The toughest animals on the Earth: desiccation tolerance and oxidative stress in tardigrades.

Tardigrades are microscopic aquatic ecdysozoans with remarkable abilities to withstand harsh physical conditions, such as dehydration or exposure to harmful highly-energetic radiations, including both solar and cosmic ionizing radiations and the vacuum of the space. They can persist in dehydrated state even for years at any life stage. Yet once external conditions become favorable they resume an unaffected active life. Nevertheless, survival of desiccated tardigrades decreases with the dehydration rate and time spent dry. On the other hand, experimental studies on Paramacrobiotus richtersi provide evidence that exposure to high temperatures, high humidity, and high oxygen partial pressure negatively affect long-term survival of anhydrobiotic tardigrades, and directly influence the time required to reactivate their metabolism. These abiotic factors produce molecular damages, which are accumulated in proportion to the time spent in the desiccated state, potentially leading to tardigrade death. Oxidative stress seems to be one of the most deleterious causes of damages due to water depletion. Experimental studies on P. richtersi indicate that the production of Reactive Oxygen Species (ROS) could occur during anhydrobiosis, being the accumulation of ROS higher in tardigrades maintained desiccated for long-time than in those maintained desiccated for one day only. Therefore anhydrobiosis needs a stringent control of oxidation processes including ROS production. Experimental studies evidence that glutathione and ROS scavenging enzymes represent a key group of molecules for desiccation tolerance in P. richtersi, where the activity of these enzymes is significantly higher in desiccated specimens than in hydrated specimens. We also suggest a role of tardigrade pigments (e.g. carotenoids) as scavengers for ROS forming during dehydration processes and/or exposition to solar radiations. These data exhibit further evidence on the role of antioxidant defenses in tolerant desiccation organisms, and the role of tardigrades as animal model to discover the secret of life without water.