Cyanolichens

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 highlights 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 ), the photosynthetic partner (the ) 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 (the thallus). 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 (chlorolichens), 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 (Agaricales). 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 (''Picea'') could support up to 38 different cyanolichen species, while Douglas fir (''Pseudotsuga'') hosted 27 species. Other conifers like western red cedar (''Thuja''), western hemlock (''Tsuga''), lodgepole pine (''Pinus'') and subalpine fir (''Abies'') 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 (photobiont) 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 (reproductive filaments). 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 (via fungal spores) tend to be more promiscuous in their photobiont selection compared to species that reproduce mainly through symbiotic propagules. In addition, species that form cephalodia (specialised structures containing cyanobacteria) 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 (in Ascomycota) or basidiospores (in Basidiomycota). 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 (referred to as chlorosymbiodemes and cyanosymbiodemes) 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.

Ecological and environmental considerations

The dispersal of cyanolichens is challenged by several factors. These include the necessity to maintain or re-establish specific symbiotic partnerships, the limited viability of propagules under adverse conditions, the need for suitable environmental conditions at establishment sites, and competition with other organisms for space and resources. The successful establishment of new thalli often depends on the presence of appropriate microhabitats that can support both partners during the early stages of development.

Additionally, various ecological factors influence reproductive success in cyanolichens. Habitat continuity is particularly critical for species that depend on local sources of compatible photobionts. Environmental parameters—including moisture availability, light intensity, and substrate characteristics—play key roles in establishment success. Furthermore, the presence of facilitating organisms, such as bryophytes that help maintain favorable microhabitats, can enhance establishment. Conversely, disturbance regimes may either create new opportunities or disrupt existing populations. A comprehensive understanding of these reproductive and dispersal mechanisms is essential for cyanolichen conservation, especially for rare species or those with specific habitat requirements.

Research history and methods

The study of cyanolichens has faced significant methodological challenges since their initial recognition as symbiotic organisms in the 1860s. While early research relied primarily on microscopic observation, modern studies employ a variety of molecular and genomic techniques to understand these complex relationships.

Identifying and isolating cyanobionts presents several unique difficulties. Unicellular cyanobionts are particularly challenging to study due to their slow growth rates and the presence of multiple free-living cyanobacterial species that may grow on or near lichen thalli. Additionally, the morphology of cyanobacteria can differ significantly between their free-living and lichenised states, complicating traditional identification methods based on visual characteristics.

Modern research increasingly relies on molecular techniques, including DNA sequencing and genomic analysis. The 16S ribosomal RNA gene sequence has become a standard tool for identifying cyanobacterial species, though researchers also employ other genetic markers such as ''rbcLX''. Despite these advances, the number of sequenced cyanobiont genomes remains relatively low compared to free-living cyanobacteria, with only about two dozen lichen cyanobiont genomes publicly available as of 2023.

In response to the need for coordinated research efforts, the International Network for research on unicellular CyanoBionts (INCb) was established. This consortium brings together lichenologists, microbiologists, and phycologists to study the diversity and ecology of unicellular cyanobionts. Their work combines traditional methods with modern genomic approaches, including metagenomics and metabolomics, to better understand these organisms.

New techniques—such as carefully isolating the photobiont from lichen tissue and analyzing small amounts of DNA—have improved researchers' ability to study cyanolichens have greatly enhanced the understanding of cyanolichens by distinguishing the symbiotic cyanobacteria from similar free‐living species. The development of specialised databases for cyanobacterial taxonomy has simultaneously enhanced the ability to classify and understand these organisms within their evolutionary context.

While early cyanolichen research focused heavily on old-growth forests, important ecological relationships in younger forest stands began to be uncovered in the 1990s. Studies during this period demonstrated that young, regenerating forests could support significant cyanolichen populations under certain conditions. Particularly significant was the discovery of nutrient transfer effects between different tree species, such as the influence of deciduous tree leachates on conifer bark chemistry. These findings helped explain how cyanolichen communities initially establish themselves in developing forests and highlighted the complex relationships between forest composition and lichen diversity. Such discoveries expanded scientific understanding beyond the traditional focus on old-growth habitats to encompass the full range of forest successional stages.

