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

Effects of nitrogen stress on uptake and translocation of organophosphate esters by watermifoil (Myriophyllum aquaticum L.) in an aquatic ecosystem

Although organophosphate esters (OPEs) and nitrogen (N) are normally present in aquatic environments, the effects of the plant uptake, accumulation, and translocation of OPEs in different levels of N remain ambiguous. To better understand these processes, watermifoil (Myriophyllum aquaticum L.) as tested plant was chosen to investigate the effects of different N levels on the uptake and translocation of OPEs by plants in matched water-sediment-plant samples. After two months, we found the root-water concentration factors, root-sediment concentration factors, and translocation factors (TFs) were significantly changed with the levels of N (p < 0.05), implying that the presence of N could alter uptake, accumulation, and translocation of OPEs in M. aquaticum, particularly the process of root absorption. Low concentrations of N could remarkably promote the uptake of OPEs by M. aquaticum. However, when the concentrations of N in water were higher than 200 mg/L, the plants' growth and OPE accumulation by M. aquaticum were obviously inhibited with the elevated N contents. Moreover, the enrichment and environmental transport of OPEs in M. aquaticum seemed to be closely associated with physicochemical parameters; the octanol-water partition coefficient had significant relationships with measured organic carbon-normalized sediment-water partition coefficients and TFs in the present study. Additionally, the substituents and structures of OPEs could also affect the accumulation and translocation of OPEs in M. aquaticum, including the chlorination degree and alkyl chain length. This study could improve our understanding of uptake and translocation of OPEs in aquatic plants under different levels of N.

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.

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.

Wetland Restoration with Hydrophytes: A Review

Restoration cases with hydrophytes (those which develop all their vital functions inside the water or very close to the water surface, e.g., flowering) are less abundant compared to those using emergent plants. Here, I synthesize the latest knowledge in wetland restoration based on revegetation with hydrophytes and stress common challenges and potential solutions. The review mainly focusses on natural wetlands but also includes information about naturalized constructed wetlands, which nowadays are being used not only to improve water quality but also to increase biodiversity. Available publications, peer-reviewed and any public domain, from the last 20 years, were reviewed. Several countries developed pilot case-studies and field-scale projects with more or less success, the large-scale ones being less frequent. Using floating species is less generalized than submerged species. Sediment transfer is more adequate for temporary wetlands. Hydrophyte revegetation as a restoration tool could be improved by selecting suitable wetlands, increasing focus on species biology and ecology, choosing the suitable propagation and revegetation techniques (seeding, planting). The clear negative factors which prevent the revegetation success (herbivory, microalgae, filamentous green algae, water and sediment composition) have to be considered. Policy-making and wetland restoration practices must more effectively integrate the information already known, particularly under future climatic scenarios.

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.

Transcriptomic Sequencing and Co-Expression Network Analysis on Key Genes and Pathways Regulating Nitrogen Use Efficiency in Myriophyllum aquaticum

Massively input and accumulated ammonium is one of the main causes of eutrophication in aquatic ecosystems, which severely deteriorates water quality. Previous studies showed that one of the commonly used macrophytes, Myriophyllum aquaticum, was capable of not only withstanding ammonium of high concentration, but also efficiently assimilating extracellular ammonium to constitutive amino acids and proteins. However, the genetic mechanism regulating such efficient nitrogen metabolism in M. aquaticum is still poorly understood. Therefore, RNA-based analysis was performed in this study to understand the ammonium regulatory mechanism in M. aquaticum in response to various concentrations of ammonium. A total of 7721 genes were differentially expressed, of which those related to nitrogen-transport, assimilation, and remobilization were highly-regulated in response to various concentrations of ammonium. We have also identified transcription factors and protein kinases that were rapidly induced in response to ammonium, which suggests their involvement in ammonium-mediated signalling. Meanwhile, secondary metabolism including phenolics and anthocyanins biosynthesis was also activated in response to various concentrations of ammonium, especially at high ammonium concentrations. These results proposed a complex physiological and genetic regulation network related to nitrogen, carbohydrate, transcription factors, and secondary metabolism for nitrogen use efficiency in M. aquaticum.

Complex regulatory network allows Myriophyllum aquaticum to thrive under high-concentration ammonia toxicity

Plants easily experience ammonia (NH₄⁺) toxicity, especially aquatic plants. However, a unique wetland plant species, Myriophyllum aquaticum, can survive in livestock wastewater with more than 26 mM NH₄⁺. In this study, the mechanisms of the M. aquaticum response to NH₄⁺ toxicity were analysed with RNA-seq. Preliminary analysis of enzyme activities indicated that key enzymes involved in nitrogen metabolism were activated to assimilate toxic NH₄⁺ into amino acids and proteins. In response to photosystem damage, M. aquaticum seemed to remobilize starch and cellulose for greater carbon and energy supplies to resist NH₄⁺ toxicity. Antioxidative enzyme activity and the secondary metabolite content were significantly elevated for reactive oxygen species removal. Transcriptomic analyses also revealed that genes involved in diverse functions (e.g., nitrogen, carbon and secondary metabolisms) were highly responsive to NH₄⁺ stress. These results suggested that a complex physiological and genetic regulatory network in M. aquaticum contributes to its NH₄⁺ tolerance.