Very simple cells, such as
Nanoarchaeum equitans, require a host to provide certain essential ingredients for life. Complex life-forms (like humans) require a whole ecosystem of other life-forms to survive (either externally, to provide food, or internally, in a symbiotic relationship).
What is the simplest cell that has been discovered so far that can live in an environment where there are no other life-forms, not even in a decomposed state? Does such an organism exist, or is all life inextricably dependent on other life?