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🐻Biology7 min read

The Tardigrade

There is an animal smaller than a full stop that can survive the vacuum of space, temperatures close to absolute zero, radiation a thousand times the lethal human dose, pressures deeper than the Mariana Trench, and decades without food or water. It has been doing this for 600 million years.

In plain English

Consider the conditions under which life cannot survive. The vacuum of space, where there is no air, no pressure, and radiation pours in from every direction. A temperature of -272°C, one degree from absolute zero. A radiation dose a thousand times higher than what would kill a human. Pressure six times greater than the deepest ocean trench. Seventy years without food or water.

Now consider that there is an animal that survives all of these. Comfortably. Routinely. Without any special preparation.

The tardigrade is between 0.1 and 1.5 millimetres long, has eight stubby legs, moves with a lurching gait that resembles a bear's, and lives in a thin film of water on moss, lichen, soil, and Antarctic ice. It is found on every continent, in every ocean, at every altitude from sea level to the high Himalayas. It is one of the most abundant multicellular animals on Earth, and almost no one has seen one.

It has been doing this, surviving in one form or another, for at least 600 million years. It predates the dinosaurs by 300 million years. It has survived every mass extinction on record.

The secret is that, when conditions become uninhabitable, the tardigrade does not die. It stops being alive. And then, when conditions improve, it starts again.

Five things to file under "wait, what?"

  • In 2007, tardigrades survived open exposure to space. The FOTON-M3 mission carried samples of tardigrades in their dormant form outside the spacecraft for ten days. They were exposed to the full vacuum of space, cosmic radiation, and UV radiation approximately 1,000 times stronger than on Earth's surface. When returned to Earth and rehydrated, a significant fraction revived and reproduced normally. They are the only animal known to have survived unprotected exposure to open space.

  • They can survive for decades without water. When tardigrades dry out, they enter a state called cryptobiosis, specifically anhydrobiosis. They retract their legs, curl into a barrel shape called a tun, expel almost all water from their cells (water content drops from 85% to below 3%), and stop all metabolic processes. In this state, they are not alive in any meaningful biological sense. They are closer to a dried spore than an animal. They can remain in this state for decades, possibly longer, and revive when moisture returns.

  • They produce a unique protein that replaces water inside their cells. In most cells, water plays a structural role: it maintains the shape of proteins and membranes. When water is removed rapidly, cells collapse and die. Tardigrades produce a protein called Dsup (Damage Suppressor) and a class of proteins called TDPs (tardigrade-specific intrinsically disordered proteins) that form a glass-like substance inside the cells, replacing the structural role of water and preventing the collapse of DNA and proteins. This is not found in any other known animal.

  • Their radiation tolerance is extraordinary. Tardigrades can survive radiation doses of up to 570,000 rads. The dose that kills 50% of humans is around 500 rads. Their Dsup protein, when inserted into human cells in laboratory experiments, reduced radiation-induced DNA damage by around 40%. Applications for human radiation therapy and potentially for long-duration space travel are being actively researched.

  • They have survived every known mass extinction. The Ordovician, Devonian, Permian, Triassic, and Cretaceous mass extinctions, the five great biological catastrophes in Earth's history, collectively eliminated up to 96% of all species. The tardigrade survived all of them. Tardigrades could survive almost any catastrophe short of the Sun expanding into a red giant, an event that will not occur for approximately five billion years.

The full story

What cryptobiosis actually is

Cryptobiosis means "hidden life." A tardigrade in cryptobiosis is not alive: its metabolism has ceased. It does not breathe. It does not eat. It does not grow. It does not reproduce. It does not respond to stimuli. The biological processes that define life have been suspended.

What remains is the structure. The proteins, the DNA, the membranes, preserved in the glassy matrix of the TDP proteins, waiting. In this state, the tardigrade is not dormant in the way a hibernating bear is dormant. A hibernating bear is still very much alive, running its metabolism at a reduced rate. A tardigrade in a tun is something else: a kind of biological pause, neither dead nor living.

When moisture returns, even decades later, the tardigrade rehydrates, its metabolism resumes, it unfurls its legs, and it walks away. No damage. No memory of the interruption. The pause was irrelevant to its continuous existence.

There are five known forms of cryptobiosis in tardigrades:

  • Anhydrobiosis: triggered by desiccation (drying out)
  • Cryobiosis: triggered by freezing
  • Osmobiosis: triggered by high salt concentration
  • Anoxybiosis: triggered by absence of oxygen
  • Chemobiosis: triggered by toxic chemicals

The Dsup protein

One of the most scientifically significant discoveries about tardigrades was the identification of the Dsup (Damage Suppressor) protein in 2016. This protein physically shields DNA from radiation damage, wrapping around it like armour and absorbing hits from ionising radiation that would otherwise cause breaks in the DNA strand.

When researchers inserted the Dsup gene into human cultured cells, those cells became more resistant to X-ray radiation. The cells produced the protein, which associated with their chromatin, and radiation-induced DNA double-strand breaks were reduced by approximately 40%.

This raises the prospect, still experimental, of engineering radiation resistance into human cells. Applications for cancer radiotherapy (protecting healthy cells while targeting tumours) and long-duration spaceflight (protecting astronauts from cosmic radiation) are both being explored.

The 2019 lunar crash

In April 2019, the Israeli spacecraft Beresheet crash-landed on the Moon while attempting the first privately funded lunar landing. Among the payloads was a capsule containing thousands of tardigrades in their dried, dormant tun form, attached to a biological archive.

The spacecraft impacted the lunar surface at approximately 500 km/h. The capsule may or may not have survived. If it did, there are tardigrades on the Moon right now, dried, inert, potentially viable, sitting in the lunar dust. Whether they could ever be revived is unclear, since they would need liquid water and a suitable environment, but they may well be structurally intact.

This raised genuine questions about planetary protection protocols, and reignited debate about what it means to "contaminate" another world with Earth life.

What they tell us about the limits of life

The tardigrade's existence expands the set of conditions under which life, or something that looks very much like life, can persist. This has direct implications for astrobiology: the study of life beyond Earth.

If a creature can survive the vacuum of space, could life hitch rides between planets on meteorites, a process called panspermia? If a creature can survive desiccation for decades, could life persist in environments we currently consider uninhabitable, the surface of Mars, the subsurface of Europa?

The tardigrade does not answer these questions. But it moves the goalposts for what a "habitable" environment needs to look like.

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