The Metabolic Response Of Duckweed To Different Nitrogen Spscies And Concentrations

With the rapid development of economy and society in china, the degree ofeutrophication caused by nitrogen and phosphorus which has become the problem ofwater pollution control is more and more serious. To conventional sewage treatmenttechnology, although their processes are mature and satisfactory, the costs of operationand management are expensive, limitted the wider application, especially to theundeveloped regions, including small towns and villages. In recent years, duckweedrestoration technology has become one of the hottest issues in the environment field forlow energy consumption, high efficiency and wide adaptability. This new ecologicaltechnology is regaded to be an innovation and popular around the world. The screening,uptake rates and resourse utilization of different duckweed species were investigated inthe previous researches. There were few studies about the nitrogen migration andtransformation mechanism in duckweed treatment system, as well as the relationshipbetween metabolic enzyme activities and nitrogen removal characteristics.The absorbtion characteristic and oxidative stress reaction of duckweed （Lemnaminor） to ammonum （NH₄⁺_-N） was investigated, and the impacts of nitrogen spsciesand concentrations on the physiological status of duckweed were studied in this paper,taking account to indicate the purification mechanism to wasterwater and providetheoretical basis and technical support for the application of duckweed restorationtechnology. The main results are as follows:1) The kinetic absorbtion characteristics of duckweed to NH₄⁺_-N could beillustrated with the Michaelis-Menten equation （M-M equation）. Accoring to the fittingresults of M-M equation, the maximum absorption rate was0.0733mg·g^-1FW·h^-1andthe affinity constant was1.4409h-1. The growth of duckweed could be influenced bydifferent concentration of NH₄⁺_-N. With increasing of concentration, the changes ofsoluble protein （SP）, chlorophyll and peroxidase （POD） activity were increased firstlyand then decreased, and the maximum value were all appeared at3mmol/L. But thechanges of SOD activity and MDA were just opposite, with the minimum value at3mmol/L. When the concentration of NH₄⁺_-N changed from0.5to3mmol/L, duckweedcan grow normally. but the indicators of enzymology showed that duckweed hadantioxidative response to NH₄⁺_-N, especially to higher concentration than3mmol/L.SOD activity can be used to indicate the stress by the high concentration of NH₄⁺_-N to duckweed.2) With the increase of time culturing in some NH₄⁺_-N treatment concentrations,the biomass of duckweed were increased with the decreasing of concentration; theuptake rate to NH₄⁺_-N and growth rate of duckweed were showed to be the same trend,increasing firstly and then decreasing. The changes of chlorophyll, SP and glutaminesynthetase （GS） activity also showed the same. The relationship between theconcentration of NH₄⁺_-N and photosynthetic pigment of duckweed was not significant,but GS activity was increased with stimulation of high NH₄⁺_-N, whereas the metabolismto NH₄⁺_-N was speed up and the synthesis to amino acids and SP were promoted.According to the results of correlation analyses, there was a significant positivecorrelation between SP and chlorophyll. Significant positive correlation between GSactivity and chlorophyll in high concentration was observed, as well as the relationshipbetween SP and Biomass in low concentration.3) With the increase of time culturing in some NO₃^-_-N treatment concentrations,the biomass of duckweed were increased with the decreasing of concentration; theuptake rate to NO₃^-_-N and growth rate of duckweed were showed to be the same trend,increasing firstly and then decreasing; Nitrate reductase （NR） activity in duckweedchanged significantly, while there was little changes to chlorophyll, SP content and GSactivity. High initial concentrations limitted the enhancement of NR activity, andpromoted the synthesis of photosynthetic pigments, whereas the adverse effect to SPand GS activity was little. According to the results of correlation analyses, a closecorrelation between SP and chlorophyll was observed in this study. The NR activitycould be improved by high NO₃^-_-N concentration, while the synthesis of chlorophylland SP in duckweed was promoted.