Dry and survive: the role of antioxidant metabolism in anhydrobiotic organisms.

Although evolution of life has turned oxygen into a vital compound for most organisms, this element can also have deleterious effects on living systems. Oxidative stress 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, common to the development of several chronic human pathologies (e.g. cancer, Alzheimer\u2019s and Parkinson\u2019s) and believed to play a major role in the ageing process. Thus antioxidant protection is essential for survival under an aerobic environment. Water too is essential for life, but some organisms have the ability to survive extreme desiccation by entering into a state of suspended animation called anhydrobiosis. These organisms are widespread throughout nature, including bacteria, protists, yeasts, plants and animals. The loss of water involves important biological processes such as changes in metabolism, alterations of cell membranes, and production of oxidative stress. Therefore, the maintenance of life in the absence of water requires a complex set of mechanisms working in close coordination, such as the accumulation of bioprotectant molecules, the activation of molecular repair mechanisms and of antioxidant and molecular chaperone systems. Oxidative stress seems to be one of the most deleterious effects of water depletion, since the susceptibility to oxidative damage may increase with dehydration. 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. Tardigrades are here presented as model system to evaluate the effective damages induced by an increase of ROS production during desiccation and to understand the role of antioxidant systems to ensure survival of living beings when in the anhydrobiotic state. Even though desiccation does not seem to have an effect on tardigrade longevity, damages are accumulated in proportion to the time spent in the desiccated state, leading to animal death. High temperatures, high humidity and high oxygen partial pressure are all factors that negatively affect tardigrade survival during long-term anhydrobiosis since they are involved in the production of oxidative stress. These abiotic conditions also directly influence the time required by animals to recover active life after a period of desiccation. Experimental studies produced evidence that enzymes (e.g. peroxidases, catalases, superoxide dismutase) and antioxidants (e.g. glutathione and carotenoids) 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 tardigrades