Cyanolichen

Bipartite and tripartite cyanolichens. (A) In the bipartite cyanolichen Peltigera scabrosa the cyanobacterial symbiont ( Nostoc ) forms a continuous layer just below the upper cortex of the lichen thallus. (B) Nephroma bellum is another example of bipartite cyanolichens. (C) In the tripartite cyanolichen Peltigera aphthosa the Nostoc symbiont is restricted to wart-like cephalodia (shown magnified) on the upper surface of the thallus, while the green algal symbiont ( Coccomyxa ) forms the photobiont layer. (D) Nephroma arcticum is another example of tripartite cyanolichens. The large cephalodia of this species are internal, but clearly visible through the upper cortex of the hydrated thallus.

Cyanolichens are lichens in which the fungal component ( mycobiont ) partners with cyanobacteria ( cyanobionts ) for photosynthesis, rather than the green algae found in most other lichens. In some cyanolichens, known as bipartite forms, the cyanobacteria form an extensive photobiont layer throughout the main thallus. Others, called tripartite 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 varied anatomy and taxonomy, cyanolichens carry out vital ecological functions. They are particularly noted for their nitrogen-fixing capabilities, contributing essential nutrients to both forest canopies and biological soil crusts in arid regions. By transforming atmospheric nitrogen into bioavailable forms, cyanolichens play a key role in ecosystem nutrient cycling and often serve as pioneer species on newly exposed substrates . 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

Nephroma arcticum (the "green-felt" lichen) is a classic tripartite lichen with both green algae and cyanobacteria. It often has conspicuous cephalodia (the wart-like structures housing cyanobacteria), illustrating how some lichens compartmentalise multiple photobionts.

Cyanolichens are lichens that apart from the basic fungal component ( mycobiont ), contain cyanobacteria, otherwise known as blue-green algae, as the photosynthesising component ( photobiont ). Overall, about a third of lichen photobionts are cyanobacteria and the other two thirds are green algae. ^[2] Some lichens contain both green algae and cyanobacteria apart from the fungal component, in which case they are called " tripartite ". Normally the photobiont occupies an extensive layer covering much of the thallus, but in tripartite lichens, the cyanobacterium component may be enclosed in pustule-like outgrowths of the main thallus called cephalodia, which can take many forms. These structures represent specialised compartments within what is now understood to be a complex micro-ecosystem rather than a simple dual organism. The cephalodia create distinct microenvironments where specific physiological interactions occur between the fungal and cyanobacterial partners. ^[3]

Cyanolichens exhibit considerable diversity in their cyanobacterial partners ( cyanobionts ), 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. ^[3] 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 . ^[3] 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. ^[4] 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. ^[3] The total diversity of cyanolichens appears to be significantly lower than that of lichens containing green algae (chlorolichens), with cyanolichens representing approximately 10% of known lichen species, though this figure may underestimate true diversity as new molecular techniques continue to reveal previously unknown cyanobiont relationships. ^[3]

Peltigera canina , "dog pelt lichen", is a common bipartite cyanolichen associated with Nostoc; it depicts the typical "leafy" morphology of many Peltigerales species and the gelatinous inner layers that become apparent when wet.

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 colonization 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. ^[5] 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. ^[5]

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. ^[6]

Establishment and reproduction

Species of Collema , such as Collema nigrescens , were historically among the earliest "jelly lichens" recognised for their cyanobacterial partners. Microscopic and later molecular studies of Collema helped clarify how morphologically similar species can differ in their cyanobionts.

In sexually reproducing cyanolichens, the fungal spores must first germinate and then find compatible photobionts in order to establish a new lichen thallus. Research has shown this process faces significant challenges, as fungal spores often fail to germinate without the presence of appropriate photobionts, and compatible photobionts may be scarce in many environments. ^[7]

Studies of cyanolichen communities have revealed that species sharing the same type of cyanobacterial photobiont often form ecological guilds. Within these guilds, some "core species" that reproduce asexually through symbiotic propagules can act as sources of compatible photobionts for "fringe species" that only produce fungal spores. This facilitation between different lichen species appears to be crucial for maintaining populations of some sexually reproducing cyanolichens. ^[7]

