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Although the evolution of life has turned oxygen into a vital chemical for aerobic organisms, this element can also have deleterious effects on living systems through the production of oxidative stress. This is a process resulting from an imbalance between the excessive production of Reactive Oxygen Species (ROS) and the limited action of antioxidant defences. It is a particularly harmful health risk factor, involved in the development of several chronic human pathologies and believed to play a major role in the ageing process. Consequently aerobic metabolism needs a stringent control of ROS. Water too is essential for life, but some organisms widespread throughout nature have the ability to survive complete desiccation by entering an anhydrobiotic state. The loss of water induces changes in metabolism, cell membrane organization, and molecular composition. In the anhydrobiotic state, high temperatures, high humidity, light exposure, and high oxygen partial pressure negatively affect organism survival and directly influence the time required to reactivate the metabolism after a period of desiccation. These abiotic factors induce damages that are accumulated in proportion to the time spent in the desiccated state, potentially leading to organism death. Oxidative stress seems to be one of the most deleterious damages due to water depletion, therefore anhydrobiosis also needs a stringent control of ROS production. Anhydrobiotic organisms seem to apply two main strategies to cope with the danger of oxygen toxicity, namely an increased efficiency of antioxidant defences and a metabolic control of both energy-production and energy-consuming processes. Experimental studies provide evidence that antioxidant defences such as ROS scavenging enzymes (e.g. peroxidases, catalases, superoxide dismutase, glutathione peroxidases) and other molecules (e.g. glutathione, carotenoids, vitamins C and E) represent a key group of molecules required for desiccation tolerance in anhydrobiotic organisms. The action of these molecules emphasises the need for redox balancing in anhydrobiotic organisms including tardigrades and chironomid larvae.
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