Algicidal activity against Skeletonema costatum by marine bacteria isolated from a high frequency harmful algal blooms area in southern Chinese coast

Four marine bacterial strains P1, P5, N5 and N21 were isolated from the surface water and sediment of Mirs Bay in southern Chinese coast using the liquid infection method with 48-well plates. These bacteria were all shown to have algicidal activities against Skeletonema costatum. Based on morphological observations, biochemical tests and homology comparisons by 16S rDNA sequences, the isolated strains P1, P5, N5 and N21 were identified as Halobacillus sp., Muricauda sp., Kangiella sp. and Roseivirga sp., respectively. Our results showed that bacterial strain P1 killed S. costatum by release of heat labile algicide, while strains P5, N5 and N21 killed them directly. The algicidal processes of four bacterial strains were different. Strains P1, N5 and N21 disrupted the chain structure and S. costatum appeared as single cells, in which the cellular components were aggregated and the individual cells were inflated and finally lysed, while strain P5 decomposed the algal chains directly. We also showed that the algicidal activities of the bacterial strains were concentration-dependent. More specifically, 10 % (v/v) of bacteria in algae showed the strongest algicidal activities, as all S. costatum cells were killed by strains N5 and N21 within 72 h and by strains P1 and P5 within 96 h. 5 % of bacteria in algae also showed significant algicidal activities, as all S. costatum were killed by strains N5, P5 and N21 within 72, 96 and 120 h, respectively, whereas at this concentration, only 73.4 % of S. costatum cells exposed to strain P1 were killed within 120 h. At the concentration of 1 % bacteria in algae, the number of S. costatum cells continued to increase and the growth rate of algae upon exposure to strain N5 was significantly inhibited.