Sessile organisms inhabiting the intertidal zones of rocky shores are exposed to constantly fluctuating and dramatic environmental changes on a daily basis. During low tide, they may be exposed to desiccation, hyper-or hypo-osmotic shock, extreme temperatures or prolonged light exposure, depending on season and latitude. The fre-quency and duration of these stresses increases with tidal elevation. Desiccation is the most important stress factor, and seaweeds that live in these zones are well adapted to significant water loss, displaying full physiological recovery dur-ing rehydration. Although the mechanisms of environmental tolerance in intertidal seaweeds are not completely under-stood, it has been suggested that reactive oxygen metabolism may play a key role. Pyropia haitanensis, a typical middle or high-intertidal species originally found in the south of China, is widely cultivated along the coasts of Fujian and Zhejiang Provinces. Owing to its high desiccation tolerance, simple morphology and ease of culture in the laboratory, P. haitanensis is an ideal material for environmental adaption studies. In this study, we observed the physiological re-sponses of blades of P. haitanensis during desiccation stress. Reactive oxygen species (ROS), including superoxide an-ion free radical ( O2-·), H2O2, reduced glutathione (GSH) and ascorbate (ASA) content, were measured, as were the activities of superoxide dismutase (SOD), catalase (CAT), glutathione reductase (GR) and ascorbate peroxidase (APX). The results show that an important boundary was reached at 60%water loss from the P. haitanensis blades. When water loss was below 60%, the major constituent of ROS produced in the P. haitanensis cell was H2O2, which has a lower relative toxicity than the more toxic O2-·, which did not change significantly. The activity of APX and levels of ASA and GSH did not change significantly, while the activities of SOD, CAT and GR were significantly down-regulated. When water loss was below 60%, as the H2O2 content decreased, the levels of the more highly toxic O2-· were signifi-cantly increased and it became the major constituent of ROS in P. haitanensis cells. Under these conditions, the activi-ties of SOD, CAT and APX were still not significantly increased, while the activity of GR and levels of ASA and GSH were significantly up-regulated. These results suggest that GR, ASA and GSH play important roles in removing ROS during serious desiccation stress, while the antioxidant enzymes SOD, CAT and APX are not involved.%失水胁迫是潮间带中高潮区坛紫菜(Pyropia haitanensis)的主要胁迫因子。本研究以坛紫菜叶状体为材料，探讨其在不同失水程度下细胞中超氧阴离子自由基( O2-·)含量、过氧化氢(H2O2)含量、超氧化物歧化酶(SOD)活性、抗坏血酸过氧化物酶(APX)活性、过氧化氢酶(CAT)活性、谷胱甘肽还原酶(GR)活性、还原型谷胱甘肽(GSH)含量和抗坏血酸(ASA)含量的动态变化过程。结果表明，坛紫菜叶状体生理适应的关键转变点在失水率达到60%时。当失水率低于60%时，细胞内的活性氧成分以毒性较低的H2O2为主， APX活性、ASA含量和GSH含量无显著变化， SOD、CAT和GR活性呈现显著下降趋势；但当超过60%时，细胞内的活性氧成分转变为以高毒性的O2-·为主，抗氧化酶SOD、CAT、APX活性无显著上升，但GR活性、ASA含量和GSH含量均有显著上升。由此间接说明，坛紫菜细胞在高度失水胁迫时所产生的活性氧成分，主要是由GR、ASA和GSH作为清除剂去除的。本研究旨在通过探讨细胞抗氧化系统在坛紫菜失水胁迫应答中的作用机制，为深入了解坛紫菜失水耐受性的生理过程提供理论依据。
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