[Lewis Thomson (OP)]

Donald has emailed in to ask this question to the forum,

"Humans and most living organisms have a finite lifespan. How about asexual unicellular bacteria? Do they eventually die if they don't reproduce? Do they have a lifespan?  If so, why would the lifespan reset with reproduction? Do HeLa cells have a lifespan? Can they persist without reproducing? Or the contagious cancers in Tasmanian devil's?  Life! What can it not do?"

Discuss your answers in the comments below...

[evan_au]

Unicellular bacteria are effectively immortal. Provided they have food and a suitable environment, they can continue to divide forever (provided they don't get too overcrowded).
- When they get overcrowded, evolution takes over; some strains survive better in crowded conditions, and outcompete those that are less fit for these resource-limited conditions.

In one sense, cancer cells have almost reverted to a unicellular state, where it is "every cell for itself", to the detriment of the body in which they originated. Most cancers are limited to one individual - any cancer cells that transfer to another individual are recognized as "alien" and destroyed by the recipient's immune system.
- However, in inbred populations, transmissible tumors can be transferred from one individual to another, without being destroyed by the other individual's immune system.
- Examples of this are the Tasmanian Devil facial tumor, Canine sexually-transmitted cancer, and a cancer of shellfish.
https://en.wikipedia.org/wiki/Clonally_transmissible_cancer

Can they persist without reproducing?

Some unicellular organisms discovered deep in a gold mine have a very low metabolic rate, and it is thought that individual may go many decades between divisions.

Unicellular organisms engage in "Horizontal gene transfer", which is a way to share useful snippets of DNA, which introduces more genetic variability than the usual cellular division. This helps them cope with extreme conditions.

See: https://en.wikipedia.org/wiki/Extremophile