Physiology

The physiological processes in cyanolichens reflect their complex nature as composite organisms, involving intricate interactions between fungal and cyanobacterial partners. These processes include specialised mechanisms for photosynthesis, water relations, and nutrient exchange.

Water relations

Cyanolichens differ fundamentally from chlorolichens in their water requirements for photosynthesis. While lichens with green algae can photosynthesise using just high atmospheric humidity, cyanolichens require liquid water to activate photosynthesis. This physiological constraint helps explain their habitat preferences and more limited distribution compared to chlorolichens.

A distinctive feature of cyanolichens is that their symbiotic cyanobacteria show enhanced resilience compared to their free-living counterparts. Studies have shown that while free-living cyanobacteria may be damaged by repeated desiccation, their lichenised counterparts demonstrate exceptional desiccation tolerance, suggesting that the fungal partner provides protective benefits beyond simple structural support. Cyanolichens display unique physiological strategies to survive fluctuating water availability. Their cyanobacterial partners, often from the order Nostocales, are typically housed in cephalodia, specialised structures that regulate oxygen levels and help maintain photobiont function during hydration cycles. These cyanobacterial symbionts have been observed to form densely packed colonies within the lichen thallus, creating hypoxic microenvironments that may facilitate nitrogen fixation while protecting them from oxidative stress.

The water relations of cyanolichens are particularly complex in species, where the cyanobacterial mucilage can absorb large amounts of water during hydration events. These lichens can undergo dramatic changes in thallus dimensions during wetting and drying cycles, sometimes expanding to several times their dry size when fully hydrated. While this ability to rapidly absorb water benefits the lichen in capturing brief periods of moisture availability, it also makes them susceptible to desiccation when conditions become dry.

Carbon and nitrogen metabolism

Cyanolichens have evolved sophisticated mechanisms for metabolite exchange between symbionts. The cyanobacterial partner provides both photosynthate (as glucose) and fixed atmospheric nitrogen (as ammonium) to the fungal partner. In tripartite lichens, where both green algal and cyanobacterial partners are present, the green algae typically handle most photosynthetic carbon fixation while the cyanobacteria focus primarily on nitrogen fixation. Studies have shown that when both photobionts are present in the , they actively contribute to photosynthesis.
The nitrogen fixation capabilities of cyanolichens are particularly enhanced in cephalodia, where the cyanobacteria show higher rates of nitrogen fixation and contain more heterocysts (specialised cells for nitrogen fixation) compared to those in bipartite cyanolichens. The presence of cephalodia in tripartite cyanolichens appears to be an evolutionary adaptation that optimises resource partitioning. Research suggests that fungal hosts with access to both green algae and cyanobacteria may benefit from a more efficient division of metabolic labour—using green algae primarily for photosynthesis while localising nitrogen fixation within cephalodia. This separation allows the fungal partner to maintain greater metabolic flexibility, particularly in environments where nitrogen availability fluctuates. This enhanced capability appears to be a direct result of the specialised microenvironment created by the cephalodia. Cephalodia-dwelling cyanobacteria are generally more rich in nitrogen-fixing heterocysts than those which live in the main photobiont layer of lichens. The symbiotic relationship provides additional benefits, as studies have shown that while free-living cyanobacteria may be damaged by repeated desiccation, their lichenised counterparts in cephalodia demonstrate remarkable desiccation tolerance, suggesting that the fungal partner provides protective benefits beyond simple structural support.

Research has revealed that lichen-symbiotic cyanobacteria possess two different nitrogen fixation pathways: a molybdenum-dependent system and a vanadium-dependent system. This dual-pathway capability provides metabolic flexibility, allowing nitrogen fixation to continue even when one of these trace elements is scarce.

Photosynthetic adaptations

The photosynthetic apparatus of cyanolichens shows several unique adaptations. Their cyanobacterial photobionts can effectively utilise wavelengths of light that filter through forest canopies, particularly green light, thanks to their phycobilin pigments. This ability can provide cyanolichens with a competitive advantage in shaded environments where other photosynthetic organisms might struggle.

The overall structure of the lichen thallus also influences photosynthetic efficiency. The thickness of different thallus layers and the development of pigments and refractive structures all affect the quantity and quality of light reaching the photosynthesising cyanobionts. Some species have evolved specialised structures to optimise light exposure while protecting the photobiont from excessive radiation.

