Cyanolichen
Cyanolichens are lichens in which the fungal component partners with cyanobacteria for photosynthesis, rather than the green algae found in most other lichens. In some cyanolichens, known as forms, the cyanobacteria form an extensive throughout the main body of the lichen. Others, called lichens, contain both green algae and cyanobacteria, with the latter often confined to specialised wart-like structures known as cephalodia. This arrangement reflects the remarkable diversity within cyanolichens, which can feature filamentous or unicellular cyanobacteria, sometimes exhibiting multiple independent evolutionary origins across different fungal lineages.
Beyond their diverse anatomy and taxonomy, cyanolichens perform vital ecological roles. Notably, they fix atmospheric nitrogen—converting it into forms that plants and other organisms can use. This nitrogen fixation is critical in both forest canopies and arid-region soil crusts, and it helps cyanolichens act as pioneer species on newly exposed. contributing essential nutrients to both forest canopies and biological soil crusts in arid regions. Their sensitivity to substrate conditions, especially the bark pH of trees, helps explain their restricted distributions, and shows the importance of mixed forest composition for sustaining cyanolichen populations.
Like other lichens, cyanolichens employ diverse reproductive strategies, including the production of sexual spores that must re-establish partnerships with compatible cyanobacteria, as well as the dispersal of symbiotic propagules containing both partners. These intricacies have long posed methodological challenges for researchers, but advancements in molecular techniques are steadily uncovering new details of cyanolichen physiology and evolutionary history. Due to their sensitivity to air pollution, habitat loss, and climate change, many cyanolichens are threatened and have been used as bioindicators to guide conservation efforts worldwide.
Types and diversity
Cyanolichens are lichens in which, in addition to the fungal partner, the photosynthetic partner is provided by cyanobacteria—organisms also known as blue-green algae. About one-third of all lichen photobionts are cyanobacteria, while the remaining two-thirds are green algae. Some lichens host both green algae and cyanobacteria alongside their fungal component. These are known as "tripartite" lichens. In most lichens, the photobiont forms an extensive layer covering much of the lichen body. However, in tripartite lichens the cyanobacteria are usually confined to small, blister-like structures called cephalodia. These cephalodia act as specialised compartments that create distinct microenvironments, where the fungal and cyanobacterial partners interact in unique ways.Cyanolichens exhibit considerable diversity in their cyanobacterial partners, which can be broadly categorised into two main types: filamentous and unicellular cyanobacteria. While filamentous forms like Nostoc and Rhizonema have been extensively studied, unicellular cyanobionts remain less well understood despite their ecological importance. The order Lichinales provides a notable example of unicellular cyanobiont diversity, containing at least ten different genera, with recent research identifying several new genera including Compactococcus and Pseudocyanosarcina.
Research into unicellular cyanobionts presents unique challenges due to their slow growth rates and the complexity of distinguishing between symbiotic and free-living forms under microscopic examination. Unlike green algal lichens, where photobionts often form distinct evolutionary lineages specific to lichen symbiosis, unicellular cyanobionts frequently cluster phylogenetically with free-living cyanobacteria. Historically, genera such as Chroococcidiopsis were thought to be major cyanobionts in various cyanolichen families including Lichinaceae, but molecular studies have revealed a more complex picture, with many previously unrecognised unicellular cyanobacterial groups participating in lichen symbioses. The total diversity of cyanolichens appears to be significantly lower than that of lichens containing green algae, with cyanolichens representing about 10% of known lichen species, though this figure may underestimate true diversity as new molecular techniques continue to reveal previously unknown cyanobiont relationships. Cyanolichens are distributed across multiple fungal lineages, occurring in at least four orders within the Ascomycota—Lichinales, Chaetothyriales, Peltigerales, and Lecanorales—as well as in the Basidiomycota. This broad taxonomic distribution suggests that the ability to form symbioses with cyanobacteria has evolved multiple times independently across distantly related fungal groups.
