Cyanolichen

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.

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 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.

[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.

[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.

This broad taxonomic distribution suggests that the ability to form symbioses with cyanobacteria has evolved multiple times independently across distantly related fungal groups.

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.

[7] 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.

[8] 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).

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.

Photosymbiodemes thus represent a flexible reproductive strategy, potentially allowing the fungal partner to adapt to varying environmental conditions by altering its photobiont associations.

While early research relied primarily on microscopic observation, modern studies employ a variety of molecular and genomic techniques to understand these complex relationships.

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.

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.

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.

[24] 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 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.

[12] The evolutionary history of cyanolichens represents multiple independent origins of symbiotic relationships between fungi and cyanobacteria, with evidence spanning hundreds of millions of years.

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.

This fossil, named Winfrenatia reticulata, consisted of a thallus composed of fungal filaments with depressions containing coccoid cyanobacteria similar to modern Gloeocapsa or Chroococcidiopsis.

[27] 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.

[31] 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.

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.

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.

[35] 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.

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.

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. [ 1 ]
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.
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.
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.
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.
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.
Sticta canariensis , a cyanolichen species. It features large, leafy lobes and prominent cyanobacterial layers.
Lobaria oregana , the "Oregon lung lichen", often hangs in forest canopies of the Pacific Northwest and fixes atmospheric nitrogen.
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 ). [ 26 ]
Cora glabrata is a basidiomycete cyanolichen, an example of one of the independent origins of lichenisation outside the more familiar Ascomycota.
Lichina pygmaea is marine/intertidal cyanolichen in the class Lichinomycetes, another evolutionarily distinct lineage.
The "western waterfan", Peltigera hydrothyria var. gowardii , is a rare aquatic cyanolichen that grows in cold, clean mountain streams of northwestern North America. Its reliance on pristine riparian habitats makes it vulnerable to pollution, habitat loss , and climate change . [ 32 ]
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.