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Molecular sensor enables water bear hardiness by triggering dormancy

Free radicals sensor triggers tardigrades to enter dehydrated tun state to withstand extreme stress

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Molecular sensor enables water bear hardiness by triggering dormancy


Tardigrade observed using a confocal fluorescent microscope. The tardigrade was overexposed to 5-MF, a cysteine selective fluorescent probe, that allows for visualization of internal organs.

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Credit: Smythers et al., 2024, PLOS ONE, CC-BY 4.0 (

Tardigrades – hardy, microscopic animals commonly known as “water bears” – use a molecular sensor that detects harmful conditions in their environment, telling them when to go dormant and when to resume normal life. A team led by Derrick R. J. Kolling of Marshall University and Leslie M. Hicks of the University of North Carolina at Chapel Hill report these findings in a new study published January 17 in the open-access journal PLOS ONE.

Water bears are famous for their ability to withstand extreme conditions, and can survive freezing, radiation, and environments without oxygen or water. They persist by going dormant and entering a tun state, in which their bodies become dehydrated, their eight legs retract and their metabolism slows to almost undetectable levels. Previously, little was known about what signals water bears to enter and leave this state.

In the new study, researchers exposed water bears to freezing temperatures or high levels of hydrogen peroxide, salt or sugar to trigger dormancy. In response to these harmful conditions, the animals’ cells produced damaging oxygen free radicals. The researchers found that water bears use a molecular sensor—based on the amino acid cysteine—which signals the animals to enter the tun state when it is oxidized by oxygen free radicals. Once conditions improve and the free radicals disappear, the sensor is no longer oxidized, and the water bears emerge from dormancy. When the researchers applied chemicals that block cysteine, the water bears could not detect the free radicals and failed to go dormant.

Altogether, the new results indicate that cysteine is a key sensor for turning dormancy on and off in response to multiple stressors, including freezing temperatures, toxins and concentrated levels of salt or other compounds in the environment. The findings suggest that cysteine oxidation is a vital regulatory mechanism that contributes to water bears’ remarkable hardiness and helps them survive in ever-changing environments.

The authors add: "Our work reveals that tardigrade survival to stress conditions is dependent on reversible cysteine oxidation, through which reactive oxygen species serve as a sensor to enable tardigrades to respond to external changes."


In your coverage please use this URL to provide access to the freely available article in PLOS ONE:

Citation: Smythers AL, Joseph KM, O’Dell HM, Clark TA, Crislip JR, Flinn BB, et al. (2024) Chemobiosis reveals tardigrade tun formation is dependent on reversible cysteine oxidation. PLoS ONE 19(1): e0295062.

Author Countries: USA

Funding: This research was supported by National Science Foundation grants awarded to L.M.H. (NSF-MCB 2149172) and D.R.J.K. (NSF-MCB 2149173). A.L.S. acknowledges funding from the North Carolina Space Grant. Marshall University students were also funded by a National Science Foundation (NSF) Grant (Award Nos. CHE1229498 and OIA1458952), the NASA West Virginia Space Grant Consortium (Grant no. NNX15AK74A). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

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