Cyanolichen

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 body of the lichen. 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 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 substrates. 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.