[anjel]

For more complicated organisms, robustness comes in the form of cellular turnover, and regenerative healing in response to injury, at least in youth. I wonder though if single celled organisms have or even need such a function.

[brazzy]

Individual cells absolutely do have mechanisms to repair damage: https://pmc.ncbi.nlm.nih.gov/articles/PMC5664224/

They can even repair their own DNA: https://en.wikipedia.org/wiki/DNA_repair