The cyanobacterial photobionts of some cyanolichen species show very limited ability to grow independently, displaying slow growth rates and an inability to produce motile hormogonia (reproductive filaments). This suggests that through evolution, some symbiotic cyanobacteria have largely lost capabilities needed for free-living existence. As a result, sexually reproducing cyanolichen species may depend heavily on obtaining photobionts from existing lichen thalli rather than from free-living cyanobacterial populations. ^[7]

The relative success of different reproductive strategies appears to influence photobiont associations. Research has shown that sexually reproducing species (those producing fungal spores) tend to be more promiscuous in their photobiont choice compared to species that reproduce primarily through symbiotic propagules. Additionally, species that produce cephalodia (specialised structures containing cyanobacteria) often associate with a wider diversity of photobionts compared to species that only form bipartite associations. ^[6]

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. ^[3]

Dictyonema sericeum , a basidiolichen. This species demonstrates that cyanolichens can form symbiotic relationships with basidiomycete fungi, not just ascomycetes, highlighting the evolutionary diversity within lichen symbioses.

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. ^[3]

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. ^[3]

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. ^[3]

New techniques such as direct photobiont picking and low-biomass DNA analysis have improved researchers' ability to study cyanolichens. These methods help distinguish between symbiotic cyanobionts and free-living species that may contaminate samples. Additionally, the development of specialised databases for cyanobacterial taxonomy has enhanced the ability to classify and understand these organisms within their evolutionary context. ^[3]

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. ^[5]

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

Leptogium cyanescens is a "gelatinous" (jelly) cyanolichen; it visibly swells and darkens when wet, demonstrating how cyanolichens require liquid water for photosynthesis and show dramatic thallus changes with hydration.

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. ^{[8] [9]}

A remarkable 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 remarkable desiccation tolerance, suggesting that the fungal partner provides protective benefits beyond simple structural support. ^[4]

The water relations of cyanolichens are particularly complex in gelatinous 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. ^[10]

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. ^[11] However, studies using zinc solutions have demonstrated that both photobionts actively contribute to photosynthesis when present together in the photobiont layer. ^[9]

Sticta canariensis , a cyanolichen species. It features large, leafy lobes and prominent cyanobacterial layers.

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. ^[4] This enhanced capability appears to be a direct result of the specialised microenvironment created by the cephalodia. ^[4] Cephalodia-dwelling cyanobacteria are generally more rich in nitrogen-fixing cells called heterocysts than those which live in the main photobiont layer of lichens. ^{[2] [1]} 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. ^[3]

Recent 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. ^[12]

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. ^[13]

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. ^[14]

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. ^[15]

The regulation of these physiological processes involves extensive molecular crosstalk between the symbionts, though many details of this communication remain poorly understood. Recent genomic studies have begun to reveal the complex network of genes involved in establishing and maintaining these symbiotic relationships. ^[16]

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

Lobaria oregana , the "Oregon lung lichen", often hangs in forest canopies of the Pacific Northwest and fixes atmospheric nitrogen.

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. This process is particularly significant in nutrient-poor environments where other sources of fixed nitrogen are limited. Recent 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. ^[17]

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. ^[16]

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. ^[12]

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. ^[18]

These lichens also form important associations with bryophytes, particularly in humid forest environments. Such associations can be mutually beneficial, with bryophytes helping to maintain moisture levels favorable for cyanolichen photosynthesis and growth. In some cases, cyanolichens and bryophytes may even share cyanobacterial partners, creating complex networks of nutrient exchange. ^[19]

Role in forest ecosystems

Peltigera aphthosa is common on soil or mossy ground in boreal forests; it is well-studied for its nitrogen fixation and its role as a pioneer species on disturbed or newly formed substrates (e.g., glacial forelands).

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. ^[18]

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. ^[5]

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. ^[12]

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. ^[16] 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. ^[9]

Evolutionary history and multiple origins

Cora glabrata is a basidiomycete cyanolichen, an example of one of the independent origins of lichenisation outside the more familiar Ascomycota.

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. ^[12]

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 coccoid cyanobacteria similar to modern Gloeocapsa or Chroococcidiopsis. Unlike modern cyanolichens, Winfrenatia lacked a protective upper cortex 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. ^[20]

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. ^[20]

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. ^[21]

Taxonomic and photobiont relationships

Lichina pygmaea is marine/intertidal cyanolichen in the class Lichinomycetes, another evolutionarily distinct lineage.

