Tardigrades – or waterbears as they are more commonly known – are a group of small, micro-animals which are known for their amazing ability to withstand extreme environments. You may have already heard of certain species of bacteria being able to survive in the depths of the oceans around sulphurous vents, or inside the blistering heat of volcanoes, but tardigrades are by far and away hardier than that. They can be found in all four corners of the earth, from the frozen wastes of the Arctic to the heights of the Himalayas. Known for their capacity to withstand temperatures from as low as -272 °C (yes – that is just one degree higher than absolute zero, commonly thought of as the lowest temperature possible!) to as high as 151 °C. They can survive wild fluctuations in pressure, hydration, radiation, and perhaps most stunning of all, the vacuum of outer space! It is no surprise that these hardy creatures have been on earth far longer than we have – indeed evidence suggests they’ve been in action for some half a billion years.
But just how do these strange eight-legged creatures do it? Fundamentally, tardigrades are able to reverse their metabolism (the chemical reactions in the body) going into what is known as a state of ‘cryptobiosis’, which they can remain in for up to ten years in some conditions. Their metabolism drops to 0.01% of normal, with cell water content reduced to just 1% of its normal levels. They do this by replacing the majority of their body’s water with a sugar called trehalose, which acts to protect the cell membranes, keeping the tardigrade stable.
Tardigrades came to much greater attention in 2007, when NASA launched a sample of them into space aboard the FOTON-M3 satellite. After twelve days of exposure to the harsh radiation of space, the satellite returned to Earth, where scientists retrieved the tardigrades and were shocked to find they had been hardly damaged by the cosmic and ultraviolet radiation they had been exposed to – although evidence was found of slight DNA damage. Scientists are not sure just how the tardigrades are able to resist such high levels of radiation, which would be deadly to any of us. Some speculate that it may be linked to the same mechanism that allows them to resist drying out. One possible way in which this may work is that by having such a low water content, the tardigrades have far fewer reactant molecules for the ionising radiation of space to act upon. As well as this, they have fantastic mechanisms in place for repairing DNA damage.
As well as the vacuum of space, tardigrades display a remarkable ability to resist dehydration. The longest surviving waterbears have been found to last for ten years in a dehydrated state, and perhaps more amazing still is an as of yet unconfirmed report of limited survival in a 120 year moss sample. Even more astonishing is the level of pressure that they can withstand. Some have been found to be able to withstand a pressure of 6000 atmospheres (6000 times higher than the standard pressure on Earth.) To put that into perspective, the Marianas Trench, the deepest trench in the world, only has a pressure of about 1000 atmospheres at its deepest point!
What do the extraordinary abilities of tardigrades mean for us? Apart from providing a fantastic example of evolutionary adaptation to a wide range of environments, the discovery of tardigrades has sparked interest in a whole range of areas. Perhaps most excitingly of all, it questions our current methodology in the search for extraterrestrial life. The remarkable ability of tardigrades to survive at such a range of conditions has led some to suggest we are being too picky with the planets and moons we choose to examine as potentially life bearing, and that life may well exist in a much greater variety of forms than we have imagined. Tardigrades’ survival ability is also of great interest to those looking at the ways in which we could ensure the safety of people in space, and we are sure to gain a lot of insight by studying these remarkable creatures.
At the end of the day, tardigrades reflect life’s ability to survive, to adapt, and indeed to flourish in even the harshest of conditions. Whilst we might look at the cheetah and its fantastic speed, or birds for their phenomenal ability to fly, when it comes down to the coolest and most impressively evolved organism on the planet, the tardigrade wins every time.