| With the rapid urbanization and industrialization, marine pollution has become a global environmental problem. Harmful algal blooms, one of marine environmental disasters, have brought about huge loses to human. Researches on controlling the blooming of the harmful algae have become an urgent work for scholars. Biological methods, especially microbiological methods, to control or mitigate the harmful algal blooms have become researches’ priority.In our study, a batch of strains were isolated from surface seawater of East China Sea, with high algicidal activity against a harmful algae Phaeocystis globosa. One of which, strain LP-10, was chosen for further analysis. Subcellular structure, physiological activity, gene expression and protein translation of target algal cells were investigated in this study. Furthermore, the culture medium and culture conditions for strain LP-10were optimized for future application. Our work will provide supports for the future development of algicidal applicable products. Main conclusions obtained in this study are listed below:1) Two isolates with high algicidal activity to P. globosa were obtained, namely LP-10and GA-5.16S rDNA sequences of both strains shared99%similarity with Bacillus. Combined with the morphologyical observation, strain LP-10and GA-5were identified as genus Bacillus.2) Strain LP-10was chosen for further study. We found the algae-lysing effect of LP-10to P. globosa was concentration-and time-dependent. Algicidal activities of bacterial cultrures at different growth phases varied significantly, it is at stable phase that LP-10showed the highest algicidal activity. Apart from P. globosa, strain LP-10could also lysed five other algal species, among which four algal species belonged to dinoflagellate that could cause harmful algal blooms. This implied that the algicidal effect of LP-10may be species-specific to some extent. LP-10was assumed to lyse algal cells by indirect attack, and the algicidal activity of LP-10 was expected from the active compounds produced by strain LP-10.3) The intracellular ROS content rised rapidly during a short time. And antioxidant system of algal cells, including SOD, POD, CAT, GSH responsed to the environmental stress. Soluble protein and carbohydrate of algal cells were observed to decrease obviously. Furthermore, the cell membrane integrity was destroyed and the membrane system of algal cells was assumed to suffered serious oxidative damage. Cell structure changes, including intracellular organelles becoming loose, thylakoid membrane becoming sparse, the cell inclusion was released, the cell volume swollen etc, indicated the normal function of algal cells have been destructed by strain LP-10.4) Not only the photosynthesis pigments were destructed, but also the maximum quantum yield (Fv/Fm) and the relative electron transport rate (rETR) decreased rapidly after algal cells were treated. These suggested that the photosynthesis capacity and efficiency of algal cells were inhibited. Gene expression profiles of psb A and rbcS, two important photosynthesis genes, revealed the expression of both genes were inhibited at24h. The content of reaction centre protein D1of PSII was also decreased after algal cells were treated. Our results suggested that photoinhibition was undertaken in algal cells and strain LP-10may be involved in the blocking of the de novo synthesis of protein D1, which lead to the dysfunction of photosynthesis system and algal cell lysis.5) The medium components and culture conditions of strain LP-10were optimized and the optimal culture medium components were:lactose0.5%, yeast extract1.5%, NaCl0.3%, and the optimal culture conditions were:pH7.5, temperature24℃, rotation speed150rpm, culture time5d, innoculation propotion3%and loading volume100mL. |
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