What is the tun state of a tardigrade?

Tardigrades can survive extreme conditions by going into a “tun” state, in which their body dries out and their metabolism drops to as little as 0.01 percent of its normal rate. When conditions return to normal, the tardigrade revives itself. A tardigrade can stay in a tun state for decades.

Damn, that's really cool.

To add, I think this bit is important as well. I’m just not smart enough to explain why.

with cysteine unavailable to the free radicals being produced, the tardigrades couldn’t form tuns.

Edit: I think it means that under stress the tardigrade forms the tun. But they also figured out that if cysteine is absent even under stress the tardigrade couldn’t form the tun.

Thanks for clarifying.

I think it goes a bit beyond that as well. Other organisms could, in theory, take some of the same measures the tardigrade takes to make itself hardier. It's possible that we don't see this all over the place because the chemical signalling is a requirement of the tun state. Without it, they're basically just dead. Without a signal to return to normal life, they'll stay like that forever. Furthermore, without the signal, they might have to rely on some other, less reliable mechanism, to enter the tun state to start with.

A signaling system that provides its cells with reliable information about environmental conditions is critical.

I think we might even find that the signaling system came first, and that nature selected for tardigrades that had some way to do something with the signal.