The Christmas Lichen & its Mysterious Kin

This unassuming splotch of color is a lichen, specifically Cryptothecia rubrocincta. Although it may not look like much, lichens are beautiful and diverse creatures, with hidden complexity.

Due to it’s red and green coloration, this particular lichen is often called the Christmas lichen, or Christmas Wreath lichen. It was one of the first lichens I found (while on a vacation in Florida; they are native to the tropical americas) as a child that piped my interest in lichens and by extent mycology. This springboarded me into a fascinating world where new discoveries are made all the time.

Lichens overall are a combination of a fungus and a photosynthetic organism. The photosynthetic organism is called a photobiont and is usually (around 85% of the time) a green algae. For around 10% of lichen species cyanobacteria photosynthesize for the lichen and in the remaining 5% the fungus is partnered with both green algae and cyanobacteria. Fairly recently a third player, a yeast, was discovered in some lichens and it is suspected it may be in many more. There is also evidence that there might be a non-photosynthetic bacteria might be contained in many lichens. These complex organismal interactions bring distinct entities into a single being.

The fungus, called a mycobiont, is almost always in the Ascomycota phylum, provides structure and housing for the photobiont. The photobiont provides energy to the organism through photosynthesis. The yeast (fungi in the phylum Basidiomycota) is still shrouded in mystery with regards to its function. Different experiments have concluded that they may help defend against pathogens, provide structure, or are a parasite of the lichen.

The species interactions can also be complex with other microorganisms. In C. rubrocincta, the yeast species Fellomyces mexicanus has discovered growing epiphytically. This means it was using the lichen as a structure to live on, but probably not directly affecting the lichens biology. There are also around 1500 species of fungi that are parasites of lichen.

Because of this unique biology, lichens are able to live almost anywhere. They can be found across the world and in many different kinds of environments, including very harsh ones. Lichens thrive across all continents and can grow on a variety of inhospitable surfaces. C. rubrocincta is often found on tree bark, but can be found on rocks or leaves. Unlike typical fungi that would be found on a tree, lichens do not parasitize the tree, but instead only use it as a substrate. Trees are a common substrate, but many lichens also favor bare rock, or sometimes exposed soil. Lichens often have a hard time competing with plants, but are great at growing places plants cannot: the arctic tundra, deserts, high elevations, bare rock and of course on other (larger) plants.

Because the environments they live in are sparse in resources, most of the nutrients and water is acquired from the atmosphere in the form of rain or dust. Cyanobacteria can also fix nitrogen into the organism when they are present.

The relationship between the mycobiont and photobiont seems mutualistic at first glance, as the structure provided by the fungi allows the photobiont to live in a wider variety of places, but some describe the relationship as parasitic. The mycobiont extracts more nutrients and energy from its partner than it returns. It also regularly stresses the photobiont enough to kill cells (at roughly the same rate it reproduces to maintain the population). There is still debate in the scientific community about what interaction is a more accurate description.

The bright color that attracted me to C. rubrocincta is pigmented by chiodectonic acid, a compound that helps the lichen tolerate the barren conditions it lives in. In Brazil this pigmentation has been used to create a vibrant red dye. Other uses for lichens include dating rocks (the organisms grow at a constant rate and some very slowly allowing date estimates in the thousands of years based on the size of a lichen), food, and medicine (both in traditional remedies and modern research).

C. rubrocincta is a crustose lichen in the Arthoniaceae family of fungi. The crustose describes its shape, forming a crust-like structure over the substrate. Other lichens can have vastly different shapes. Some grow in gelatinous blobs, others in wispy hair like strands, and others yet in three dimensional branching structures. The family is important to keep track of as there is a wide range of fungal partners in lichens. There are 5 main orders of Ascomycota that are lichenized and account for the mycobiont in 98% of lichens. Three of these orders also contain un-lichenized fungi.

The world of lichens is a fascinating look into how complex and otherworldly biology can be. We are constantly learning more about lichens and there is still so much we don’t know, despite them being around us constantly. I’m excited for the future frontier as more about the cryptic organisms is discovered.

References

“Cryptothecia Rubrocincta — Alchetron, the Free Social Encyclopedia.” Alchetron.com, 19 Feb. 2018, alchetron.com/Cryptothecia-rubrocincta.

Robinson, Keith. “Yeast Emerges as Hidden Third Partner in Lichen Symbiosis.” Purdue University News, www.purdue.edu/newsroom/releases/2016/Q3/yeast-emerges-as-hidden-third-partner-in-lichen-symbiosis.html.

Salt, Alun, and Alun Salthttp. “Plectocarpon Lichenum: the Parasite That Makes Itself Comfortable When It Moves into a Lichen “ Botany One.” Botany One, 15 Jan. 2018, www.botany.one/2018/01/plectocarpon-lichenum-parasite-makes-comfortable-moves-lichen/.

Tuovinen, Veera, et al. “Two Basidiomycete Fungi in the Cortex of Wolf Lichens.” Current Biology, Cell Press, 17 Jan. 2019, www.sciencedirect.com/science/article/pii/S0960982218316543.

Volk, Tom. “Cryptothecia Rubrocincta, the Christmas Lichen.” Cryptothecia Rubrocincta, the Christmas Lichen — Tom Volk’s Fungus of the Month for December 2006, botit.botany.wisc.edu/toms_fungi/dec2006.html.

“What Is a Lichen?” What Is a Lichen? | The British Lichen Society, www.britishlichensociety.org.uk/about-lichens/what-is-a-lichen.

This story was first written for a Cornell Mycology course.