| The aquaculture industry in China is in a growth phase and sea kelp is one of the most commercialized seaweed species so far. However, the key link restricting the production growth in artificial breeding is the heat-resistance and the disease-resistance decreasing. It is of important meanings to study the mechanisms of the heat-stressing occurrence and to find effective tecliniques to raise the heat-resistance in sea kelp farming.2 strains ot Laminar ia japonic a cultured widely in Shandong adlittoral: the heat-resistant strain of Laminaria japonica 901, shorter form of 901, and the comparatively heat-sensitive strain of Rongcheng No.l, shorter form of R.C, were used to analyze the changes of the activities of antioxidant system and the isozyme differentiations under various treating conditions.901 was exposed to high temperature to analyze its physiological and biochemical characteristics changes. Results demonstrated that the growth was inhibited and the photosynthesis rate decreased with the temperature increment. The infection rate of alginic acid decomposing bacteria increased simultaneously Meanwhile, the reactive oxygen species contents and the membrane relative permeability increased in contrast that the phospholipid content decreased in microsomal membrane. Results indicated that high temperature could lead to membrane damages and such damage was really relative to the accumulation of reactive oxygen species that initiated deesterification.Reactive oxygen species had close correlation with antioxidant system ability. To investigate the reasons of heat-resistance, the ROS contents, the membrane stability and the antioxidant system ability of 901 and RC exposed to different living conditions were examined. Results showed that:When living in normal conditions, the soluble protein contents of them two did nol show significant difference, while the contents of soluble polysaccharide, chlorophyll of 901 were much higher than that of R.C (*p<0.05) . Meanwhile, the soluble amino acid content of901 was higher than that of R.C but not on the significant level. There was no significant difference between 901 and RC in total antioxidant ability and POD activity. However, the Vitamin C (Vt.C) content in non-enzymic antioxidant system and glutathione peroxidase (GSH-px) activity in enzymic antioxidant system of 901 were much lower than that of R.C (*p<0.05 ) . The other contents' activities investigated, such as SOD, CAT, PAL and PPO, showed no significant differences. Furthermore, the vertical polyacrylamide gel electrophoresis ( PAGE ) was used to identify the isozyme differentiation of 901 and RC. Isozymes of PRX, MDH, ME, PPO and GDH presented differences in forms of numbers of bands, relative mobility (Rr) and staining intensity. PRX and ME could be used as the distinctive indicator to discriminate the 2 strains of Laminaria japonica cultured in normal conditions. It seemed that the heat-resistance might have nothing with the background contents of antioxidant system.Then the materials were exposed to high temperature. ROS contents kept constant in 901 while accumulated rapidly in RC, suggesting that ROS might play an important role in membrane stability. The protecting enzymes had various ways to resistant the heat stress, some increased the enzymatic activities and some enhanced the stability. However, enzymatic activities in RC were more vulnerable to heat stress and decreased markedly compared with 901, hinting that heat might have stronger effect on enzymatic system of RC. This might be the reason of heat-resistant difference in 901 and RC. Result also showed the 50% nocuity time of the chl. could be used as the most sensitive indicator to indicate the heat-resistant ability of Laminaria japonica.It has been known that Ca2+ could protect the cell membrane from injury. So the pretreatment of Ca2+ was used to prove the inference above mentioned. Ca2+ really took effect on alleviating the increment of ROS and MDA contents under heat-stressing condition. Such reaction played an importan...
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