Metabolite exchange

The exchange of metabolites between partners requires sophisticated cellular machinery. In most cyanolichens, thin-walled fungal hyphae penetrate the gelatinous sheaths of cyanobionts but usually without forming direct contact with the cell walls. This intimate association allows for efficient transfer of nutrients while maintaining the integrity of both partners. Notable exceptions exist in some basidiomycete lichens, where intracellular haustoria actually penetrate the cyanobacterial cells.

The regulation of these physiological processes involves extensive molecular crosstalk between the symbionts, though many details of this communication remain poorly understood. Genomic studies have begun to reveal the complex network of genes involved in establishing and maintaining these symbiotic relationships.

Ecological roles

Cyanolichens play several crucial roles in terrestrial ecosystems, particularly through their contributions to nutrient cycling, soil stabilisation, and ecological interactions. Their impacts range from local microhabitat effects to ecosystem-level processes.

Nitrogen fixation and nutrient cycling

A primary ecological function of cyanolichens is their contribution to ecosystem nitrogen budgets through biological nitrogen fixation. Unlike green algal lichens, cyanolichens can convert atmospheric nitrogen into biologically available forms through their cyanobacterial partners. The nitrogen fixation process is carried out by an enzyme complex called nitrogenase, which converts atmospheric (N₂) into ammonia (NH₃). This energy-intensive process requires 8 electrons and 16 ATP molecules to convert a single N₂ molecule. Since nitrogenase is sensitive to oxygen, the cyanobacteria form heterocysts where nitrogen fixation occurs. These heterocysts lack the oxygen-producing components of photosynthesis, creating the necessary anaerobic environment for nitrogen fixation.

In cyanolichens, the fixed nitrogen is initially converted to the amino acid glutamine within the heterocysts, which is then exported to other cells for further processing. Studies using isotope tracing have shown that much of this fixed nitrogen is transferred to the fungal partner, with some species showing transfer rates of over 90%. The fungal partner then converts the nitrogen into other forms such as glutamate and alanine. This process is particularly significant in nutrient-poor environments where other sources of fixed nitrogen are limited. Research has revealed that lichen-associated ''Nostoc'' can employ both molybdenum- and vanadium-dependent nitrogen fixation pathways, providing metabolic flexibility in environments where different trace elements may be scarce.

The contribution of cyanolichens to ecosystem nitrogen inputs can be substantial. In boreal forests, for example, canopy-dwelling cyanolichens can fix several kilograms of nitrogen per hectare annually. This fixed nitrogen becomes available to other organisms through various pathways, including leaching during rainfall, decomposition of lichen tissue, and consumption by herbivores. Environmental factors such as moisture, temperature, and light influence nitrogen fixation rates in cyanolichens. While nitrogen fixation can occur at temperatures as low as , optimal rates typically occur between . The process requires sufficient moisture and moderate light levels, with very high light intensities potentially inhibiting nitrogen fixation.

Soil stabilisation and crust formation

In arid and semi-arid regions, cyanolichens are often important components of biological soil crusts, where they contribute significantly to soil stability and fertility. These crusts help prevent soil erosion, enhance water retention, and increase soil organic matter content. Studies in desert environments have shown that cyanolichen-containing soil crusts can have significantly higher carbon and nitrogen fixation rates compared to crusts lacking cyanolichens. For instance, research in the Mojave Desert has demonstrated that areas with well-developed cyanolichen crusts show markedly higher rates of both carbon and nitrogen fixation compared to other crust types.

Ecological interactions

Cyanolichens form critical components of forest ecosystems through their interactions with other organisms. They support diverse invertebrate communities that feed on or shelter within their thalli, which in turn provide essential food sources for both resident and migratory birds. This role is particularly important in mature and old-growth forests, where studies have shown that invertebrate abundance can be significantly higher on trees with abundant lichen growth compared to those without.

