Whether You Lichen It or Not, They Lichen You

Characteristics and Forms of Lichen Presented on and in the Human Body

Lichens are fascinating composite organisms, primarily comprising a symbiotic association of two distinct species: a fungus (mycobiont) and a photosynthetic partner (photobiont), usually an algae or cyanobacteria. Traditionally, lichens have been recognized for their ability to colonize some of the harshest environments on Earth, from desolate Antarctic tundras to bare rocky terrains. Less known, however, is the existence and characteristics of lichens that have adapted to survive in and on the unique ecosystem of the human body.

One should distinguish the lichen-forming fungi from the medically recognized condition called “lichen” in humans, such as lichen planus, lichen planopilaris or lichen sclerosus, which are not associated with the symbiotic organisms found in natural environments but are dermatological conditions characterized by skin lesions. The use of “lichen” in these instances refers to the similarity in appearance to lichen in nature. This essay will focus on the characteristics of true lichen in and on the human body.

The human body represents a distinct and specialized habitat for lichen species due to the specific microclimate, availability of nutrients, and constant interaction with the human immune system. Here, lichens have not only adapted to survive but have also diversified into numerous forms, exhibiting various morphologies and reproductive strategies.

Of their morphologies, lichen on the human body can be divided into three main types. The first is the crustose form, which grows flush against the skin. This form is characterized by its encrusting thallus, or body, that adheres tightly to the substrate, making it almost inseparable from the human skin. The second type is the foliose form, which resembles leaf-like structures. They are somewhat flat, and unlike crustose lichens, they can be gently removed from the skin. Lastly, the fruticose form resembles miniature shrubs, displaying a branched or bushy structure. Fruticose lichens are relatively rare in the human ecosystem, likely due to the environment’s constant change and relative instability.

Reproductive strategies of lichens in and on the human body are primarily asexual, involving the production of specialized propagules like soredia and isidia. Soredia are tiny balls of algal cells surrounded by fungal hyphae, which can be dispersed by minor disturbances and form new lichens elsewhere in or on the body. Isidia are cylindrical outgrowths that can break off to initiate new colonies.

Living in and on the human body also poses unique challenges to lichens, including elevated temperatures, varying humidity, constant shedding of skin cells, and the dynamic host microbiome. Think dry sauna. Lichens have adapted to these conditions by developing specialized structures and biochemical compounds. Many lichens produce unique metabolites, collectively termed as lichen substances, which have antibacterial and antifungal properties, helping them to fend off potential competitors and to resist the host’s immune responses.

While the study of human-associated lichens is still in nascent stages, early evidence suggests a complex interaction with the host. They can potentially influence the skin’s microbiota and even participate in nutrient cycles, such as nitrogen fixation.

In conclusion, lichens that reside on the human body showcase a complex array of adaptations to a unique habitat. The understanding of their roles, interactions, and implications on human health remains an intriguing area of research, with potential applications in dermatology, microbiology, and pharmaceuticals.