Nitrate uptake capacity of duckweed lemna Minor L. upon the laboratory conditions

Kỷ yếu Hội nghị: Nghiên cứu cơ bản trong “Khoa học Trái đất và Môi trường” DOI: 10.15625/vap.2019.000198 534 NITRATE UPTAKE CAPACITY OF DUCKWEED LEMNA MINOR L. UPON THE LABORATORY CONDITIONS Nguyen Quy Hao 1 , Tran Ngo Hoang Dung 1 , Bui Thi Nhu Phuong 2 , Phan The Huy 2 , Dao Thanh Son 1* , Bui Xuan Thanh 1 1 Faculty of Environment and Natural Resources, Ho Chi Minh City University of Technology, VNU-HCM, Vietnam. 268 Ly Thuong Kiet street, district 10, Ho Chi Minh City, Vietnam. 2 Institute for Environmental and Resources, VNU - HCM *Corresponding author’s email: dao.son@hcmut.edu.vn ABSTRACT Organic pollution, nutrient increase and eutrophication of aquatic environment are common problems in the world in general and Vietnam in particular. Phytoremediation is one of the most priority means to improve the surface water quality because of the friendly environment, low cost and high efficiency. In this study, we evaluated the nitrate uptake capacity of duckweed (Lemna minor L.) from Vietnam upon the laboratory conditions. The results showed that after 9 days of incubation, the duckweed could uptake from 10 - 23% of the nitrate concentration. Based on the biomass, the nitrate uptake capacity of the duckweed was around 24.87 mg nitrate/ g wet weight of the plant. The high nitrate uptake capacity of the duckweed suggests further researches at higher scales (e.g. pilot) before application. Keywords: Duckweed, nitrate, phytoremediation. 1. INTRODUCTION Duckweed (Lemna minor L.) is a water plant, which is widely distributed throughout the world, especially in tropical and subtropical regions. They are often found in lakes or swamps. Duckweed is also found in places with high nutrient content (nitrogen, phosphorus) such as domestic and agricultural wastewater, septic tanks [1]. Duckweed is an important food source for some aquatic organisms such as waterfowl, fish and poultry [2], [3]. Duckweed plays an important role in overcoming the excess of mineral nutrients in ponds by biological methods because they grow quickly and absorb most of these substances, namely nitrogen and phosphorus [4]. Using duckweed to evaluate environmental quality has also achieved many scientific and practical achievements [5]. Research on duckweed in our country is still modest and sporadic with a few published works. Besides, duckweed shows great potential in cleaning the aquatic environment [3]. Organic pollution is a common problem in fresh water bodies in Vietnam. The water environment in Vietnam, in general, has been and is polluted organically. The objective of this study was to evaluate nitrogen uptake capacity of Vietnamese duckweed in laboratory conditions. 2. MATERIAL AND METHODS 2.1. Material Duckweed is collected from a polyculture pond with fish culture in District 12, Ho Chi Minh City. Duckweed (Lemna minor L.) is being maintained at the Ecological Toxicology Module, Department of Environment and Resources, Ho Chi Minh City University of Technology. Duckweed is reared in a modified nutrient solution Z8 [6] under laboratory conditions with a temperature of 27 ± 2 ° C, pH = 7, light intensity of about 3,000 Lux at the water surface and a light and dark cycle. 12 am: 12 pm [2]. 2.2. Methods In the study, the experiment was designed to include the Z8 medium with duckweed. Two flasks were prepared and carried out in parallel, the first containing 250mL of Z8 medium and 125 Hồ Chí Minh, tháng 11 năm 2019 535 duckweed. Water samples used to measure nitrate (50 mL) for the experiment were collected 3 times, which are on the first day, on the 5th day and the last day of the experiment (day 9). The number of duckweeds in the experimental vessels at day 5 and day 9 of the experiment was also recorded. The average weight of a duckweed was also determined by randomly weighing 30 duckweeds (2-leaf plants each) from the experimental batches. The second flask of the experiment contains 300 ml of Z8 medium and 150 duckweeds. The third one only has 300 ml of Z8 medium with no duckweeds added in parallel to verify the change of nitrate (in Z8) in the absence of duckweed. The experiment lasted for 9 days and by the last day, there was enough duckweed to cover the surface area of the flasks. 2.3. Analytical methods Using analytical methods for nitrate according to TCVN 6180:1996. 