These lichens also form important associations with bryophytes ( non-vascular land plants), particularly in humid forest environments. Such associations can be mutually beneficial, with bryophytes helping to maintain moisture levels favourable for cyanolichen photosynthesis and growth. Recent studies indicate that cyanolichens are not simply two-partner systems but are embedded within a broader microbiome of additional fungi and bacteria. Some secondary fungal species, detected within cyanolichen thalli, may play roles in nutrient cycling or structural reinforcement. This microbial complexity suggests that cyanolichens function as microbial ecosystems, rather than solely as binary fungal–cyanobacterial symbioses. In some cases, cyanolichens and bryophytes may even share cyanobacterial partners, creating complex networks of nutrient exchange.

Role in forest ecosystems

Cyanolichens play a crucial role in forest hydrology and fire ecology. Their ability to absorb and retain significant amounts of water helps maintain higher humidity levels within forest canopies. This moisture retention capacity can contribute to reducing forest fire risk by helping to maintain higher moisture levels in the forest environment, particularly during dry periods.

An important ecological phenomenon known as the "dripzone effect" occurs when nutrient-rich leachates from deciduous trees, particularly species of poplar (''Populus''), significantly enhance cyanolichen colonisation on nearby conifers. This effect occurs when nutrients, especially calcium, from deciduous tree canopies wash down during rainfall and increase the pH of bark on neighbouring conifer trees. The dripzone effect is most pronounced in humid regions not affected by acid rain and can be particularly important in young forest stands where cyanolichens are first becoming established. This relationship demonstrates how deciduous trees can indirectly facilitate nitrogen fixation in forest ecosystems by promoting cyanolichen colonisation on conifers that might otherwise be too acidic to support these sensitive organisms. The presence of this facilitative relationship has important implications for understanding forest ecosystem dynamics and the distribution patterns of cyanolichen communities.

Role in succession

Cyanolichens often serve as pioneer organisms in primary succession, particularly on newly exposed rock surfaces or disturbed soils. Their ability to fix nitrogen and contribute to soil development makes them important facilitators for the establishment of other organisms. In mature ecosystems, particularly old-growth forests, specific assemblages of cyanolichen species can indicate long-term habitat continuity and are often used as indicators of ecological integrity.

Evolution and systematics

The evolutionary history of cyanolichens represents multiple independent origins of symbiotic relationships between fungi and cyanobacteria, with evidence spanning hundreds of millions of years. This complex history has resulted in considerable diversity in both fungal and cyanobacterial partners. Growing evidence suggests that the ancestor of the Peltigerales had only cyanobionts as photobionts, with chlorobionts (green algae) being added later in their evolutionary history. The presence of tripartite lichens with both types of photobionts in their photobiont layer demonstrates a level of versatility in the mycobionts that allowed them to successfully incorporate both partners.

Evolutionary history and multiple origins

The oldest confirmed fossil of a stratified cyanolichen dates to the Lower Devonian period, demonstrating the ancient origins of these symbioses. While lichen fossils are generally rare, amber specimens from the Tertiary period have preserved examples of several extant genera, providing insights into their evolutionary persistence. Molecular dating studies suggest that the initial diversification of the Pezizomycotina (Ascomycota), which includes most lichen-forming fungi, occurred during the Ordovician period, with subsequent lineage diversification continuing throughout the Phanerozoic.

The oldest conclusively identified cyanolichen fossil comes from the Early Devonian Rhynie chert of Scotland, dated to approximately 400 million years ago. This fossil, named '' Winfrenatia reticulata'', consisted of a thallus composed of fungal filaments with depressions containing cyanobacteria similar to modern ''Gloeocapsa'' or ''Chroococcidiopsis''. Unlike modern cyanolichens, ''Winfrenatia'' lacked a protective upper and showed a more primitive level of integration between the fungal and bacterial partners. The fossil demonstrates that lichen symbioses involving cyanobacteria had already evolved by the Early Devonian, though the fungal partner appears to have been a member of the Zygomycetes rather than the Ascomycetes that form most modern lichen symbioses.

Early cyanolichen evolution may have been driven by the need for the photobiont to retain liquid water, as evidenced by thick mucilaginous sheaths around cyanobacterial cells in the earliest known fossil lichens. The symbiosis may have also provided protection from herbivory and high ultraviolet radiation in early terrestrial environments.

