Final answer:
The specimen likely represents a prokaryotic cell, fitting the size range and lacking a membrane-bound nucleus, which are characteristics of bacteria and archaea.
Step-by-step explanation:
The unknown specimen you've collected from the Cueva de Villa Luz caves exhibits characteristics that provide vital clues to its classification. With a size of 0.5 micron, a cell membrane, and ribosomes, but without mention of a nucleus or other specialized organelles, it's likely that your specimen is a prokaryotic cell. Prokaryotic cells, which encompass bacteria and archaea, typically have sizes within the 0.1 to 5.0 µm range, making the size of your specimen fitting. In addition, they all possess a plasma membrane and ribosomes, as you've noted in your specimen. The absence of a membrane-bound nucleus and other organelles in your description also points towards this classification.