Survival in extreme environments - on current knowledge of adaptations in tardigrades

Tardigrades are microscopic animals found worldwide in aquatic as well as
terrestrial ecosystems. They belong to the invertebrate superclade Ecdysozoa,
as do the two major invertebrate model organisms: Caenorhabditis elegans
and Drosophila melanogaster. We present a brief description of the tardigrades
and highlight species that are currently used as models for physiological
and molecular investigations. Tardigrades are uniquely adapted to a
range of environmental extremes. Cryptobiosis, currently referred to as a
reversible ametabolic state induced by e.g. desiccation, is common especially
among limno-terrestrial species. It has been shown that the entry and exit of
cryptobiosis may involve synthesis of bioprotectants in the form of selective
carbohydrates and proteins as well as high levels of antioxidant enzymes and
other free radical scavengers. However, at present a general scheme of
mechanisms explaining this phenomenon is lacking. Importantly, recent
research has shown that tardigrades even in their active states may be extremely
tolerant to environmental stress, handling extreme levels of ionizing
radiation, large fluctuation in external salinity and avoiding freezing by
supercooling to below )20 C, presumably relying on efficient DNA repair
mechanisms and osmoregulation. This review summarizes the current
knowledge on adaptations found among tardigrades, and presents new data
on tardigrade cell numbers and osmoregulation.