Symbioses between fungi and cyanobacteria have evolved independently multiple times throughout fungal evolution. While the vast majority of cyanolichens involve ascomycete fungi, a few notable examples exist among the basidiomycetes, such as ''Dictyonema''. The independent evolution of these partnerships is evidenced by their different anatomical structures and varying degrees of integration between partners. Some lineages appear to have lost the ability to form lichen symbioses over time, suggesting that the evolution of lichenisation is not unidirectional.

Taxonomic and photobiont relationships

Most cyanolichen species belong to the ascomycete class Lecanoromycetes, particularly within the order Peltigerales. Within this group, some families (such as Collemataceae, Nephromataceae, Pannariaceae, and Peltigeraceae) predominantly form associations with Nostoc cyanobacteria, while others partner with different cyanobacterial genera. The Lichinomycetes, a primarily tropical class of ascomycetes, form bipartite symbioses almost exclusively with non-nostocalean cyanobacteria such as '' Gloeocapsa''.
The evolution of photobiont relationships reflects distinct patterns between green algal lichens and cyanolichens, terms that were formally distinguished when Vernon Ahmadjian introduced the term 'chlorolichen' in 1989 as a counterpart to 'cyanolichen'. Unlike green algal lichens, where photobionts typically form distinct evolutionary lineages adapted to lichen symbiosis, cyanolichen photobionts are more closely related to free-living cyanobacteria. Rather than forming tightly co-evolving partnerships, cyanobacterial symbionts in cyanolichens are often recruited from environmental populations or from other cyanolichens. This supports the concept of 'fungal farming', where mycobionts selectively acquire and exchange photobionts in response to environmental conditions rather than evolving alongside a single photobiont lineage.

While ''Nostoc'' is the predominant cyanobiont in most cyanolichens, some groups, such as ''Dictyonema'' basidiolichens, associate with a distinct cyanobacterial genus, ''Rhizonema''. Research has shown that ''Rhizonema'' exhibits much lower diversity compared to its fungal partners, further reinforcing the idea of repeated photobiont acquisition rather than co-speciation. This distinction highlights how green algal photobionts tend to co-evolve with their fungal hosts, whereas cyanobacterial symbionts are more frequently exchanged across different lichen lineages.

Further studies indicate that ''Rhizonema'' differs from many other lichenised cyanobacteria in that it has not been found in a free-living state, suggesting it may be an obligate symbiont. Although initially recognised in ''Dictyonema'', ''Rhizonema'' has been identified in other cyanolichen lineages, indicating a broader ecological role. Additionally, its morphology appears to be influenced by the fungal partner, displaying either filamentous or pseudocolonial forms depending on the lichen species. These findings reinforce the concept that mycobionts selectively recruit photobionts based on functional compatibility rather than through strict co-evolution.

Genetic adaptations and contemporary diversity

Molecular studies have shown that symbiotic lifestyle has led to specific genetic adaptations in both partners. For example, cyanobionts in lichenised states often show modified gene expression patterns compared to their free-living relatives, particularly in genes related to nitrogen fixation and carbon metabolism. The fungal partners have evolved specialised structures and molecular mechanisms for engaging with their photobionts, though many aspects of these adaptations remain poorly understood.

Modern molecular techniques continue to reveal previously unrecognised diversity in cyanolichen partnerships. Recent studies suggest that traditional morphological species concepts may significantly underestimate true species diversity, particularly in tropical regions. This has led to ongoing revision of many taxonomic groups and a growing appreciation for the complexity of these symbiotic relationships.

Further research using advanced genomic and phylogenetic methods is likely to continue revealing new insights into the evolutionary history and diversity of cyanolichens. The discovery of new species and symbiotic relationships remains an active area of research, particularly in poorly studied regions and habitats.

Conservation

Cyanolichens face numerous conservation challenges worldwide, with many species experiencing population declines due to environmental changes and human activities. Their sensitivity to environmental conditions makes them particularly vulnerable to habitat alterations while simultaneously making them valuable as ecological indicators.

