in the hydrologically stable vegetation period is primarily a function of the local physical instream condition. Using site-specific preference curves and a two-dimensional hydraulic model, it was possible to determine minimum annual flows that might prevent the excessive growth and channel encroachment caused by Myriophyllum aquaticum.

[Effects of Wastewater Nitrogen Concentrations and NH4+/NO3- on Nitrogen Removal Ability and the Nitrogen Component of Myriophyllum aquaticum (Vell.) Verdc]

Solution culture experiments were conducted to investigate the effect of wastewater nitrogen levels and NH₄⁺/NO₃^- on nitrogen removal ability and the nitrogen component of Myriophyllum aquaticum. Experiments with three nitrogen levels and NH₄⁺/NO₃^- were set up as follows:20, 100, and 200 mg·L^-1and NH₄⁺/NO₃^- 1:0, 0.5:0.5, and 0:1. The results showed that the biomass of plants increased fastest during the first week. The plants treated with NH₄⁺/NO₃^-=1:0 with nitrogen levels of 20 and 100 mg·L^-1 and those treated with NH₄⁺/NO₃^-=0.5:0.5 with a nitrogen concentration of 200 mg·L^-1 exhibited higher biomass than the others. The removal rates of water total nitrogen, ammonium nitrogen, and nitrate nitrogen during the first week were the maximum for all treatments and increased with water nitrogen levels. There were no significant differences in the removal rate between ammonium nitrogen and nitrate nitrogen with a nitrogen level of 20 mg/L, while with nitrogen levels of 100 and 200 mg·L^-1, the nitrate removal rates were higher than those for ammonium nitrogen. The Myriophyllum aquaticum nitrogen accumulation and its contribution rate to nitrogen removal from water and sediment were all increased with water nitrogen levels and increased fastest during the first week. The contribution rate of nitrogen accumulated by plants with NH₄⁺/NO₃^-=0:1 was the highest with nitrogen levels of 20 mg·L^-1, while plants with NH₄⁺/NO₃^-=0.5:0.5 were the highest with nitrogen levels of 100 and 200 mg·L^-1. The protein, amino, and nitrate nitrogen contents in Myriophyllum aquaticum plants were all increased by increasing water nitrogen levels with a ranking of protein content > amino nitrogen content > nitrate nitrogen content. The protein concentrations in plants with NH₄⁺/NO₃^-=1:0 and NH₄⁺/NO₃^-=0.5:0.5 were higher regardless of water nitrogen levels, while the amino nitrogen concentration in plants with NH₄⁺/NO₃^-=1:0 and the nitrate nitrogen content in plants with NH₄⁺/NO₃^-=0:1 were higher than the others. It was concluded that the nitrogen removal ability of Myriophyllum aquaticum was improved by raising water nitrogen levels under the tested condition, which indicates that Myriophyllum aquaticum could purify high nitrogen wastewater. Myriophyllum aquaticum is an ammonium-nitrophile, but had the strongest capacity for growing and removing wastewater nitrogen exhibited with higher than 100 mg·L^-1 nitrogen levels only with equal NH₄⁺ to NO₃^-. The nitrogen component concentrations of protein, amino, and nitrate in Myriophyllum aquaticum plant were all affected by the ratio of NH₄⁺/NO₃^-.

The adaptability of a wetland plant species Myriophyllum aquaticum to different nitrogen forms and nitrogen removal efficiency in constructed wetlands

Constructed wetlands (CWs) cultivated with Myriophyllum aquaticum showed great potential for total nitrogen (TN) removal from aquatic ecosystems in previous studies. To evaluate the growth characteristics, photosynthetic pigment content, and antioxidative responses of M. aquaticum, as well as its TN removal efficiency in CWs, M. aquaticum was treated with different levels of ammonium (NH₄⁺) and nitrate (NO₃^-) for 28 days. The results indicated that M. aquaticum had strong nitrogen stress tolerance and was more likely to be suppressed by high levels of NH₄⁺ than NO₃^-. High levels of NH₄⁺ also led to inhibition of synthesis of photosynthetic pigments and increased peroxidase activity in plant leaves, which was not found in the NO₃^- treatments. High levels of both NH₄⁺ and NO₃^- generated obvious oxidative stress through elevation of malondialdehyde content while decreasing superoxide dismutase activity in the early stage. A sustainable increase of TN removal efficiency in most of the CWs indicated that M. aquaticum was a candidate species for treating wastewater with high levels of nitrogen because of its higher tolerance for NH₄⁺ and NO₃^- stress. However, the increase of TN removal efficiency was hindered in the late stage when treated with high levels of NH₄⁺ of 26 and 36 mmol/L, indicating that its tolerance to NH₄⁺ stress might have a threshold. The results of this study will enrich the studies on detoxification of high ammonium ion content in NH₄⁺-tolerant submerged plants and supply valuable reference data for proper vegetation of M. aquaticum in CWs.

