Science For Everyone - The Amazing Tardigrade

Copyright © Karl Dahlke, 2022

In the last chapter, we subjected some insects, a cat, and even a few humans, to sustained high ɡ forces, novel situations for which we are ill-prepared. In earlier chapters we sent humans high above the clouds, where the air is thin, and down below the sea, where the pressure is high. However, the king of survival, in extreme situations, has to be the tardigrade, or more accurately, tardigrada, comprising over 1300 species. Let's meet this little invertebrate, then place him in some rather harsh conditions.

As we saw with flies, animals have a survival advantage when they are small - and the tardigrade is tiny, barely half a millimeter long when fully grown. It has 8 legs and a segmented body, though it is not a spider, and not an arachnid. The legs may terminate in claws, or suction disks, but you will need a microscope to see these details. They feed on plant cells, or algae, or even smaller invertebrates. The bulk of their segmented body is for digestion. There is no heart, no spiracles, no respiratory organs. Gas exchange is sufficient for this tiny creature. Nor are there nephridia, for filtering and excreting waste. By remaining small, and keeping its metabolic rate low, it doesn't neeed the surface area expanders that we find in other animals. In many ways, the tardigrade is simpler than an insect, and this may help it survive harsh and unusual conditions.

Tardigrades are found everywhere on earth, from mountain tops to the deep sea. They are also very old, perhaps 500 million years old, which could explain why evolution has enabled them to survive so many harsh conditions. They passed through all five mass extinctions unscathed. We can comfort ourselves with that explanation, but the vacuum of space? Really?

Their go-to trick is desiccation, followed by a near perfect stasis. They can reduce their water content to 1% of normal, and once dehydrated, their metabolic rate is less than 0.01% of normal. In this state, they can go without food or water for 30 years. Once rehydrated, they forridge for food, and reproduce, as if nothing had happened.

Let's perform a sanity check on the math. This is always a good idea when presented with a surprising or astounding claim. Thirty years is 30 × 365 days, or ten thousand days, and the tardigrade's metabolic rate is reduced by a factor of ten thousand, so this is comparable to you or me going a day without food or water. The math lines up. Of course it isn't a perfect analogy. Things happen to the tardigrade, across thirty years, that don't happen to you or me in just one day. Radiation is the first example that springs to mind. If you received thirty years of normal background radiation, from the rocks around you and the sky above, in one second, while youre molecules were frozen in time and unable to repair the damage, that might be a concern. We'll talk about this and other factors below.

As mentioned earlier, tardigrades can survive the vacuum of space. This was verified by experiment in September, 2007, when tardigrades were sent into low earth orbit. The FOTON-M3 mission exposed them to the vacuum of space for ten days. When they were brought back to earth, 68% of the subjects came back to life and were unharmed. However, the combination of vacuum and solar ultraviolet radiation was generally not survivable.

At the other end of the scale, most tardigrades can withstand 1,200 atmospheres of pressure, as might be found in the deep sea, and some species can withstand 6,000 atmospheres, nearly six times the pressure of water in the deepest ocean trench.

When dehydrated, a tardigrade can withstand 1,000 times more radiation than most other animals. This includes gamma rays and heavy ions. Even when hydrated, they still tolerate high doses of ultraviolet radiation, as might be found at high elevations. a specialized, damage suppressor protein protects their DNA from ionizing radiation.

The range of temperatures that they can withstand, when desiccated, is jaw-dropping. With rare exceptions, delicate biological molecules shake apart at temperatures near the boiling point. That's why surgical instruments were traditionally boiled, to ensure complete sterilization. However, a tardigrade can survive 145 °c, or 300 °f, for several minutes. That is hot enough to cook a roast. At the other extreme, some species can survive at 1 degree above absolute 0. That's colder than any place in the universe. I'm not sure how this experiment was performed. If there was any air in the chamber, it would liquify, and then freeze around the tardigrades, embedding them in a sheet of frozen air. Such a freeze-thaw cycle would probably damage their external tissues. More likely, the air was removed first. We know they can survive in a vacuum, so that is not a concern. Then the temperature was reduced, all the way down to 1 degree above absolute zero. When gently warmed, and rehydrated, these little creatures came back to life.

Bacteria

There are some species of bacteria that can outcompete the tardigrade under certain conditions. These extremophiles do not just survive, they live, and function, and reproduce. Some bacteria can thrive in the near boiling temperatures of a hot spring, or in high acidity, or high saline conditions. Along with acid, dehydration, and vacuum, deinococcus radiodurans is remarkably tolerant of radiation. It can live in the radioactive sludge produced by a nuclear reactor. In fact, some of these bacterial strains have been genetically engineered to remove mercury and other heavy metals from nuclear waste. It is the world's toughest bacterium, as per the Guinness Book of World Records.

Deinococcus radiodurans is a relatively large, spherical bacterium. Cells tend to clump together in groups of four, known as a tetrad. Fortunately for us, they don't cause disease.

Deinococcus radiodurans can absorb 500,000 rads in a short burst, repair its DNA within 24 hours, and soldier on. For comparison, a chest x-ray, or a ten-day moon mission, is 10 rads, 500 rads will kill a human, and 400,000 rads will kill a tardigrade.

Inspired by their resiliance, I wondered if these bacteria, or similar strains, might be used to seed life throughout the galaxy. This is one of the 9 short stories in my science fiction book.

Bacteria Can Almost Live Forever

If you want a bacterium to hunker down for 10 million years, you have to solve the radiation problem. Revving up the background radiation by a factor of 10 million isn't survivable, by any living organism. I don't care what DNA repair tricks you have up your sleeve.

Most of this background radiation comes from above, in the form of cosmic rays. These energetic particles come from outer space, and are, by and large, deflected by earth's magnetic field, and absorbed by our protective atmosphere. Those that get through can jostle our DNA, but the effect is so small, that something else, even old age, is bound to kill us first. Multiply this radiation by 10 million however, and it's a different story.

Bacteria can escape this barrage by hiding 100 meters underground, or 1,000 meters under water. That takes care of the sky. If the nearby rocks are also low in radioactivity, they could remain in suspended annimation, or survive at an insanely low metabolic rate, for 100 million years, ready to come back to life when conditions are favorable once again.

Coal is a good place to find these ancient microbes. Remember that coal began as organic matter, teeming with bacteria. If any of them survive the heat and pressure, they are entombed in carbon for millions of years. Carbon, near the beginning of the table of elements, is not radioactive, save some trace atoms of carbon 14, and even those quiet down after a few thousand years. In contrast, the high elements, like uranium, can remain radioactive for a billion years. Thus our intrepid bacteria can hide, and wait,for eons, if the surrounding rocks consist of carbon and silicon and oxygen and iron and other stable elements.

It's almost impossible for us to imagine an organism surviving in stasis, or living ultraslowly, for 200 million years, and yet it could be so. Truth is strranger than fiction. read more about these long lived microbes here.