The ability of cyanolichens to colonise different tree species is strongly influenced by substrate conditions, particularly bark chemistry. Studies have shown that cyanolichens generally require bark with a pH greater than 5.0 to successfully establish and maintain viable populations. This pH requirement helps explain why some tree species support more diverse cyanolichen communities than others, as conifer bark tends to be naturally acidic. Successful cyanolichen colonisation often depends on various mechanisms that can increase bark pH, such as nutrient enrichment from nearby deciduous trees or the presence of other buffering substances. Research in humid inland forests of British Columbia has documented this variation, finding that spruce could support up to 38 different cyanolichen species, while Douglas fir hosted 27 species. Other conifers like western red cedar, western hemlock, lodgepole pine and subalpine fir generally supported fewer species, though this may be partly due to their relative scarcity in younger forest stands.
Studies of tropical cyanolichen communities have revealed complex ecological networks formed through photobiont sharing. Over half of mycobiont species share photobionts with other fungal species, often across different genera or even families, leading to the formation of ecological networks called "photobiont-mediated guilds". Within these guilds, some mycobiont species are strict specialists that associate exclusively with a single photobiont variant, while others are more generalist in their associations. The most extensive symbiont networks documented have involved dozens of fungal species from multiple genera associating with numerous Nostoc photobiont variants.
Establishment, reproduction, and dispersal
For sexually reproducing cyanolichens, the process begins when fungal spores not only germinate but also must locate a compatible photosynthetic partner to form a new lichen. This extra step renders their reproduction more challenging than that of lichens which reproduce asexually by dispersing both partners together. Fungal spores frequently fail to germinate without the presence of appropriate photobionts, and in many environments, compatible photobionts may be scarce.Research into cyanolichen communities has shown that species sharing the same type of cyanobacterial photobiont often form ecological guilds. Within these guilds, "core species" that reproduce asexually through symbiotic propagules can serve as reservoirs of compatible photobionts for "fringe species" that rely solely on fungal spores. Such facilitation appears crucial for sustaining populations of sexually reproducing cyanolichens. Moreover, the cyanobacterial photobionts of some cyanolichen species show very limited capacity for independent growth—they often grow slowly and are unable to produce motile hormogonia. This observation suggests that, over evolutionary time, certain symbiotic cyanobacteria have lost much of the functionality required for a free-living existence. Consequently, sexually reproducing cyanolichen species may depend heavily on acquiring photobionts from existing lichen thalli rather than from free-living populations.
The success of a cyanolichen's reproductive strategy also appears to affect its photobiont associations. Species reproducing sexually tend to be more promiscuous in their photobiont selection compared to species that reproduce mainly through symbiotic propagules. Species that form cephalodia often associate with a wider diversity of photobionts than those forming strictly bipartite associations.
Reproductive strategies
Many cyanolichen species employ specialised vegetative structures that enable the simultaneous dispersal of both fungal and cyanobacterial partners. These symbiotic propagules take several forms, including:Finger-like outgrowths called isidia that contain both partners,
Microscopic packets called soredia, in which fungal hyphae enclose cyanobacterial cells, and
, small thallus fragments that break off to establish new individuals.
Such methods ensure the continuation of successful symbiotic partnerships, typically resulting in clonal reproduction of the entire lichen. The relative importance of each method varies among species and habitats.
In contrast, many cyanolichen fungi reproduce sexually by producing or basidiospores. However, this approach poses a significant challenge: after dispersal, the fungal spores must locate and establish new partnerships with appropriate cyanobacterial partners. Fungal partners may acquire compatible cyanobionts either by recruiting free-living cyanobacteria from the surrounding environment or by sourcing them from existing lichen thalli, even those of other species. The overall success of sexual reproduction largely depends on the availability of suitable photobionts; some fungal species are highly selective, while others can associate with a broader range of partners.
Some cyanolichens also form unique structures called, in which a single fungal species produces different morphological forms depending on whether it partners with green algae or cyanobacteria. These structures may occur as separate thalli with distinct photobionts or as combined thalli in which both types of photobionts are present in different regions. Photosymbiodemes thus represent a flexible reproductive strategy, potentially allowing the fungal partner to adapt to varying environmental conditions by altering its photobiont associations.