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.
Terminology and conceptual origin
The concept of the borderline lichen emerged from long-standing debates over what constitutes a lichen in systematic treatments of lichenized fungi. Historically, many definitions centred on the presence of a thallus: a vegetative body composed of fungal and algal components differing from either partner grown alone. A widely used definition proposed by Hawksworth in 1988 described a lichen as a stable, self-supporting association of a and a in which the mycobiont is the "exhabitant" (the exterior layer) and the alga an interior but extracellular inhabitant.
Borderline cases challenged this model because their fungal partners may be intermixed with, or even within, the photobiont's tissues rather than clearly forming an exterior layer. In a marine context, Kohlmeyer and co-authors used "borderline lichens" for loose, often submerged associations in which the fungal partner does not develop a well-differentiated and is not clearly exhabitant under prevailing definitions. They argued that these associations should still be recognized as lichens because they show specialization involving derived ascomycete orders and because the fungus modifies the algal partner's morphology. In 2001, separate groups led by Kováčik and Pereira and by Lud and co-authors independently investigated the developmental morphology of polar maritime species and concluded that simple organizational levels did not preclude lichen status. Shortly afterwards, the term "optional lichenization" was introduced by Wedin and co-authors in 2004 for fungi that switch between borderline lichenized and saprotrophic modes depending on the substrate.
In a broader synthesis, Lücking and co-authors noted that some lichen-like associations fit the mycobiont–photobiont framework but remain unstable, show little morphogenetic effect, or blur with other categories such as lichenicolous fungi on sterile thalli. They emphasized morphogenesis (a novel morphology not found in the separate bionts) and the photobiont body plan as useful criteria for delimiting borderline and marginal cases. Sanders reviewed these conceptual boundaries, noting that while the "exhabitant" model (where the fungus forms an exterior layer) is standard, many lichens are structurally simpler, especially species growing within bark or rock where the fungus may simply contact algal surfaces without forming specialized tissues. The term "primitive lichen" appears in some older literature as a near-synonym, but modern lichenology generally prefers "borderline" to describe a boundary between different structural and nutritional strategies rather than implying an evolutionary hierarchy.
+ Key terminology in borderline lichenology Term Year Proposed/popularised by Context Mycophycobiosis 1972 Kohlmeyer and Kohlmeyer Fungi living inside multicellular algae without morphological change Exhabitant definition 1988 Hawksworth Fungal partner as the outer layer of the symbiosis Borderline lichen 2004 Kohlmeyer, Hawksworth and Volkmann-Kohlmeyer Specialised loose symbioses with modified algal morphology Optional lichenization 2004 Wedin, Döring and Gilenstam Fungi switching between saprotrophy and loose lichenization
Diagnostic characteristics
The primary characteristic of borderline lichens is the absence of a structured, stratified thallus. In complex lichens, the fungus builds specialized tissues such as an upper and lower cortex, which protect the photobiont from desiccation and excessive light. The internal structure is typically (layered). Borderline lichens lack these features. Instead of forming (dense, tissue-like structures formed by fused hyphae), the fungus maintains an irregular, filamentous network that interweaves with the photosynthetic colonies. In marine borderline lichens, the partners may show specialization and altered photobiont morphology, yet still lack the well-differentiated fungal tissues often treated as characteristic of lichens (except in reproductive structures such as ascomata).
These associations sit between better-defined categories. They differ from mycophycobioses—associations where a fungus lives within an alga without altering its form—because the fungus shows specialization and the photobiont shows noticeable structural changes. What separates borderline lichens from simple fungal infestations is the morphogenetic influence of the mycobiont: the fungal partner triggers changes in the arrangement, division, or morphology of the algal cells. In the association between Mastodia tessellata and Prasiola, for example, the fungus causes the foliose (leaf-like) alga to separate into packets of cells, a pattern not seen in free-living Prasiola.
Borderline lichens often lack the diverse secondary metabolites found in macrolichens. Kohlmeyer and co-authors noted that the Mastodia–Prasiola association does not form secondary lichen substances, consistent with reduced lichen-like organization in the vegetative thallus. However, the fungal partner still produces characteristic reproductive structures such as perithecia, apothecia, and , which may be well-developed even when the vegetative thallus is absent or inconspicuous.
