Thermotolerance experiments on active and desiccated states of Ramazzottius varieornatus emphasize that tardigrades are sensitive to high temperatures

Abstract

Global warming is already having harmful effects on habitats worldwide and it is therefore important to gain an understanding of how rising temperatures may affect extant animals. Here, we investigate the tolerance to high temperatures of Ramazzottius varieornatus, a tardigrade frequently found in transient freshwater habitats. Using logistic modelling on activity we evaluate the effect of 24 hour temperature exposures on active tardigrades, with or without a short acclimation period, compared to exposures of desiccated tardigrades. We estimate that the 50% mortality temperature for non-acclimated active tardigrades is 37.1 °C, with a small but significant increase to 37.6 °C following acclimation. Desiccated specimens tolerate much higher temperatures, with an estimated 50% mortality temperature of 82.7 °C following 1 hour exposures, but with a significant decrease to 63.1 °C following 24 hour exposures. our results show that metabolically active tardigrades are vulnerable to high temperatures, yet acclimatization could provide a tolerance increase. Desiccated specimens show a much higher resilience-exposure-time is, however, a limiting factor giving tardigrades a restricted window of high temperature tolerance. tardigrades are renowned for their ability to tolerate extreme conditions, but their endurance towards high temperatures clearly has an upper limit-high temperatures thus seem to be their Achilles heel.

New insights into the limited thermotolerance of anhydrobiotic tardigrades

“…To address this question, we investigate the effect of 1 week temperature exposures on desiccated R. varieornatus. Our results are compared and discussed with respect to our previously reported data on the effect of high temperature on the anhydrobiotic tuns of R. varieornatus [18].…”

“…The controls were then kept for 1 hour at RT, rehydrated with 2 ml of ultrapure water for another hour at RT and, finally, transferred to 5ºC. Moreover, selected data obtained from a study we published recently (see Neves et al 2020 [18]) were also considered in the analyses performed here. In this previous study, desiccated specimens of Ramazzottius varieornatus were exposed to temperatures of 40, 50, 60, 65 or 70ºC for 24 hours following a methodological setup similar to the one described above.…”

“…ca. 100 tuns per temperature) and exposed to high temperatures for 1 week or 24 hours (the latter data are from Neves et al 2020 [18]). Scoring of activity was performed on fully hydrated tardigrades, 2 hours, 24 hours and 48 hours after heat shock.…”

“…These observations indicate that high temperatures destabilize and denature molecules essential for the survival of desiccated and ametabolic tardigrades. Exposure-time is clearly a limiting factor giving the desiccated tardigrades a restricted window of high temperature tolerance [18].…”

Mechanisms of Desiccation Tolerance: Themes and Variations in Brine Shrimp, Roundworms, and Tardigrades

“…Although tardigrades are often championed for tolerance of a broad range of extremes, survival of some of these stresses can rely on prior exposure to desiccation. For example, tardigrades like Ramazzottius varieornatus are only able to survive at high temperatures when they are in the desiccated state (Neves et al, 2020). Thus, the mechanisms of cellular protection that tardigrades engage during desiccation may explain at least in part the tolerance of other extremes.…”