Nature

The lichen’s general structure is composed of layers of fungus threads and green and/or blue-green cyanobacteria alga that together form the interior of the thallus. The thallus is so integrated into the fungus/alga mix that only recently has it it been studied as a single organism in the lichen. The thallus lies against the substrate and has no clearly-defined stems or leaves.

Lichens grow slowly, live a long time, are stress tolerant, and come in many colors and shapes (some even resemble minute forests). The outer layer inside the thallus is the cortex and provides a little protection and color, in some species. The algal layer usually tells us, by color, the kind of alga a lichen has. When dry, it is usually gray or matches the cortex color but, when wet, those cells become transparent and the algal cells underneath show their color. Cyanobacteria can be a layer under the upper cortex or in small pockets on top of the cortex. If there is a green algal layer already present, it will give the lichen a dark green, brown, or black color. In some lichens however, there are no layers of fungus and alga. The individual components are mixed together in one big uniform layer and the resulting growth form is gelatinous. These types are called jelly lichens—definitely qualifying as a unique shape.

How do lichens attach themselves to a substrate? There are a couple of ways. Rhizomes are fungal filaments that extend out and hold the lichen down to whatever it is sitting on—the substrate. The other way is the “holdfast,” an extension of the lichen thallus. Some lichens have a central peg or holdfast that attaches to the substrate—typically a rock—kind of like an umbilical cord.

About one billion people globally eat lichens as part of their diet. They are used to prepare indigenous foods, beverages, spices and animal feed in various cultures around the world.
Since the fungus can protect its algae, the normally water-requiring lichens can live in dry, sunny climates without dying as long as there are occasional rain showers or flooding to let them recharge and store food for the next drought period. Further, lichens provide a means to convert carbon dioxide in the atmosphere through photosynthesis into oxygen, which we all need to survive.

In recent years, lichens have become more frequent subjects for scientific research. Their unique symbiotic association between alga/cyanobacterium and a fungus produce secondary metabolites that offer promising drug leads. To quote an article in an August 2022 Journal of Ethnopharmacology entitled “Lichens: An Update on their Ethnopharmacological Uses and Potential as Sources of Drug Leads”: “Natural products are low molecular weight molecules produced by living organisms (plants, animals, and microbes) and have been traditional leads for medicines for ages. They have a historic significance as important novel compounds which are useful as drugs, models for synthetic/semisynthetic structure modifications and optimization, biochemical and pharmacological probes…”. “Natural product molecules and/or their synthetic modifications, have been particularly useful for chemotherapy of cancer and malaria.” “…introduction of new therapeutic agents has not kept pace with the increase in the evolution of multi-drug resistant microbial strains, making many infections untreatable. There is therefore a dire need for novel drugs.”

So that ho-hum lichen we routinely walk by without noticing has something pretty special about it that is well-worth our attention and appreciation.

Photos by Jim Burkstrand