Interaction Between Chlorella Vulgaris And Associated Heterotrophic Bacteria And Response To Enhanced UV-B Radiation

In current mariculture,some cultured species were strengthened to excess,and other components were weakened and eliminated.A simple ecological structure and blocked material cycle result in a ruinate environment.Environmental improvement and optimization of cultural mode are very important to the healthy and stable development of aquaculture and has been one of the most serious problems aroused general concern in the world nowadays..Enhanced UV-B(280nm-320nm) radiation resulting from ozone depletion is one of global environmental problems.Not only marine phytoplankton,But also marine heterotrophic bacterioplankton can be affected by enhanced UV-B radiation.A common diet microalgae-Chlorella vulgaris was selected to serve as experimental materials and the relationship between C.vulgaris and associated heterotrophic bacteria was estimated under controlled laboratory conditions and change of relationship when stressed by enhanced UV-B radiation.The results could supply experimental base to answer the effect of enhanced UV-B radiation on aquatic ecosystem,while provide theoretical evidences to ecological restoration technology in mariculture.Results showed:1.The ecotoxicological method was used to determine the different effect of antibiotics on Chlorella vulgaris and associated heterotrophic bacteria,worked out a practicable method for establishing axenic Chlorella vulgaris.The results showed that: Penicillin,Gentamycin,Kanamycin and Neomycin were feasible antibiotics for removing bacteria in Chlorella vulgaris medium;after removing mould by taking monoclonal Chlorella vulgaris on the plate,Penicillin,Gentamycin,Kanamycin were added to cultural system of diluted and filtered Chlorella vulgaris in turn,which the concentrations were 100IU respectively.Axenic Chlorella vulgaris was obtained after twice treatment.2.The results about effect of enhanced UV-B radiation on Chlorella vulgaris pre and post removing bacteria indicated that there was not significantly difference between axenic and non-axenic Chlorella vulgaris by comparing their population growth.But the growth rhythm was varied obviously;the lag phase,log phase and stationary phase were prolonged significantly;and maximal cell density increased. When the dose of UV-B radiation was from 0.2 J/m2 to 0.8J/m2,axenic and non-axenic Chlorella vulgaris were promoted and the non-axenic Chlorella vulgaris had a better growth compared with the growth of axenic Chlorella vulgaris under the same dose of UV-B radiation.While two experimental groups were inhibited when the doses of UV-B radiation exceed 1.6J/m2,and axenic Chlorella vulgaris was presented more obvious inhibitory effect under the same dose of UV-B radiation.3 The microbiological and ecotoxicological method was used to study associated heterotrophic bacterial composition and quantitative character in different growth phase of Chlorella vulgaris.The results are as follows:An obvious selectivity of Chlorella vulgaris presented an obvious selectivity to heterotrophic bacteria, community composition including Bacillus,Pseudomonas,Flavobaterium and Alcaligense.Gram negative bacteria occupied absolute advantage position;the value was about 91%;Pseudomonas was the dominant bacteria in the lag phase,stationary phase and decreased phase Chlorella vulgaris and Bacillus was dominant bacteria in log phase of Chlorella vulgaris;the order of total amount of bacteria from high to low was decreased phase＞stationary phase＞lag phase＞log phase.4 Response of associated heterotrophic bacterial composition and quantitative character in different growth phase of Chlorella vulgaris to enhanced UV-B radiation was determined in lab.No change of bacteria composition under the dose of UV-B radiation from 0.2 J/m2 to 1.6J/m2;while Aeromonas was killed completely when the dose of UV-B radiation reached 3.2 J/m2.Pseudomonas was the dominant bacteria when UV-B radiation from 0.2 J/m2 to 0.8 J/m2;while Bacillus was the dominant bacteria if the dose of UV-B radiation was more than 1.6 J/m2.The total amount of bacteria was decreasing gradually with enhanced UV-B radiation.5 The ecotoxicological and statistics method was selected to examine interaction between Chlorella vulgaris and associated heterotrophic bacteria.Results indicated that the change of relative growth rates of Chlorella vulgaris and associated heterotrophic bacteria were always opposite;there were an enhanced inhibitory effect to associated heterotrophic bacteria with cell density of Chlorella vulgaris increasing; associated heterotrophic bacteria could promote the growth of Chlorella vulgaris when its density was low,while an inhibitive effect was presented to Chlorella vulgaris when cell density of associated heterotrophic bacteria exceed 10×106cell/ml. Different action mechanisms were exhibited between Chlorella vulgaris and associated heterotrophic dominant single bacteria,Chlorella vulgaris exhibited inhibitive effect to bacteria-H1(Bacillus)through interference competition and exploitation competition to bacteria-H2(Pseudomonas);while bacteria-H1 and H2 presented exploitation competition and interference competition to Chlorella vulgaris respectively.6 Interactions between Chlorella vulgaris and associated heterotrophic bacteria were changed by enhanced UV-B radiation.The lag phase of Chlorella vulgaris in all experimental groups were shortened,log phase and stationary phase were prolonged,and maximal cell density was promoted after UV-B irradiation,which the dose of UV-B radiation was under 0.8J/m2;the optimal enhancement action was presented in 0.4J/m2experimentaI group;the growth of Chlorella vulgaris were inhibited when the doses of UV-B radiation exceed 1.6J/m2.The growth of associated heterotrophic bacteria was not changed obviously when the dose of UV-B radiation was lower than 0.4 J/m2;UV-B radiation could produce inhibition effect to heterotrophic bacteria in co-cultural system when dose of UV-B radiation was higher than 0.8J/m2.Combined action of UV-B and Chlorella vulgaris enhanced inhibitory effect to associated heterotrophic bacteria,and inhibitive effect were weakened stressed by UV-B and associated heterotrophic bacteria.Inhibitive effects of Chlorella vulgaris Filtrate after treatment by UV-B radiation on bacteria-H1were lowered,and had not significant change to bacteria-H2;the growth of Chlorella vulgaris was not changed when cultured with filtrate of bacteria-H1 and H2,which stressed by UV-B radiation.7 An associated heterotrophic dominant single bacteria-H2 was selected to study its mechanism of quorum sensing and factors that affected quorum sensing by using ecotoxicological method in co-cultural system with Chlorella vulgaris.The results showed that self-regulation and inhibitive effect to Chlorella vulgaris of bacteria-H2 were conducted through excreting chemic substance.Density of bacteria-H2 and concentration of chemic substance excreted by bacteria-H2 were necessary conditions, which decided whether the regulation mechanism could be carried out or not.Initial density of Chlorella vulgaris,bacteria-Hlstress and enhanced UV-B radiation could affect quorum sensing of bacteria-H2,but there were different action mechanism among them.Chlorella vulgaris inhibited bacteria-H2 through exploitation competition;bacteria-H1 can use the chemic substance excreted to medium by bacteria-H2,by which the H1 regulated quorum sensing;UV-B radiation also affected the growth of bacteria-H2 by control the rate of cell division of bacteria-H2.