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 . ^[22]

The evolution of photobiont relationships shows intriguing patterns. Unlike green algal lichens, where photobionts often form distinct evolutionary lineages specific to lichen symbiosis, cyanolichen photobionts frequently cluster phylogenetically with free-living cyanobacteria. This suggests different evolutionary dynamics in cyanobacterial versus green algal symbioses. Recent molecular studies have revealed previously unrecognised diversity in both partners, particularly among unicellular cyanobionts. ^[14]

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. ^[15]

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. ^[16]

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. ^[16]

Reproduction and dispersal

Cyanolichens employ diverse strategies for reproduction and dispersal, involving both sexual and asexual methods. The maintenance and re-establishment of the symbiotic relationship presents unique challenges that have led to various adaptive solutions.

Reproductive strategies

Many cyanolichen species reproduce through specialised vegetative structures that allow 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 consisting of fungal hyphae wrapped around cyanobacterial cells, and lobules , which are small thallus fragments that break off and establish new individuals. These methods ensure the continuation of established successful partnerships and typically result in clonal reproduction of the entire symbiotic unit. The effectiveness of these strategies varies among species and habitats, with some species relying more heavily on one method over others. ^[23]

Many cyanolichen fungi reproduce sexually through the production of ascospores (in Ascomycota) or basidiospores (in Basidiomycota). This process presents a particular challenge as the fungal spores must locate and establish new partnerships with appropriate cyanobacterial partners after dispersal. The fungal partner can obtain compatible cyanobionts through different routes, either by recruiting free-living cyanobacteria from environmental populations or by acquiring cyanobionts from existing lichen thalli, including those of other species. The success of sexual reproduction depends largely on the availability of compatible photobionts in the new environment. Studies have shown that some fungal species are highly selective in their choice of photobiont, while others can associate with a broader range of partners. ^[23]

Some cyanolichens can form unique structures called photosymbiodemes , where a single fungal species creates different morphological forms depending on whether it partners with green algae or cyanobacteria. These structures can exist either as separate thalli with different photobionts (known as chlorosymbiodemes and cyanosymbiodemes) or as combined thalli where both types of photobionts occur in different portions of the same structure. Photosymbiodemes represent a reproductive strategy where the fungal partner maintains flexibility in its photobiont associations, potentially allowing adaptation to different environmental conditions. ^[24]

Ecological and environmental considerations

The dispersal of cyanolichens faces several key challenges. These include the need to maintain or re-establish specific symbiotic partnerships, limited viability of propagules under adverse conditions, requirements for appropriate environmental conditions at the site of establishment, and competition with other organisms for space and resources. Success in dispersal and establishment often depends on the presence of suitable microhabitats that can support both partners during the critical early stages of development. ^[25]

Various ecological factors influence reproductive success in cyanolichens. Habitat continuity is particularly important for species that rely on local sources of compatible photobionts. Environmental conditions, including moisture availability, light levels, and substrate characteristics, play crucial roles in establishment success. The presence of facilitating organisms, such as bryophytes that may help maintain suitable microhabitats, can enhance establishment success. Disturbance regimes can either create new opportunities for establishment or disrupt existing populations. Understanding these reproductive and dispersal mechanisms has important implications for cyanolichen conservation, particularly for rare species or those with specific habitat requirements. ^[12]

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

Human-induced environmental changes have resulted in significant threats to cyanolichen populations globally. Air pollution, particularly from sulphur dioxide and nitrogen compounds, has severely impacted many species, leading to local extinctions in heavily industrialised regions. ^[26] 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. ^[18] 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. ^[26]

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. ^[27]

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. ^[26]

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. ^[26]

Conservation strategies

Erioderma pedicellatum , the boreal felt lichen, is a critically endangered cyanolichen found in boreal forests of Atlantic Canada and Scandinavia. It is extremely sensitive to air pollution and habitat loss, making it a prime example of conservation challenges.

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. ^[26] 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. ^[28]

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. ^[18] 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. ^[18]

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. ^[5]

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. ^[26]

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. ^[26]

References

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Cited literature

• Rikkinen, Jouko (2015). "Cyanolichens". Biodiversity and Conservation. 24 (4): 973–993. doi:10.1007/s10531-015-0906-8.