Some symbiotic relationships are obligate, meaning that both symbionts entirely depend on each other for survival. For example, many lichens consist of fungal and photosynthetic symbionts that cannot live on their own. Others are facultative (optional): they can, but do not have to live with the other organism.

Answer this question, if possible (if impossible, reply "unanswerable"): What organism demonstrates only the obligate symbiotic relationship?
unanswerable