Lichen
When partnerships are very close, it is difficult to assign benefits to either partner. Their success is as a pair (or trio). Sometimes the partners can exist as separate organisms: certainly their close relatives can. Probably most of the algae and all of the cyanobacteria can survive on their own, though in more restricted habitats. The details vary according to the specific species or strains involved. The man who first realised the nature of lichen, Simon Schwedener, thought of the partnership as one of controlled parasitism. Obviously, the photobiont provides photosynthetic products: carbohydrates as sugar alcohols (green algae) or glucose (cyanobacteria). The bacteria also convert atmospheric nitrogen (N2) to ammonium ions (NH4+) which the fungus can use in amino acids for proteins.p26