Borderline lichen

Borderline lichen is a term used in lichenology for structurally simple, symbiotic associations between a fungus and a photosynthetic partner (an alga or cyanobacterium). Unlike typical lichens, these associations lack a differentiated thallus (vegetative body) and a protective outer cortex. Instead, the fungus forms a loose network of mycelia intertwined with the photosynthetic cells. A defining feature is that the fungal partner triggers visible changes in the arrangement or form of the photobiont's cells, distinguishing borderline lichens from simple fungal colonization. The term was introduced in 2004 by Kohlmeyer, Hawksworth, and Volkmann-Kohlmeyer for certain marine and maritime associations, including Mastodia tessellata and Collemopsidium pelvetiae, that resemble lichens yet lack the organized fungal tissues expected under standard lichen definitions. The concept has since been extended to include fungi that switch between free-living and loosely lichenized modes depending on their substrate (termed "optional lichenization"), bryophyte-associated cyanobacterial systems in which fungal–photobiont contact occurs without organized thallus development, and associations between stress-tolerant fungi and algae in nutrient-poor environments such as caves in the Atacama Desert.

Borderline lichens do not form a single evolutionary group. Instead, they represent a polyphyletic assemblage of fungi from distantly related lineages that have independently evolved symbiotic lifestyles. They have been proposed as present-day analogues of early stages of thallus evolution, before more complex lichen organization arose. Their simple structure has also made them a focus of debate over whether the lichen symbiosis is best understood as a mutualism or as a form of controlled parasitism, because the costs and benefits to each partner are considered more readily observable than in structurally complex lichens. These associations are often described as occupying the transitional zone between free-living organisms and fully integrated lichen symbioses.