Dry and survive: the role of antioxidant defences in anhydrobiotic organisms

Although evolution of life has turned oxygen into a vital compound for aerobic organisms, this element can also have deleterious effects on living systems being involved in the production of oxidative stress. This is a process resulting from an imbalance between excessive production of reactive oxygen species (ROS) and limited action of antioxidant defenses. 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 in anhydrobiotic state. The loss of water involves important biological processes such as changes in metabolism, modifications of cell membranes, and production of ROS. In anhydrobiotic state, high temperatures, high humidity, light exposure and high oxygen partial pressure negatively affect organism survival and directly influence the time required by them to recover active life after a period of desiccation. These abiotic factors induce damages that are accumulated in proportion to the time spent in the desiccated state, leading to organism death. Oxidative stress seems to be one of the most deleterious damages due to water depletion, therefore anhydrobiosis needs a stringent control of ROS production too. Anhydrobiotes seem to apply two main strategies to cope with the danger of oxygen toxicity, namely an increasing efficiency of antioxidant defences and a metabolic control of both energy-production and energy-consuming processes. Experimental studies produced 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 tardigrades. The action of these molecules emphasises the need for redox balancing in anhydrobiotic organisms including tardigrades and sleeping chironomid larvae.