Threats

Cyanolichens are highly sensitive to human-induced changes in the environment. For instance, air pollutants like sulphur dioxide and nitrogen compounds have led to local extinctions in industrial regions, while habitat loss and climate change further threaten these organisms. The impact is particularly severe because sulphur dioxide becomes more toxic under acidic conditions, making cyanolichens especially vulnerable in areas with acid rain. This has led to documented losses of species in many industrialised regions, where sensitive species like ''Lobaria'' have disappeared even from otherwise suitable habitats. Habitat loss and fragmentation, especially of old-growth forests, have disrupted populations of many epiphytic cyanolichens that depend on long-term habitat continuity. Climate change presents an emerging threat, potentially affecting the delicate balance of these symbiotic relationships through changes in temperature and precipitation patterns.

In semi-arid regions, soil-dwelling cyanolichens face particular challenges from overgrazing and soil disturbance. These impacts can be especially severe for biological soil crusts, where cyanolichens play crucial roles in ecosystem stability. The loss of these communities can trigger cascading effects on soil fertility and erosion control.

Conservation challenges vary significantly by region. In Europe, many cyanolichen species are red-listed due to historical impacts of air pollution and habitat loss, though some populations have shown recovery following air quality improvements. Tropical regions harbour high cyanolichen diversity, but many species remain poorly documented and may be threatened by rapid habitat conversion. In arid regions, the conservation of soil crust communities has become increasingly important as these ecosystems face mounting pressures from climate change and land use intensification.

Bioindicators

The sensitivity of cyanolichens to environmental conditions has led to their widespread use as bioindicators. Many species serve as early warning systems for environmental degradation, particularly regarding air quality and forest ecological continuity. For example, ''Lobaria pulmonaria'' is frequently used as an indicator species to identify forests of high conservation priority in Europe, while various cyanolichen species are used to monitor air quality in urban and industrial areas.

Conservation strategies

Conservation efforts for cyanolichens focus on both habitat protection and species-specific interventions. The preservation of old-growth forests has proven crucial for maintaining populations of many epiphytic species, particularly in boreal and temperate regions. Among the most urgent conservation priorities are species facing severe population declines. For instance, ''Erioderma pedicellatum'' has experienced dramatic declines despite conservation efforts in both Europe and North America. Its landmark addition to the IUCN Red List in 2003 marked the first time a lichen received this level of recognition, making it an enduring symbol for lichen conservation globally.

Key conservation measures include maintaining pressure on governments to regulate air pollution, particularly from coal-fired power plants that contribute to acid rain, and preserving old-growth forest stands that harbour rare species. Practical conservation measures include timing forestry operations to avoid spring months when lichen spores are being released, limiting the use of herbicides in regenerating forests that can affect lichen reproduction, and ensuring the retention of "legacy trees" that can serve as sources for recolonisation. The preservation of mature trees is particularly crucial as studies have shown that many cyanolichen species take centuries to establish significant populations, even in otherwise suitable habitats.

Management strategies must consider the important role of mixed forest composition in supporting cyanolichen populations. The presence of deciduous trees, particularly ''Populus'' species, in young forest stands can be critical for initial cyanolichen establishment through their facilitation of suitable bark chemistry on neighbouring conifers. This relationship is particularly important in regions not affected by acid rain, where such natural facilitation processes can still function effectively. Forest management practices that maintain an appropriate mix of deciduous and coniferous trees can help preserve these ecological relationships and support cyanolichen diversity. In areas where acid rain has disrupted these natural processes, the loss of this facilitation effect has likely contributed to cyanolichen decline, suggesting that more active intervention may be needed to maintain cyanolichen populations.

Future prospects

The future of cyanolichen conservation depends on addressing both immediate threats and long-term challenges. Climate change adaptation strategies are becoming increasingly important, as shifting environmental conditions may affect the distribution and viability of many species. Improved understanding of cyanolichen biology, particularly their reproductive requirements and habitat needs, continues to inform more effective conservation approaches. The development of new molecular tools for identifying and monitoring populations offers promising opportunities for more targeted conservation efforts.

International collaboration in research and conservation planning has become increasingly important, particularly for widespread species and those in poorly studied regions. Success stories in air quality improvement and habitat protection demonstrate that conservation efforts can be effective when properly implemented and maintained.

References

Cited literature

*{{cite journal , last=Rikkinen , first=Jouko , title=Cyanolichens , journal=Biodiversity and Conservation , volume=24 , issue=4 , year=2015 , doi=10.1007/s10531-015-0906-8 , pages=973–993

Lichenology