Myriophyllum aquaticum Constructed Wetland Effectively Removes Nitrogen in Swine Wastewater

Removal of nitrogen (N) is a critical aspect in the functioning of constructed wetlands (CWs), and the N treatment in CWs depends largely on the presence and activity of macrophytes and microorganisms. However, the effects of plants on microorganisms responsible for N removal are poorly understood. In this study, a three-stage surface flow CW was constructed in a pilot-scale within monospecies stands of Myriophyllum aquaticum to treat swine wastewater. Steady-state conditions were achieved throughout the 600-day operating period, and a high (98.3%) average ammonia removal efficiency under a N loading rate of 9 kg ha^-1 d^-1 was observed. To determine whether this high efficiency was associated with the performance of active microbes, the abundance, structure, and interactions of microbial community were compared in the unvegetated and vegetated samples. Real-time quantitative polymerase chain reactions showed the abundances of nitrifying genes (archaeal and bacterial amoA) and denitrifying genes (nirS, nirK, and nosZ) were increased significantly by M. aquaticum in the sediments, and the strongest effects were observed for the archaeal amoA (218-fold) and nirS genes (4620-fold). High-throughput sequencing of microbial 16S rRNA gene amplicons showed that M. aquaticum greatly changed the microbial community, and ammonium oxidizers (Nitrosospira and Nitrososphaera), nitrite-oxidizing bacteria (Nitrospira), and abundant denitrifiers including Rhodoplanes, Bradyrhizobium, and Hyphomicrobium, were enriched significantly in the sediments. The results of a canonical correspondence analysis and Mantle tests indicated that M. aquaticum may shift the sediment microbial community by changing the sediment chemical properties. The enriched nitrifiers and denitrifiers were distributed widely in the vegetated sediments, showing positive ecological associations among themselves and other bacteria based on phylogenetic molecular ecological networks.

Effects of live Myriophyllum aquaticum and its straw on cadmium accumulation in Nasturtium officinale

The purpose of this study is to determine whether the allelopathy of living Myriophyllum aquaticum and its straw has the same effects; two pot experiments were conducted to study the effects of intercropping using M. aquaticum and its straw on the growth and cadmium (Cd) accumulation of Nasturtium officinale. Different planting ratios (1:3, 2:2 and 3:1) of N. officinale and M. aquaticum led to an increase in the biomass of both plant species and increased the Cd content in roots and shoots of N. officinale, but led to a reduction in the Cd content in roots and shoots of M. aquaticum. When the intercropping ratio of N. officinale and M. aquaticum was 3:1, the Cd amount in whole plants reached the maximum of 293.96 μg pot^-1. Mulching the straw of M. aquaticum roots on the soil surface increased the biomass of N. officinale, but mulching the straw of M. aquaticum stems and leaves led to a decrease. Mulching the straw of roots, stems and leaves of M. aquaticum reduced Cd content and amounts in roots and shoots of N. officinale. Intercropping with M. aquaticum can improve the Cd uptake ability of N. officinale, but mulching M. aquaticum straw can reduce its Cd uptake ability.

[Physiological Characteristics and Nitrogen and Phosphorus Uptake of Myriophyllum aquaticum Under High Ammonium Conditions]

Ammonium nitrogen (NH₄⁺-N) at high concentrations is toxic to plants. In order to explore the NH₄⁺-N tolerance of Myriophyllum aquaticum (M. aquaticum) and its ability of nitrogen (N) and phosphorus (P) uptake, this study used a nutrient solution with three NH₄⁺-N levels (70, 210, 420 mg·L^-1) to incubate M. aquaticum for 21 d. The characteristics of plant physiology and N and P uptake of M. aquaticum were measured. At NH₄⁺-N of 70 mg·L^-1, M. aquaticum grew healthily, and shoot height and biomass linearly increased with the increase incubation time. Relative shoot height and biomass of M. aquaticum were 40.56 cm and 17.82 g·hole^-1 on day 21, respectively. Compared to the control with 70 mg·L^-1 ammonium, malondialdehyde (MDA) content of M. aquaticum was significantly increased; chlorophyll and soluble sugar contents were also high at NH₄⁺-N of 210 mg·L^-1. M. aquaticum suffered from the NH₄⁺-N stress. However, the stress of 210 mg·L^-1 NH₄⁺-N did not affect its normal growth and there was no significant difference in the relative growth rate of the shoot height and biomass compared with the control. At NH₄⁺-N of 420 mg·L^-1, MDA contents of M. aquaticum doubled and the shoot height and biomass growth rate were only 27.4% and 17.9% of those for 70 mg·L^-1 NH₄⁺-N, indicating that M. aquaticum was subjected to serious stress that caused unhealthy growth or even death. At three NH₄⁺-N levels, the ranges of N and P content of M. aquaticum were 30.7-53.4 mg·g^-1 and 3.8-7.7 mg·g^-1, respectively, which indicated that M. aquaticum had a high uptake capacity of N and P. M. aquaticum is an ideal wetland plant that has a good application prospect for constructed wetlands in biological treatment of high-ammonia wastewater.

