Phosphorus Uptake Efficiency Of Duckweed And Its Mechanism Under Different Growth Conditions

With the fast development of economy and population, water body eutrophication has become one of the most important environmental problems. Phosphorus is known to play an important role in water eutrophication. In order to improve water quality it is crucial to remove phosphorus from eutrophic water effectively. In recent years, ecological remediation methods using aquatic plants have aroused widespread interest. One of the most efficient and economical ways is the use of duckweed for eutrophic water body treatment. However, studies to date mainly deal with duckweed growth capacity and mechanisms of nitrogen uptake, while knowledge concerning mechanisms of phosphorus uptake and controlling factors still remains limited. In this paper, phosphorus purification capacity of duckweed were studied by determining phosphorus uptake efficiency, uptake kinetics, characteristics of phosphorus metabolism enzymes in plants and cell ultrastructure changes under the influence of various factors(varieties, light intensity, temperature, pH, phosphorus concentration and water type). The absorption efficiency and mechanisms of phosphorus by duckweed under different conditions were determined, which provided an important theoretical reference for the development and utilization of duckweed in the phosphorus restoration of eutrophic water bodies. The main results were as follows:(1) Lemna aequinoctialis had a higher relative growth rate than Spirodela oligorrhiza and Spirodela polyrrhiza, with a leaflets’doubling time of3.23days, while the doubling times of Spirodela oligorrhiza and Spirodela polyrrhiza were4.15and4.81days respectively. However, the phosphorus uptake capacity of Lemna aequinoctialis was siginificantly lower than that of Spirodela oligorrhiza and Spirodela polyrrhiza. The high phosphorus accumulation in Lemna aequinoctialis was a result of biomass increase. The Km and Vmax values for phosphorus uptake of Lemna aequinoctialis were lower than those of Spirodela oligorrhiza and Spirodela polyrrhiza, indicating a stronger affinity to phosphorus of Lemna aequinoctialis Spirodela polyrrhiza had the highest Km and Vmax values for phosphorus uptake, which might lead to a better performance of Spirodela polyrrhiza in purifying wastewater with a higher phosphorus concentration. Spirodela polyrrhiza had the highest ACPase and H+-ATPase activity, followed by Lemna aequinoctialis and Spirodela oligorrhiza. Morphologically, Spirodela oligorrhiza and Spirodela polyrrhiza had tightly arranged chloroplasts thylakoid membranes, and the chloroplasts were surrounded by many mitochondrias, while chloroplasts in Lemna aequinoctialis had relatively loose structure, with accumulated starch and vesicles. Cell ultrastructure of Spirodela oligorrhiza and Spirodela polyrrhiza was of great benefit to light use and transport of assimilation substance in plants. Therefore, Spirodela polyrrhiza had higher phosphorus absorption capability than Spirodela oligorrhiza and Lemna aequinoctialis, making it more suitable for purifying high-phosphorus polluted water.(2) RGR value of Spirodela polyrrhiza increased with the increase of light intensity. Spirodela polyrrhiza fronds remained in a high expanding speed even in60001x high light intensity. Total absorbed phosphorus amount of Spirodela polyrrhiza increased with the increase of light intensity, while phosphorus uptake capacity was the lowest at60001x high light intensity. There was no significant difference between phosphorus uptake capacity with light intensity of30001x and15001x. With the increase of light intensity Vmax for phosphorus uptake of Spirodela polyrrhiza increased, while no change was observed for Km. Phosphorus affinity of Spirodela polyrrhiza with30001x and60001x light intensity were slightly higher than15001x. ACPase activity of Spirodela polyrrhiza also increased with the increase of light intensity. H+-ATPase activity firstly decreased, and then increased slowly during the cultivation period. At the later period of cultivation, H+-ATPase activity of Spirodela polyrrhiza with weak light intensity were higher than strong light intensity. When supplied with strong light intensity, chloroplasts in mesophyll cell of Spirodela polyrrhiza increased. Though the number of chloroplasts decreased with weak light intensity, grana lamella of chloroplasts thickened to increase its photosynthesis reaction area.(3) The highest and lowest RGR values of Spirodela polyrrhiza were determined at25℃and10℃respectively. The growth of duckweed was negatively influenced more by low temperature than by high temperature. Total absorbed phosphorus in Spirodela polyrrhiza increased with temperature, while the highest phosphorus uptake capacity was detected at10℃, which indicated that biomass increase was the reason for high cumulation of absorbed phosphorus in25℃and35℃treatments. Vmax for phosphorus uptake of Spirodela polyrrhiza increased with the increase of temperature. Km value of phosphorus affinity changed at different temperatures:25℃>10℃>35℃. ACPase activity of Spirodela polyrrhiza increased with the increase of temperature, however, the stressing effect caused by a long period of time with low temperature may also lead to a higher phosphorus affinity. H+-ATPase activity of Spirodela polyrrhiza under different temperature was25℃>10℃>35℃. In terms of cell ultrastructure, Spirodela polyrrhiza was under stress to a certain degree in10℃and35℃treatments, as shown by shriveled. Chloroplasts and large cell cavity at10℃, and dissolved chloroplast envelope, exuded stroma and increased Plastoglobulus at35℃.