Based on the given description, the organism found in the hot spring, possessing a cell membrane but lacking peptidoglycan in its cell wall and containing circular DNA and RNA, would be classified as an "archaea."
Archaea are a distinct domain of microorganisms that are separate from bacteria and eukaryotes. They are characterized by their ability to live in extreme environments, such as hot springs, acidic environments, and salt flats. Archaea have unique cell wall compositions that can vary, and some do not contain peptidoglycan, unlike bacteria. They can have circular DNA, similar to bacteria, but their genetic and biochemical characteristics differentiate them from both bacteria and eukaryotes.
Therefore, based on the provided information, the organism found in the hot spring would most likely be classified as an archaeon.