Final answer:
Lichens are considered a controlled parasitism rather than saprophytic because the algal partner contributes food from photosynthesis and is harmed by the fungus.
Step-by-step explanation:
Lichens are considered a controlled parasitism rather than saprophytic. This is because the algal partner contributes food from photosynthesis, the algal partner is living, and the algal partner is harmed by the fungus. The fungal partner can survive without the algal partner, and the algal partner is not free to leave at any time.