High temperatures due to global warming will be dramatic even for tardigrades

Global warming, a serious aspect of global climate change , is already causing a good range of negative impacts on many habitats of our planet. it’s thus of the utmost importance to know how rising temperatures may affect animal health and welfare.

A research group from Department of Biology, University of Copenhagen has just shown that tardigrades are very susceptible to long-term heat exposures. the small animals, in their desiccated state, are best known for his or her extraordinary tolerance to extreme environments.

In a study published recently in Scientific Reports, Ricardo Neves and Nadja Møbjerg and colleagues at Department of Biology, University of Copenhagen present results on the tolerance to high temperatures of a tardigrade species.

Tardigrades, commonly referred to as water bears or moss piglets, are microscopic invertebrates distributed worldwide in marine, freshwater and terrestrial microhabitats.

Ricardo Neves, Nadja Møbjerg and colleagues investigated the tolerance to high temperatures of Ramazzottius varieornatus, a tardigrade frequently found in transient freshwater habitats.

“The specimens utilized in this study were obtained from roof gutters of a house located in Nivå, Denmark. We evaluated the effect of exposures to heat in active and desiccated tardigrades, and that we also investigated the effect of a quick acclimation period on active animals,” explains postdoc Ricardo Neves.

Rather surprisingly the researchers estimated that for non-acclimated active tardigrades the median lethal temperature is 37.1°C, though a brief acclimation periods results in alittle but significant increase of the median lethal temperature to 37.6°C. Interestingly, this temperature isn’t faraway from the currently measured maximum temperature in Denmark, i.e. 36.4°C. As for the desiccated specimens, the authors observed that the estimated 50% mortality temperature is 82.7°C following 1 hour exposures, though a big decrease to 63.1°C following 24 hour exposures was registered.

The research group used logistic models to estimate the median lethal temperature (at which 50% mortality is achieved) both for active and desiccated tardigrades.

Approximately 1300 tardigrade species are described thus far . The body of those minute animals is barrel-shaped (or dorsoventrally compressed) and divided into a head and a trunk with four pairs of legs. Their linear unit varies between 50 micrometers and 1.2 millimeters. aside from their impressive ability to tolerate extreme environments, tardigrades also are very interesting due to their close evolutionary relationship with arthropods (e.g., insects, crustaceans, spiders).

As aquatic animals, tardigrades got to be surrounded during a film of water to be in their active state (i.e., feeding and reproducing). However, these critters are ready to endure periods of desiccation (anhydrobiosis) by entering cryptobiosis, i.e., a reversible ametabolic state common especially among limno-terrestrial species. Succinctly, tardigrades enter the so-called “tun” state by contracting their anterior-posterior body axis, retracting their legs and rearranging the interior organs. This provides them with the capacity to tolerate severe environmental conditions including oxygen depletion (anoxybiosis), high toxicant concentrations (chemobiosis), high solute concentration (osmobiosis) and very low temperatures (cryobiosis).

The extraordinary tolerance of tardigrades to extreme environments includes also heat endurance. Some tardigrade species were reported to tolerate temperatures as high as 151°C. However, the exposure time was only of half-hour . Other studies on thermotolerance of desiccated (anhydrobiotic) tardigrades revealed that exposures above 80°C for 1 hour resulted in high mortality, with most specimens dying at temperatures above 103°C. It remained, yet, unknown how anhydrobiotic tardigrades handle exposures to high temperatures for long periods, i.e., exceeding 1 hour.

“From this study, we will conclude that active tardigrades are susceptible to high temperatures, though it seems that these critters would be ready to acclimatize to increasing temperatures in their natural habitat. Desiccated tardigrades are far more resilient and may endure temperatures much above those endured by active tardigrades. However, exposure-time is clearly a limiting factor that constrains their tolerance to high temperatures,” says Ricardo Neves.

Indeed, although tardigrades are ready to tolerate a various set of severe environmental conditions, their endurance to high temperatures is noticeably limited and this might actually be the Achilles heel of those otherwise super-resistant animals.

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