Ammonia stress on nitrogen metabolism in tolerant aquatic plant-Myriophyllum aquaticum

Ammonia has been a major reason of macrophyte decline in the water environment, and ammonium ion toxicity should be seen as universal, even in species frequently labeled as "NH₄⁺ specialists". To study the effects of high NH₄⁺-N stress of ammonium ion nitrogen on tolerant submerged macrophytes and investigate the pathways of nitrogen assimilation in different organisms, Myriophyllum aquaticum was selected and treated with various concentrations of ammonium ions at different times. Increasing of ammonium concentration leads to an overall increase in incipient ammonia content in leaves and stems of plants. In middle and later stages, high concentrations of NH₄⁺ ion nitrogen taken up by M. aquaticum decreased, whereas the content of NO₃^- ion nitrogen increased. Moreover, in M. aquaticum, the activities of the enzymes nitrate reductase, glutamine synthetase and asparagine synthetase changed remarkably in the process of alleviating NH₄⁺ toxicity and deficiency. The results of the present study may support the studies on detoxification of high ammonium ion content in NH₄⁺-tolerant submerged macrophytes and exploration of tissue-specific expression systems.

Allelopathic Effects of Myriophyllum aquaticum on Two Cyanobacteria of Anabaena flos-aquae and Microcystis aeruginosa

Allelopathy has been proposed as a sustainable means to control undesired algal growth and to reduce blooms threatening freshwater systems worldwide. In this study, the allelopathic effects of Myriophyllum aquaticum and its exudate on two typical bloom-forming cyanobacteria, Microcystis aeruginosa and Anabaena flos-aquae, were investigated under laboratory conditions. The growth of the cyanobacteria was strongly inhibited by live M. aquaticum while the primary addition of M. Aquaticum exudates had a significant inhibitory effect on the growth of M. aeruginosa but not A. flos-aquae. The results suggested that the persistent release of allelochemicals from live M. aquaticum was needed to effectively constrain the growth of A. flos-aquae. Analysis of cyanobacterial physiological indexes indicated that M. aquaticum produced an inhibitory effect on SOD enzyme activity of A. flos-aquae, while it affected membrane lipid peroxidation in M. aeruginosa. The results show the potential of M. aquaticum to mitigate cyanobacterial blooms in coexistence systems.

[Effect of Wastewater Nitrogen Concentrations on Nitrogen Removal Ability of Myriophyllum aquaticum]

Myriophyllum aquaticum, which is an important plant for constructed wetlands, has powerful purification ability for wastewater, however, the relationship between nitrogen removal ability of Myriophyllum aquaticum and wastewater nitrogen concentrations is still unclear. In this study, pot culture experiment was conducted to investigate the effect of wastewater nitrogen levels on nitrogen removal ability of Myriophyllum aquaticum. 7 nitrogen levels were set up as following:2, 5, 10, 20, 100, 200, 400 mg·L^-1. The results showed that when the wastewater nitrogen concentration was not higher than 20 mg·L^-1, Myriophyllum aquaticum with 20 mg·L^-1 of nitrogen concentration grew best in the first 3 weeks; the removal rates of total and ammonia nitrogen were nearly 100% after one week, while the nitrate nitrogen concentrations were very low and varied little; the nitrogen contents of Myriophyllum aquaticum had no significant change, the upper part nitrogen content was higher than the underneath, Myriophyllum aquaticum could also remove nitrogen from the sediment. When wastewater nitrogen concentrations were 100-400 mg·L^-1, Myriophyllum aquaticum with 200 mg·L^-1 of nitrogen concentration grew best from 4th to 5th week; the removal rates of total nitrogen were 76.5%, 71.5% and 48.1% in the three treatments, and the removal rates of ammonia nitrogen were 99.6%, 99.3% and 60.2% respectively, while the removal rates of nitrate nitrogen were all about 50% and there was no significant difference among treatments; the nitrogen contents of Myriophyllum aquaticum increased with nitrogen levels, but the difference between upper part and underneath was not remarkable, showing uniform distribution; nitrogen accumulations by Myriophyllum aquaticum and sediment accounted for 27.9%-48.4% and 12.2%-24.4% of total nitrogen loss in wastewater. Therefore, the nitrogen removal ability of Myriophyllum aquaticum should be inhibited by higher wastewater nitrogen level, the ammonia nitrogen removal rate was significantly higher than nitrate, the mechanism of Myriophyllum aquaticum nitrogen accumulation and distribution should also be affected by wastewater nitrogen level, and further research is needed.

