Life cycle, life-cycle or lifecycle?

My medical dictionaries (Dorland's Illustrated, 30th Ed.; Saunders Comprehensive Veterinary Dictionary) either list it as two words (Dorland's) or not at all (Saunders). So I would say in medical literature fields, where the context would be something like the life cycle of a parasite, it's still two words.

And I'm not sure that lifecycle and life-cycle are nearly as common as life cycle. Here's the NGram for life cycle vs. lifecycle vs. life-cycle.

Four online dictionaries I consulted (Merriam-Webster, Oxford Dictionaries, Macmillan, and Dictionary.com) all agree on the open form life cycle.

In particular, the Dictionary.com definition in the link uses the open form for all the senses life cycle.

My feeling is that if referring to a single concept, it should be a single or closed word, i.e. lifecycle.

On the other hand, when referring to separate concepts, the open usage would be more appropriate. For example, the various stages of an insect's life would be referred to as a life cycle.

It eventually comes down to context and that is the responsiblity of the individual writer/author/editor, etc.