3. RESULTS AND CONCLUSIONS 3.1. The growth of duckweed and the change of nitrate concentration in the experiment As shown in 3.2. Nitrate uptake capacity of duckweed The results of fresh weight and average calculation (n = 30) showed that the average fresh weight of 1 duckweed was 0.827 ± 0.1102 mg. Thus, considering the nitrate uptake capacity of duckweed based on the number of duckweed and changes in nitrate concentration in Table 2, the nitrate uptake capacity of duckweed in the current study is about 24.87mg nitrate/g fresh weight duckweed. Goopy and Murray (2003) reported that N uptake capacity of the aquatic plants (Lemnaceae) can be up to 6,110 kg N/ha/year. However, the information on N uptake capacity of duckweed (Lemna minor) per unit of fresh weight of duckweed, as far as we understand, is not available. Therefore, it is necessary to study and calculate the absorption capacity of each individual aquatic plant species to evaluate the effectiveness of nutrient treatment before putting them into application under specific conditions. The results of this study show that nitrate uptake rate is very positive, making it a basis for conducting research at a higher level (such as pilot) before putting into practical application. Table 1, the number of duckweeds increases steadily. It is possible that the number of duckweeds and the surface area affect the production rate of duckweeds under laboratory conditions. This needs to be studied further to determine the quantitative impact if any. Considering the changes in nitrate concentration in the experiment, the rate of nitrate uptake ranged from 10-23%, which is not high. This rate is still very low compared to the total nitrogen treatment rate of water hyacinth (Eichhornia crassipes) in the pilot with hog farm wastewater, which went up to 65% [7]. The major differences between this experiment and the ones carried out by Nguyet et al., (2015) are (1) differences in plant objects (duckweed and water hyacinth) for nutrient treatment; (2) research scale (laboratories and pilots); and (3) the nature of nutrient- containing solution (artificial Z8 medium and pig waste after decomposition by microorganisms). 3.2. Nitrate uptake capacity of duckweed The results of fresh weight and average calculation (n = 30) showed that the average fresh weight of 1 duckweed was 0.827 ± 0.1102 mg. Thus, considering the nitrate uptake capacity of duckweed based on the number of duckweed and changes in nitrate concentration in Table 2, the nitrate uptake capacity of duckweed in the current study is about 24.87mg nitrate/g fresh weight duckweed. Goopy and Murray (2003) reported that N uptake capacity of the aquatic plants (Lemnaceae) can be up to 6,110 kg N/ha/year. However, the information on N uptake capacity of duckweed (Lemna minor) per unit of fresh weight of duckweed, as far as we understand, is not available. Kỷ yếu Hội nghị: Nghiên cứu cơ bản trong “Khoa học Trái đất và Môi trường” 536 Therefore, it is necessary to study and calculate the absorption capacity of each individual aquatic plant species to evaluate the effectiveness of nutrient treatment before putting them into application under specific conditions. The results of this study show that nitrate uptake rate is very positive, making it a basis for conducting research at a higher level (such as pilot) before putting into practical application. Table 1. Changes in number of duckweeds and nitrate content in the experiment Z8 medium (mL) Day of the experiment Number of duckweeds Nitrate concentration (N- NO3 - mg/L) 1 150 47 300 mL 5 245 45 9 402 36 1 125 47 250 mL 5 285 46 9 363 42 Table 2. Nitrate uptake capacity of duckweed in laboratory conditions Z8 medium (mL) Nitrate uptake capacity of duckweed (mg nitrate/g duckweed) 5 days of experiment 9 days of experiment 300mL 9,87 33,08 250 mL 4.24 16,65 Average 7.05 24,87 REFERENCES [1]. C. A. Lembi, (2014). The Biology and Management of Algae, in BIOLOGY AND CONTROL OF AQUATIC PLANTS - A Best Management Practices Handbook, West Lafayette: Aquatic Ecosystem Restoration Foundation, pp. 97-104. [2]. APHA (2012). Standard methods for the examination of water and wastewater, 22nd edition. Edited by E. W. Rice, R. B. Baird, A. D.Eaton and L. S. Clesceri. [3]. R. A. Leng, (1999). Duckweed: A tiny aquatic plant with enormous potential for agriculture and environment. FAO. [4]. J. P. Goopy and P. J. Murray, (2003). A review on the role of duckweed in nutrient reclamation and as a source of animal feed,” Asian-Australasian J. Anim. Sci., 16(2), pp. 297-305. [5]. T. Le Huu and H. Le Huy, (2016). 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