These single-celled survivors, called extremophiles, don't merely endure environments too serve for other life forms.
-called extremophiles is closed in by commas because the commas act as an appositive
Appositives are words that add information to a noun or noun phrase that precede them. They are always enclosed by commas.
In your example the added information = what the single celled survivors are called.
Here are a few more examples.
Liverpool**, my home town,** is a wonderful place.
My friend**, called Fred,** is not very clever.
English**, my favourite subject,** is very challenging.
Appositives function to add more information and to make our language more interesting.
I hope that helps you.
Source: Advanced Grammar Course Manual. BC Ministry of Education, Canada.