What makes a Tardigrade so tough?

If you have heard of Tardigrades, then you know three things:

  1. They can survive in almost any conditions.
  2. That includes hot springs.
  3. Also space.

If have not heard of tardigrades, allow me to explain.

A tardigrade looks like this.

tardigrade

Cute, isn’t it?

It can grow to around half a millimetre and usually lives on a diet of algae, small invertebrates, and smaller tardigrades. A single litre of water can contain as many as 25000 of them, and they are found pretty much everywhere. These organisms, sometimes referred to as ‘Water Bears’ or ‘Moss Piglets’ (no, seriously) were first discovered in 1773 by German scientist Johann Goeze, and given the name ‘Tardigrada’ three years later by Italian scientist Lazzaro Spallanzani. ‘Tardigrada’ translates to ‘slow-stepper,’ but only because in the 1770s they must have been unaware of its other attributes, or they would have called it ‘Durocomecazzo’ (with apologies to the Italian language.) However, tardigrades cannot be considered extremophiles, as an extremophile is an organism that actively chooses to live in a harsh environment. Given the choice, a tardigrade would happily curl up on a wet bog and venture nowhere near a sulphuric spring, but could still survive there long enough to get away from it, in the same way that the A1 is just quick enough to get you out of The North before any lasting damage occurs to you.

Now let us revisit the most well known facts about Tardigrades. These facts can be very readily found on a wikipedia page (this one,) so instead I will add some perspective.

Temperature:

  • Tardigrades can survive temperatures of 151 Celsius for a few minutes. This is the lowest boiling point of Paraffin, although it is unclear how efficient burning tardigrades would be as a fuel source.
  • Tardigrades can survive temperatures of -20 Celcius for 30 years. That in itself is so impressive it does not need perspective, but according to a page on Live Science, presumably written by a robot:

    “A person usually expires when their body temperature drops to 70 degrees F (21 degrees C).”

  • Tardigrades can survive temperatures of -200 Celcius for a few days, which means they could go for a long weekend on the surface of Saturn. Saturn doesn’t have a surface, but you get what I mean.

Pressure:

  • For those wondering why going deeper in water means a higher pressure, it is because everything is pulled towards the centre of the earth by gravity, and the deeper you go, the greater the amount (and therefore weight) of water there is on top of you. A human being can withstand 3-4 atmospheres of pressure, which means a standard person could probably swim to a depth of around 100 feet (30.48 metres) before being crushed to death by the water. A tardigrade can survive 1200 atmospheres. this equates to roughly 17600 psi or 4.5 times the force of a saltwater crocodile’s bite. A human bite is roughly equivalent to 13 atmospheres, so your jaw exerts the same force as the ocean at 130 metres deep. Good job. I’ve forgotten what my point was.
  • Some species of tardigrade can survive 6000 atmospheres, which is 6 times the pressure of the lowest point of the Mariana Trench, 10994 metres down.

Radiation:

  • Gamma rays are produced when the centre of an atom (nucleus) decays because there are too many excess bits (subatomic particles) hanging around for there to be enough energy to keep them all together. Gamma rays are produced in the most energetic parts of the universe, such as stars, pulsars and supernovae. Humans have harnessed the energy of gamma rays in the medical field in order to sterilise equipment. Gamma rays are very good at killing things. A dose of 5-10gy could be fatal to a human, and a tardigrade can withstand over 5200 of them.

In short, tardigrades will still be here when we are not.

However, as impressive as all this fact spewing may be, it doesn’t answer the question, why are they so tough?

The answer mostly comes down to retain their cell structure. The main problem with extremes of temperature or pressure or radiation is that it affects the amount of water you can keep in a cell. Water is vital to nearly all the reactions that go on inside cells, as well as retaining its structure. The cell dries out, cell functions stop, its bad news all round. So what is to be done?

For a while, the tardigrade’s solution was thought to be down to one thing.

trehalose

This is Trehalose. Not to be confused with this.

trejolose

Which is Trejolose.

Assuming anyone is still reading, trehalose is a non-reducing sugar that was thought to be responsible for retaining cell structure, acting as a crutch for the cell membrane not to fold in on itself and burst. The tardigrade then suspends its metabolism, and when water is reabsorbed back into the cell the trehalose dissolves back into the water. Simple, elegant, false.

A study into tardigrade

What does this protein and my life have in common?

1a5r_SUMO-1_protein

They are both intrinsically disordered.

Intrinsically disordered proteins, or IDPs, are proteins (no really) that do not exist in one fixed three-dimensional shape. There is a separate post entirely as to why the discovery of these changed the fundamentals of cellular biology, but for now lets just say that tardigrades can produce them, as the aforementioned post would have to be written by someone that isn’t just listing off snippets from someone else’s research papers.

When the water concentration drops, the majority of tardigrade species start to produce IDPs. The rest of tardigrade species already produce them at a consistent level, regardless of surrounding water concentration. As each cell desiccates, the IDPs inside them form amorphous solids. I have spent 25 minutes trying to think of an amorphous solid joke, but then realised my mum is going to read this, so lets move on.

The IDPs then vitrify, which is to say they become a glass like structure which maintains the shape of the cell without the need for water, in very much the same way as trehalose was thought to do.

fx1

This picture explains in 10 seconds what I have just failed to do for 10 minutes.

The tardigrade protects itself against drying out on two fronts. Not only does it utilise IDPs, but also employs the services of DSUPs. DSUPs, or damage suppressors, are proteins exclusive to tardigrades that prevent the breakdown of the DNA helices in cells. If a cells DNA can remain intact, it can continue to code for IDPs and the cells can stay relatively functional, in the same way that the sleep pattern of a third year undergraduate is relatively functional.

I could go into more detail but it is nearly 2am and I have based this post around papers that feature quotes such as this:

To do this we mapped the chemical environment of the covalent bond between each backbone amide nitrogen and its attached proton based on the heteronuclear single-quantum coherence (HQSC) spectra of the protein.

and this:

Subcellular localization prediction using WoLF PSORT suggested nuclear localization for both proteins, and a putative nuclear localization signal is predicted in both proteins at a similar position near C-terminus by using cNLS Mapper software.

And if that doesn’t make you feel ill then you are a sociopath or a chemist, and I don’t know which one makes me more uncomfortable.

Sources:

“Tardigrades Use Intrinsically Disordered Proteins to Survive Desiccation”

https://www.cell.com/molecular-cell/fulltext/S1097-2765(17)30133-8?_returnURL=https%3A%2F%2Flinkinghub.elsevier.com%2Fretrieve%2Fpii%2FS1097276517301338%3Fshowall%3Dtrue#%20

DNA Protection Protein, a Novel Mechanism of Radiation Tolerance: Lessons from Tardigrades

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5492148/

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s