Preliminary Study Of The Biological Control On The Cyanobacteria Using Lymnaea Sp.

Algal bloom is a phenomenon that algae (mainly referring to cyanobacteria) explosively grow caused by eutrophication in freshwater. The cyanobacteria blooms become more frequent due to the economic growth, and has been taken as a threat to human health and the aquatic ecosystem stability. Environmentalists have been paid much attention to study the prevention and treatment strategies for cyanobacterial bloom. Now there are some methods had been tried to practice, including to biological manipulation, which has been taken as the most promising way.Focusing on algal blooms, through laboratory simulating, this thesis tried to use Lymnaea sp. as a tools to control cyanobacteria bloom. Based on the theory of non-classical biological manipulation, the applicablity of the proposed method was verified, and the mechanism of the present method was also preliminarily studied. In detail, the effect of applying this method was verified; the treating effects of Lymnaea sp. and Cipangopaludina cathayensis against Microcystis aeruginosa (a tyical cyanobacterium) were compared; the quantitative relation between the density of Lymnaea sp. and M. aeruginosa ensuring the inhibition effect was examined; in addition, the total nitrogen changes in the water and cells under the treatment of Lymnaea sp., and the mechnisms of the inhibition effect of Lymnaea sp. against M. aeruginosa were finally studied. In this thesis, through the study on the new biologic manipulation species, Lymnaea sp., we aim on this application of this method to help the prevention and the controlling for algal blooms. It not only has economic significance and practical operability, and no secondary pollutions, but also helps to improve water quality and environmental protection, which should possess the significance in environmental science and ecologyl. The main results of this thesis is as following:(1) Lymnaea sp.grows in the environment of the water temperature around20-23℃. It is hermaphrodite, oviparous.Its eggs are wrapped in a the eggs bag. Under laboratory conditions, the egg sac sizes vary normally including about80-90eggs. If not being in good conditions, it produce circular eggs or short egg sac including only about ten eggs. The eggs was2-3rows in the egg sac. The eggs attach to the tank wall or aeration stone, or the plants in water. The period from eggs to being completely hatched is about7-12d. The food in laboratory can be vegetables (cabbage, lettuce). Small amount of water plants help its habitat.(2) Laboratory studies have found that the OD value at630nm wavelength of algal cells can represent Microcystis aeruginosa cell density. In a certain density range, chlorophyll total mass concentration of Microcystis aeruginosa absorbance (OD630nm), and the cell densities have the linear relationship in one another. The linear correlation coefficient for chlorophyll total mass concentration and the density of algal cells. the algae cell densiy and absorbance (OD630nm), and the chlorophyll total mass concentration and absorbance (OD630nm) is R2=0.9949, R2=0.9995, R2=0.9922, respectively. Therefore, OD value can be an accurate representative parameter for the cell density. The usage of OD value rather than the cell density will reduce the complexity of the experimental operation.(3) The verification of the inhibititon on the cyanobacteria growth by Lymnaea sp.. The results show that the Lymnaea sp. can effectively inhibit the growth of mixed cyanobacteria as well as single Microcystis aeruginosa.:the experiment beginning algae density of mixed algal is3.5×105Cell·ml-1, adding the Lymnaea sp.2days, algae cell density decreased rapidly (T5about1.13×105Cell·ml-1、T15about0.82×105Cell·ml-1), algae cell density of not adding Lymnaea sp. is on the rise (10.56×105Cell·ml-1); the experiment beginning algae density of Microcystis aeruginosa. is1.76×106Cell·ml-1, adding the Lymnaea sp.7days, algae cell density is zero, algae cell density of not adding Lymnaea sp. is on the rise (3.92×106Cell·ml-1). Microcystis aeruginosa was selected as the object to avoid the population’s uncertainty in mixing cyanobacteria. In the experiments comparing the effects of Lymnaea sp. and Cipangopaludina cathayensis against Microcystis aeruginosa, we found that Lymnaea sp. was more tolerant than Cipangopaludina cathayensis in harsh enviroments, therefore, with promising potential.(4) Quantitative relationship study on Lymnaea sp. against Microcystis aeruginosa. The experimental design includes the two programs, the first of which used fixed Lymnaea sp. density and various Microcystis aeruginosa densities while second of which used various Lymnaea sp. density and finxed Microcystis aeruginosa densities. In detail, for the first one, we put10indi·L-1adults of Lymnaea sp. in all three treatmentswith initial cell densities of Microcystis aeruginosa with approximately4.38×106Cell·ml-1,14.25×106Cell·ml-1,25.55×106Cell·ml-1respectively. For the second one, initial cell densities of Microcystis aeruginosa were all approximately6.91×107cell·ml-1, we put10、20、30indi·L-1adults of Lymnaea sp. into three respective treatments. In both two experiments, Lymnaea sp. can effectively inhibit the growth of algae. The significant difference were not observed in these two experiments. The critical ratio should be further studied.(5) Study on the mechanism of the Lymnaea sp. controlling Microcystis aeruginosa. It was found that Lymnaea sp. culture medium also has activity agsinst algae, that is, after excluding Lymnaea Lymnaea, the culture medium can inhibit the growth of Microcystis aeruginosa. Therefore, the activity of the culture medium after different treatments were examined. After filtered by0.22μm,0.45μm membrane, no activity was obsered. The substances on the membrane have activity. So symbiotic microbios were isolated and examined, the results of which indicate no inhibition effects was found for the bacteria, and fungi. In addition to a small amount of M. aeruginosa cells in the previous treatments, a kind of unknown swimming small organism was frequently found. The density of this small organism incresed with the elongation of incubation time, but decreased after2～3d when algae cells completely disappeared. Therefore, this unknown small organism may be one of the reasons inhibiting algae growth. Under a microscope, it was usually accompanied by M. aeruginosa cells, with round or oval shapes and about1mm diameter. A flagellum with length of1.5folds of its body. It can swim in water or attach on the bottle, and can not survive without water after hours. According to the mophology and habites, the unknown small organism should belong to zoomastigina for flagellate. However, the hibits and its mechanism of controlling algae growth need the in-depth study.(6) Total nitrogen (TN) after Lymnaea sp. treatment did not change too much. However, due to the decrease of the algae cell density, the utilization of nitrogen by algae also decreased. Therefore, the reduction of TN in control was more signicant than that in treatment.(7) The excretion of Lymnaea sp. fed with M. aeruginosa was also examined. No algal cells were oberved in the excretion by microscope.. And the occurrence of algal cell growth was also not observed after it was cultured in BG-11medium. These results showed Lymnaea sp. completely digested algae cells, which indicates that it has significant importance for biological manipulation.In this paper experimental study, preliminary understanding the Lymnaea sp.’s life habits, and OD value can be an accurate representative parameter for the cell density. It was found that, the Lymnaea sp. can effectively inhibit the growth of mixed cyanobacteria and Microcystis aeruginosa., Lymnaea sp. was more tolerant than Cipangopaludina cathayensis in harsh enviroments, but the critical ratio should be further studied; It was found that Lymnaea sp. culture medium has activity agsinst algae, maybe the Lymnaea sp.parasitic, symbiotic microorganisms in culture medium has activity agsinst algae; the Lymnaea sp. may affect the concentration of total nitrogen in water; and the excretion by microscope. Almost does not contain algae cell.