Phosphorus removal from lagoon-pretreated swine wastewater by pilot-scale surface flow constructed wetlands planted with Myriophyllum aquaticum

Although constructed wetlands (CWs) are used as one relatively low-cost technology for livestock wastewater treatment, the improvement of phosphorus removal in CWs is urgently needed. In this study, a three-stage pilot-scale CW system consisting of three surface flow CWs (SFCWs; CW1, CW2, and CW3) in series from inlet to outlet was constructed to treat swine wastewater (SW) from a lagoon. The CWs were planted with Myriophyllum aquaticum. Considering different inlet loading rates, three strengths of swine wastewater (low: 33% SW, medium: 66% SW, and high: 100% SW) were fed to the CW system to determine total phosphorus (TP) removal efficiency and clarify the important role of plant harvest. Results from the period 2014-2016 indicate that the three-stage CW system had mean TP cumulative removal efficiencies and removal rates of 78.2-89.8% and 0.412-0.779gm^-2d^-1 respectively, under different inlet loading rates. The TP removal efficiency and removal rate constant had temporal variations, which depended on temperature condition and the annual growth pattern of M. aquaticum. The harvested phosphorus mass was 15.1-40.9gm^-2yr^-1 in the CWs except for CW1 with high strength SW, and contributed 22.5-59.6% of TP mass removal rate by the SFCWs. The TP removal was mainly by adsorption and precipitation in the substrate in CW1 but by uptake and multiple harvests of M. aquaticum in CW2 and CW3. The results suggest the three-stage CW system planted with M. aquaticum is suited for removing high TP concentrations from swine wastewater with a high removal efficiency. However, TP removal in high strength SW amounted to 70.1±23.3%, and the outflow concentration of 17.0±14.9mgL^-1 was still high. Optimal loading rates for high strength SW still need to be investigated for the CW system presented here.

Nitrogen removal in Myriophyllum aquaticum wetland microcosms for swine wastewater treatment: 15 N-labelled nitrogen mass balance analysis

BACKGROUND

Ecological treatments are effective for treating agricultural wastewater. In this study, wetland microcosms vegetated with Myriophyllum aquaticum were designed for nitrogen (N) removal from two strengths of swine wastewater, and ¹⁵ N-labelled ammonium (NH₄⁺ -N) was added to evaluate the dominant NH₄⁺ -N removal pathway.

RESULTS

The results showed that 98.8% of NH₄⁺ -N and 88.3% of TN (TN: 248.6 mg L^-1 ) were removed from low-strength swine wastewater (SW1) after an incubation of 21 days, with corresponding values for high-strength swine wastewater (SW2) being 99.2% of NH₄⁺ -N and 87.8% of TN (TN: 494.9 mg L^-1 ). Plant uptake and soil adsorption respectively accounted for 24.0% and 15.6% of the added ¹⁵ N. Meanwhile, above-ground tissues of M. aquaticum had significantly higher biomass and TN content than below-ground (P < 0.05). ¹⁵ N mass balance analysis indicated that gas losses contributed 52.0% to the added ¹⁵ N, but the N₂ O flux constituted only 7.5% of total gas losses. The dynamics of NO₃^- -N and N₂ O flux revealed that strong nitrification and denitrification occurred in M. aquaticum microcosms, which was a dominant N removal pathway.

CONCLUSION

These findings demonstrated that M. aquaticum could feasibly be used to construct wetlands for high N-loaded animal wastewater treatment. © 2016 Society of Chemical Industry.

