Anhydrobiosis is an astounding strategy that allows certain insects, crustaceans, nematodes, rotifers and tardigrades to survive severe dry and/or extreme cold or hot conditions they often encounter. During anhydrobiosis the animal enters an almost completely desiccated state which stabilizes its membranes and other cellular structures, preventing otherwise lethal damage caused by environmental extremes. Two, not mutually exclusive, hypotheses propose to explain the mechanisms underlying anhydrobiotic macromolecular stabilization. First, the water-replacement hypothesis proposes that accumulated proteins and non-reducing sugars (e.g. trehalose) interact via hydrogen bonds with macromolecules to replace water, thus stabilizing these structures. The alternative vitrification (glass-forming) hypothesis proposes that hydrophilic molecules enter a glassy state during desiccation and this biological glass immobilizes macromolecules, thus preventing denaturation or other structural disruptions.
Steffen Hengherr, Roger Worland and colleagues, from the Universität Stuttgart and the British Antarctic Survey tested the effect of anhydrobiotic glass forming on the heat tolerance of nine Tardigrada (water bears – a sister taxon to arthropods and onychophorans) species. Glass, in this context, doesn't refer to commonly known silica-based window glass but to an amorphous (non-crystalline) biomolecular matrix forming a thermodynamic liquid, i.e. a liquid with an extremely high viscosity, corresponding to a physical solid.
Using tardigrade species from Germany, north-western United States, Kenya and Alaska, Hengherr and Worland's team gradually desiccated individuals until only 5–7% of body mass consisted of water. They then exposed the anhydrobiotic tardigrades to temperatures from 60 to 110°C for 1 h, rehydrated them and assessed survival. They also used differential scanning calorimetry (DSC) to measure glass transition temperatures – temperatures where the biomolecular matrix changes from a highly viscous glassy state to a less viscous ‘rubbery’ state, or the ‘melting point’ of the biological glass.
When fully hydrated these tardigrades are unlikely to survive >40°C. However, during anhydrobiosis the Alaskan species' survival declined only after exposure to 60°C. Five other species could survive 80°C and two others survived 90°C. One extremely tolerant species, Milnesium tardigradum, could survive exposure up to 100°C. In six species, excluding M. tardigradum, glass transitions were clearly detected. These glass transition temperatures corresponded closely to the thermal survival temperatures, linking these transitions to possible survival-reducing spatial disarrangements of biomolecules.
While anhydrobiosis confers markedly higher survivability in tardigrades, the lesser anhydrobiotic heat protection in the Arctic species nevertheless suggests an environmental correlate to the degree of anhydrobiotic protection. Furthermore, previous tardigrade anhydrobiosis research detected no increase in the concentration of trehalose – a key role-player in the water-replacement mechanism's anhydrobiotic biomolecular stabilization in non-tardigrade taxa. In the six species showing glass transition temperatures this supports the vitrification hypothesis. In two species the authors acknowledge that glass transitions could be below the DSC instrument's sensitivity due to small body sizes. And in M. tardigradum's case undetectable glass transitions and undetectable trehalose concentration increases suggest other, as yet unknown, mechanisms conferring unusually effective anhydrobiotic protection against environmental extremes. This study emphasizes our limited understanding of anhydrobiosis and poses further research challenges regarding this remarkable phenomenon.