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Thomas Lunde Sørensen-Hygum:
Extreme stress tolerance in tardigrade

Date: 31-12-2019    Supervisor: Nadja Møbjerg

Tardigrades are a fascinating group of microscopic metazoans with many species that are able to deal with temporary periods of extreme stress by entering a dormant state known as cryptobiosis. In this state, tardigrades suspend their metabolism and wait until the stress has passed. By entering cryptobiosis, tardigrades are able to colonize terrestrial habitats that these otherwise aquatic organisms simply could not live in. Cryptobiosis is at present not well understood, and tardigrades are ideal model organisms to try to get a better understanding of cryptobiosis. While it is true that some tardigrades can enter cryptobiosis, this is not the case for others. The distribution of cryptobiotic abilities within the tardigrade phylogeny is debated, partly because most previous studies have focused on limno-terrestrial eutardigrades. Cryptobiosis is not a uniform phenomenon - it is differentiated into categories based on the inducing factor, i.e, anhydrobiosis (desiccation), anoxybiosis (lack of oxygen), chemobiosis (environmental toxicants, cryobiosis (freezing), and osmiobiosis (high concentrations of salts). Currently there is little knowledge on how much the various forms of cryptobiosis differ from each other in terms of underlying physiology. The aim of my PhD thesis has been to obtain and analyze data on dehydration tolerance from a marine heterotardigrade in order to broaden the perspective on cryptobiosis within tardigrades, while also investigating different forms of extreme stress. Studies on dehydration tolerance have been complemented by a comparative study on chemobiosis using copper as a model toxicant and a study on radiation tolerance in the species Echiniscoides sigismundi.

In the course of the studies, it has been possible to provide a new modelling framework for interpreting data from studies on cryptobiosis and stress experiments generally based on the dose-response modelling strategy from toxicology. The studies of cryptobiosis in E. sigismundi have demonstrated that this tardigrade does not require tun formation to recover from dehydration, and that it tolerates extremely rapid dehydration, both things that had not previously been thought possible. Further E. sigismundi has proven to be a very capable cryptobiont, especially in the sense that it is extremely flexible in how few requirements for successful entry into cryptobiosis it has. We have contributed to the knowledge of toxicology in tardigrades by presenting data on copper tolerance across four different species. The work has also contributed to an already large body of literature on radiation tolerance in tardigrades.