+ Comparison of structural traits: borderline vs. stratified lichens Feature Borderline lichen Stratified/macrolichen Thallus cortex Absent or poorly developed Well-differentiated and Internal stratification Absent () Distinct layers () Fungal organization Loose mycelia Complex Secondary chemistry Limited; mostly melanin Diverse (acids, depsides, depsidones) Substrate penetration Often or Typically or
Evolutionary origins and classification
Borderline lichens represent a polyphyletic assemblage of fungi that have independently evolved symbiotic lifestyles. Recent analyses suggest that the lichen-forming lifestyle evolved at least 20 to 30 times independently in more than five different classes of Ascomycota and several orders of Basidiomycota. Borderline and optional lichenization are described as scattered across distant fungal groups, including marine-intertidal lineages such as Collemopsidium and substrate-plastic lineages in Stictidaceae.
In many cases, borderline lichens represent single species within genera or families that are otherwise composed of non-lichenized, saprotrophic, or parasitic fungi, suggesting that the transition to a lichenized mode of nutrition can occur with minimal structural modification. Grube and Wedin interpreted such facultative, substrate-dependent lichenization as a present-day analogue of early stages of thallus evolution, before more advanced forms of organization arose. Muggia and co-authors similarly argued that studying simple borderline forms by observation and experiment may inform understanding of how fungal–algal interactions transition from loose contact to stable, structured lichen symbioses.
The order Collemopsidiales
A significant development in the classification of borderline lichens occurred in 2016 with the establishment of the order Collemopsidiales within the clade Dothideomyceta. Pérez-Ortega and co-authors described this order to accommodate the family Xanthopyreniaceae, which includes the marine borderline-lichen genus Collemopsidium and the lichenicolous genus Zwackhiomyces. Using five fungal fossils as calibration points (including Paleopyrenomycites devonicus), they inferred a crown age of approximately 230 million years for Collemopsidiales, placing its diversification in the Triassic, shortly after the Permian–Triassic extinction event. The exact phylogenetic position of the family within the Dothideomyceta remains uncertain: six molecular markers were insufficient to assign it with certainty to either Arthoniomycetes or Dothideomycetes.
Transitions from saprotrophy
The family Stictidaceae illustrates the fluid boundary between saprotrophic and borderline lichenized lifestyles. Some species in this family exhibit optional lichenization, switching from a saprotrophic mode on dead wood to a borderline lichenized mode associating with Coccomyxa algae on bark, depending on the substrate. Ancestral character state analysis for this group indicates that a saprotrophic ancestor was most likely, with lichenization occurring independently in various lineages, suggesting that the evolutionary step between these fungal lifestyles is smaller than previously anticipated.
Example systems
Collemopsidium species
The genus Collemopsidium (family Xanthopyreniaceae) comprises marine borderline lichens that typically associate with filamentous cyanobacteria of the genus Hyella. Rocky seashores are the primary habitat: species in this genus inhabit the midlittoral to supralittoral zones and frequently grow as endoliths, boring into calcareous rocks or the shells of marine organisms. Their thalli are often entirely immersed within the substrate, with only the , black perithecia visible as minute dots on rock or shell surfaces.
A 2016 study using two molecular markers (nrLSU and mtSSU) and the general mixed Yule–coalescent model (GMYC) established that the global diversity of marine Collemopsidium is far greater than previously recognized: approximately 26 putative species were inferred worldwide, compared with six morphological species recognized previously in Europe. Bayes factor comparison strongly supported the higher-diversity model. The study also found that rock-boring ability has been acquired and lost multiple times throughout the evolutionary history of the group, indicating that this trait evolved in parallel in different lineages within Collemopsidiales. The maritime borderline lichen Collemopsidium pelvetiae, which grows on the brown seaweed Pelvetia canaliculata, was also treated by Kohlmeyer and co-authors as a borderline lichen example.
Hortaea werneckii and Dunaliella atacamensis