Tardigrades also called water bears or moss piglets are organisms found on Earth, known to be present in extreme environments. These microscopic invertebrates were being carried in dehydrated form on an Israeli space craft which crash landed on the moon. The enduring nature of these organisms has made them the perfect candidate for space research. Although infectious potential of the organism has not been described, astrobiologists are suspicious of hazardous implications of introducing a new organism on the moon.
The idea of what exactly a tardigrade is boggles the common man. It’s not a bacteria or an insect but an animal, basically a eukaryote that feeds on moss. They are 50 to 2,100 μm in length. They are segmented with a functional digestive and nervous system. They have 5 segments beginning with a head and have 4 pairs of legs with claws. The body is covered by a cuticle and may or may not have plates depending on the species.They consume algae, bacteria, fungi or nematodes depending on the species which are broadly fresh water and terrestrial.Parthenogenesis is also known to occur in some species. Their life cycle begins with eggs laid into the environment, followed by moulting 2-7 times after hatching. Adults are known to live for several months. Genes are spread through horizontal gene transfer.Currently, around 90 to 1,200 tardigrade species are known.
They are known to survive in the worst of conditions, below freezing to above 120°C, high pressure and high vacuum conditions. These organisms have a special skill of shrinking their body size, hence decreasing their surface area and in turn reducing transpiration of water by 50%. This condition also called the ‘tun’ state is one of the crypto biotic states that the organism enters to protect itself, similar to hypobiosis. In this crypto biotic state, tardigrades have shown resistance to hydrogen sulphide, carbon dioxide, ultraviolet light, and X-rays. The tun state also prevents destruction of internal and external organs.
Paramacrobiotusrichtersi ,Ramazzottiusoberhaeuseri, Richtersiuscoronifer and Milnesiumtardigradum are some of the species extensively studied for space research.Desiccation tolerance of tardigrades has been attributed exclusively to Intrinsicdisordered protein(IDP), Cytoplasmic Abundant Heat Soluble (CAHS) and Secondary Abundant Heat Soluble (SAHS) protein. CAHS and SAHS by modifying their inherent structure into alpha helicesaid in adapting to extreme environments. Altering enzyme activity and fat content are other routes through which tardigrades acquire their unique traits.Genomic analysis showed that tardigrades had 17.4% foreign genes, another unusual feature contributing to its uniqueness.
Genomic studies of the tardigrades are being undertaken, so as to incorporate resistant genes into other living beings. The special nature of tardigrade proteins has also been suggested for vaccines and medicines that require cold chain maintenance.
- Weronika, E. and Łukasz, K., 2017. Tardigrades in space research-past and future. Origins of Life and Evolution of Biospheres, 47(4); 545-553.