Halophilic prokaryote/halophile and Domain Archaea.
These are the clues that led to my answer.
No membranous organelles: Prokaryotes are known for this!
Cell wall lacks peptidoglycan:
This is common among eukaryotes and archaea. Since the organism is UNICELLULAR then it is not eukaryotes, so we are left with Archaea.
" grow well on 25% salt supplemented agar."
Halophile is an organism that thrives in saline conditions, so that means in HIGH SALT concentrations.