We collected the diseased blades of Laminaria japonica from Yantai Sea Farm from October to December 2002, and the alginic acid decomposing bacterium on the diseased blade was isolated and purified, and was identified as Alteromon as espejiana. This bacterium was applied as the causative pathogen to infect the blades of L. japonica under laboratory conditions. The aim of the present study was to identify the effects of the bacterium on the growth of L. japonica, and to find the possibly effective mechanism. Results showed that: (1) The blades of L. japonica exhibited symptoms of lesion, bleaching and deterioration when infe cted by the bacterium, and their growth and photosynthesis were dramatically sup pressed. At the same time, the reactive oxygen species(ROS) generation enhanced obviously, and the relative membrane permeability increased significantly. The c ontents of malonaldehyde(MDA) and free fatty acid in the microsomol membrane gre atly elevated, but the phospholipid content decreased. Result suggested an obvio us peroxidation and deesterrification in the blades of L. japonica when infected by the bacterium. (2) The simultaneous assay on the antioxidant enzyme activiti es demonstrated that superoxide dismutase (SOD) and catalase(CAT) increased grea tly when infected by the bacterium, but glutathione peroxidase(Gpx) and ascorbat e peroxidase(APX) did not exhibit active responses to the bacterium throughout t he experiment. (3) The histomorphological observations gave a distinctive eviden ce of the severity of the lesions as well as the relative abundance in the bacte rial population on the blades after infection. The bacterium firstly invaded int o the endodermis of L. japonica and gathered around there, and then resulted in the membrane damage, cells corruption and ultimately, the death of L. japonica.
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