Nitrogen removal and mass balance in newly-formed Myriophyllum aquaticum mesocosm during a single 28-day incubation with swine wastewater treatment

The aim of this research was to assess the applicability of Myriophyllum (M.) aquaticum for swine wastewater treatment. Nitrogen (N) removal processes were investigated in M. aquaticum mesocosms with swine wastewater (SW), 50% diluted swine wastewater (50% SW), and two strengths of synthetic wastewater, 200 mg [Formula: see text] L(-1) (200 [Formula: see text] ) and 400 mg [Formula: see text] L(-1) (400 [Formula: see text] ). During a 28-day incubation period, the average [Formula: see text] and TN removal rates were 99.8% and 94.2% for 50% SW and 99.8% and 93.8% for SW, which were greater than 86.5% and 83.7% for 200 [Formula: see text] , and 73.7% and 74.1% for 400 [Formula: see text] , respectively. A maximum areal total nitrogen (TN) removal rate of 157.8 mg N m(-2) d(-1) was found in M. aquaticum mesocosms with SW. During the incubation period, the observed dynamics of [Formula: see text] concentrations in water and gene copy numbers of ammonia-oxidizing archaea (AOA), ammonia-oxidizing bacteria (AOB), nirK and nirS in soil unraveled strong nitrification and denitrification processes occurring in M. aquaticum mesocosms with swine wastewater. The N mass balance analysis indicated that plant uptake and soil N accumulation accounted for 17.9-42.2% and 18.0-43.8% of the initial TN load, respectively. The coupled nitrification and denitrification process was calculated to account for, on average, 36.8% and 62.8% of TN removal for 50% SW and SW, respectively. These findings demonstrated that the N uptake by M. aquaticum contributed to a considerable proportion of N removal. In particular, the activities of ammonia-oxidizing and denitrification microbes responsible for nitrification and denitrification processes in M. aquaticum mesocosm accelerated [Formula: see text] and TN removal from swine wastewater.

Inter-laboratory trial of a standardized sediment contact test with the aquatic plant Myriophyllum aquaticum (ISO 16191)

A whole-sediment toxicity test with Myriophyllum aquaticum has been developed by the German Federal Institute of Hydrology and standardized within the International Organization for Standardization (ISO; ISO 16191). An international ring-test was performed to evaluate the precision of the test method. Four sediments (artificial, natural) were tested. Test duration was 10 d, and test endpoint was inhibition of growth rate (r) based on fresh weight data. Eighteen of 21 laboratories met the validity criterion of r ≥ 0.09 d(-1) in the control. Results from 4 tests that did not conform to test-performance criteria were excluded from statistical evaluation. The inter-laboratory variability of growth rates (20.6%-25.0%) and inhibition (26.6%-39.9%) was comparable with the variability of other standardized bioassays. The mean test-internal variability of the controls was low (7% [control], 9.7% [solvent control]), yielding a high discriminatory power of the given test design (median minimum detectable differences [MDD] 13% to 15%). To ensure these MDDs, an additional validity criterion of CV ≤ 15% of the growth rate in the controls was recommended. As a positive control, 90 mg 3,5-dichlorophenol/kg sediment dry mass was tested. The range of the expected growth inhibition was proposed to be 35 ± 15%. The ring test results demonstrated the reliability of the ISO 16191 toxicity test and its suitability as a tool to assess the toxicity of sediment and dredged material.

Bioavailability of copper in contaminated sediments assessed by a DGT approach and the uptake of copper by the aquatic plant Myriophyllum aquaticum

The assessment of the potentially harmful effects of metals on biota depends on the speciation and bioavailability of the metals. In the present study, the authors investigated Cu accumulation and toxicity in the aquatic plant Myriophyllum aquaticum after exposure to artificial sediments varying in peat or ferric hydroxide content and spiked with Cu (5-200 mg kg(-1)). Modeling of the kinetic diffusive gradient in thin film (DGT) measurements revealed fast and slow Cu resupply from the solid phase for sediment formulated with and without peat, respectively. Myriophyllum aquaticum proved to be sensitive to Cu, as the Cu accumulation and growth differed depending on the sediment composition and Cu concentration. Comparing the Cu accumulation in M. aquaticum with total dissolved concentration, free concentration, and concentration in solution derived from DGT measurements (CDGT), Cu concentrations revealed that CDGT concentrations were a better predictor of accumulation than the others. However, the relatively weak correlation observed (r(2)  = 0.6) and the fact that plant uptake does not increase proportionally to DGT fluxes suggest that Cu uptake in plants was not diffusion limited. Thus, the free Cu concentrations near the root surface were sufficient to meet the plant's demand during the experiment. Furthermore, labile complexes that continuously resupply the Cu(2+) pool may also contribute to the concentrations available for plant uptake. In the range of Cu concentrations investigated in the present study, saturation of uptake processes as well as toxicity are considered responsible for the poor DGT prediction of plant uptake.