(4) RGR values of Spirodela polyrrhiza at different pH were pH6> pH5> pH7> pH8> pH9. The highest total cumulative absorbed phosphorus in Spirodela polyrrhiza was found at pH6, while phosphorus uptake capacity was not affected by pH. Vmax for phosphorus uptake of Spirodela polyrrhiza was pH6> pH7> pH8>pH5> pH9, and Km was pH5> pH9> pH8> pH7> pH6. H+-ATPase and ACPase activities of Spirodela polyrrhiza were higher at pH6, pH7and pH8than other treatments. The number of chloroplasts in mesophyll cell of Spirodela polyrrhiza decreased significantly at pH5, with loose strucutre of thylakoidas lamella. At pH6and pH7, Spirodela polyrrhiza had more chloroplasts, solvend and organelles, and tightly arranged thylakoids, which assured a high photosynthetic efficiency. Duckweed was damaged by high concentration of OH-under pH8and pH9treatments, especially at pH9. As a result, the number of chloroplasts decreased dramatically, while the membranes of chloroplasts and thylakoid were collapsed and many plastoglobulus ermerged.(5) Spirodela polyrrhiza could maintain a high RGR in water with0.1mg/L to15mg/L of phosphorus. The growth of Spirodela polyrrhiza was restricted, and RGR value was low as phosphorus concentration in water was above45mg/L. The highest phosphorus removal efficiency of Spirodela polyrrhiza was reached approximately70%when phosphorus concentration in solution was ranged between0.3mg/L and3mg/L. ACPase activity of Spirodela polyrrhiza increased when phosphorus was lower than0.3mg/L. However, long time exposure to low phosphorus would limit the growth of Spirodela polyrrhiza, which thus reduced the difference of ACPase activities betweenlow and high phosphorus concentration treatments. H+-ATPase activity with high phosphorus treatment was higher than low phosphorus treatment. Under low phosphorus treatment, many starch grains appeared in chloroplasts and the transport of photosynthetic products was interrupted. As soon as phosphorus concentration of outer solution was increased, starch grains disappeared. When phosphorus concentration reached45mg/L, plasm olysis started and structure of chloroplasts was destroyed, indicated by disintegrated membranes, lamella of thylakoidas arranged in disorder, and decreased nutrients assimilatory efficiency.(6) Difference existed in phosphorus uptake efficiency of three duckweek species in different types of eutrophic water. In landscape water with low nitrogen and phosphorus concentrations, total absorbed phosphorus in Lemna aequinoctialis was higher than in Spirodela oligorrhiza and Spirodela polyrrhiza. This is due to the increase of biomass. However, total absorbed phosphorus and phosphorus uptake capacity of Spirodela polyrrhiza was the highest in domestic sewage and agricultural waste water with higher nitrogen and phosphorus concentration. Spirodela polyrrhiza showed a high phosphorus uptake capacity in high phosphorus concentration environment. In landscape water, chlorophyll content of Lemna aequinoctialis was higher than the other two species, whereas in domestic sewage it was the lowest. With the increase of nitrogen and phosphorus concentration in water, chlorophyll content of Spirodela oligorrhiza and Spirodela polyrrhiza increasedand protein content also increasedin all three duckweed species. ACPase and H+-ATPase activity of Spirodela polyrrhiza, which increased with the increase of phosphorus concentration of outer water environment, was much higher than that of Lemna aequinoctialis and Spirodela oligorrhiza in domestic sewage and agricultural waste water treatments. In high phosphorus water environment, phosphorus removal efficiency, cumulative uptake capacity, and growth rate of Spirodela polyrrhiza were higher than low phosphorus environment, making it a better candidate for treating domestic sewage and agricultural waste water with a high phosphorus concentration. Lemna aequinoctialis had a high growth rate and strong phosphorus uptake capacity in low phosphorus water environment, suitable for low-phosphorus landscape water treatment.(7) Hill equation had a higher precision compared with Michaelis-Menten equation, when correlating H2PO4-uptake rate of three duckweed species and phosphorus concentration in solution. When simulating the relation curve between H2PO4-uptake rate of Spirodela polyrrhiza and phosphorus concentration in solution under different environmental conditions, Hill equation showed a better result than Michaelis-Menten equation. The relationship between H2PO4-uptake rate of duckweed and phosphorus concentration in solution followed a sigmoid curve.