Introduction: Extremophiles are organisms that have adapted or at least are tolerant to extremely harsh environments. Most extremophiles are single cellular organisms with simple structure, such as archaea and bacteria. Tardigrade were found in harsh environments such as Antarctica as well as in urban area, e.g., from activated sludge in a sewage treatment plant. All tardigrades require surrounding water to grow and reproduce, but some limno terrestrial species are able to tolerate almost complete dehydration. When the surrounding water evaporates, tolerant tardigrades lose almost all body water enter a metabolically inactive dehydrated stage called anhydrobiosis. The dehydrated tardigrades withstand various extreme conditions that normally disallow the survival of most other organisms: for example, low and high temperatures from (-273 °C to nearly 100 °C), high hydrostatic pressure (7.5 GPa), immersion in organic solvent and exposure to high dose of irradiation.
Methods: We reviewed about 22 articles were conducted from 2019 to 2022 in the world and Iran. We searched some key words such as astrobiology, extremophiles, tardigrade, HSP70, microbes in space in sciencedriect, Elsevier, PubMed and SID.
Results: Tardigrades, known colloquially as water bears or moss piglets, are a phylum of eight legged segmented micro animals. The 0.1 mm to 1.2 mm large animal, was firstly described by German zoologist Goeze in 1730. Tardigrade is Latin and means slow moving, because of their appearance and clumsy way of moving they are often referred to as water bears. Despite called HSP the induction is not only due to temperature effect but also to a whole other series of environmental stress factors. These proteins are highly conserved and ubiquitous in all organisms. They play a crucial role in folding newly synthesized proteins, intracellular protection against protein denaturing factors, folding, unfolding of damaged proteins, assembly of multi protein complexes, transport, sorting of proteins into correct subcellular compartments, cell cycle control and signaling and protection of cells against stress, apoptosis. HSPs are classified based on their molecular weight: HSP10, HSP40, HSP70, HSP90. Genes encode these proteins are conserved in all of the organisms they have been found, including tardigrades. Due to their function in the cell, HSPs are believed to enhance tolerance in cryptobiosis. We observed different levels of expression of HSP70 genes across consecutive states of cryptobiosis in the tardigrade milnesium tardigradum and from the three recognized HSP70 isoforms, HSP70Π was the most relevant in recovering from anhydrobiosis (HSP70) induction in rehydrating tardigrade richtersius coronifer shows a similar gene expression pattern as observed in m. tardigradum. I expected to observe down regulation in HSP70 like 1 gene in the transitional state compared to other observed anhydrobiosis states active, preconditioned and dry. Previous studies with anhydrobiotic tardigrade m. tardigradum showed downregulation of HSP70 isoform 1 in the transitional state, suggesting the gene having no direct role in anhydrobiosis. In addition, HSP70 showed downregulation in the transitional stage in tardigrade R. coronifer.
Conclusion: we showed that in m. tardigradum three isoform of HSP70 are up regulated during a heat shock. However only isoform 2 was significantly induced in the transitional stage between the active and cryptobiotic stage and was found in the anhydrobiotic stage. Yeast studies showed that HSP70 does not protect the yeast from dehydration stress during desiccation. Thus, HSP70 is probably only translated after rehydration to fold newly synthesized protein and does not protect the cell during desiccation.
Keywords: astrobiology, extremophiles, tardigrade, HSP70, microbes in space