I was wondering if Rahn need to breathe. If so, I imagine they can breathe through their skin but I couldn't find anything on the wiki. I know the Galotians don't because the lore page talks about how they didn't understand other species needing to breathe when they were first interacting.
Considering that they are single-celled organisms, it depends on whether or not they're Aerobic, Anaerobic or Facultative. The codex does not specify. The need for a source of moisture may suggest that they are Aerobic or Facultative, thus needing oxygen.