lichen symbiosis (noun, pl. lichen symbioses;ecology term) – a symbiosis in which one or several mycobionts, most often ascomycetes, rarely basidiomycetes, form a network of hyphae that embrace one or several populations of photobionts, either eukaryotic algae or cyanobacteria or both. Some forms of symbiotic associations are typically not treated as lichen symbioses. For example, the symbiosis formed by the zygomycete Geosiphon pyriforme is more accurately referred to as endocytobiosis, i. e., a specialized form of endosymbiosis, where the cyanobacterium cells of the genus Nostoc inhabit the coenocytic hyphae. In contrast, photobiont cells of lichens always remain extracellular; though sometimes penetrated by haustorial hyphae, the photobiont cells do never occur within the hyphal cells. Also, the associations of symbiotic fungi growing inside thalli of brown algae (Phaeophyta) are distinctly different from lichen symbioses. These symbioses, where the alga, not the fungus, forms the main structure that is inhabited, are more accurately called mycophycobioses.