Blue-gray rosette lichen and common sunburst lichen. Photo by R. Hodnett.Wikimedia Commons
(Click on the following link to listen to an audio version of this blog …. Lichens
Lichens have been described and discussed since the times of the ancient Greeks and Romans. Aristotle, Dioscorides, Theophrastus and Pliny all mentioned lichens in their writings and considered them to be an odd type of plant. Lichens were not of any great economic value or seemingly important at all, so they received very little attention or discussion.
Some of the earliest descriptions of lichens postulated that they were, in fact, merely excretions produced by rocks, or soil or trees, or that they were decomposition products oozing out of “higher” types of plants. The term “vegetable juice” was use to describe them. They were considered to have almost no complex structure at all.
Rocella tintoria, N. Nagel. Wikimedia Commons
A group of lichens that were valued for thousands of years, though, and which were described by Aristotle, are the rock lichens (Roccella tinctoria) from which a valuable purple dye was extracted. Other lichens of large or unusual conformations were also occasionally mentioned by observers and naturalists. But little detail about these species were developed. Some lichens that had shapes or colors that corresponded to the pseudo-scientific ideas called the “doctrine of similarities.” This doctrine contended that a plant that looked like a particular organ of the human body or in some way resembled physical aspects of some human disease would be of benefit to that organ or to curing that disease. So, a lichen that resembled a lung (Lobaria pulmanaria) was collected and used to treat lung diseases, and a lichen that has pustule-like eruptions all over it (Peltigera aphthosa) was used to treat thrush (a yeast infection of the mouth and/or throat). Similarly, a yellow lichen (Xanthoria parietina) was used to treat jaundice.
Starting in the 18th Century, lichens were almost universally considered to be some type of plant. They were variously classified as algae, as fungi (which then were considered to be a type of plant), as a liverwort or as a moss. Linnaeus in his 1751 work Philosophica Botanica mentions 86 lichen species. It was somewhere around this time, though, that Linnaeus disdainfully referred to lichens as the “poor trash” (“rustici pauperimi”) of vegetation. Over the next one hundred years, another 1000 lichen species would be described.
Calopaca marina. R. Griffiths, Wikimedia Commons
In 1869 the Swiss botanist, Simon Schwendover, proposed that lichens were not plants and that they were also not the singular organism that they had previously been considered to be. He referred to lichens as “dual organisms” made of a fungus with algae living within the fungus’ cytoplasm. He used extensive metaphors to explain the relationship of fungi and the algae in the lichen. The algae, according to Schwendover, were variously “slaves” or kept “damsels” or “parasites with the wisdom of a statesman.” The fungi were, again according to Schwendover, “masters” or “tyrants” that kept and enslaved the algae inside of their cellular masses.
Schwendover inspired few converts to his lichen hypotheses. He was widely ridiculed and dismissed. His language made it difficult to seriously consider or accept his ideas, but even more fundamentally, his rejection of the increasingly popular, and distinctly Darwinian concept of “life divergence” (living organisms separating from each other as they live and evolve (Origin of Species had been published just ten years earlier, in 1859)) and his advocacy of an opposing concept of “life convergence” (living organisms fusing together), was, philosophically too extreme for many to tolerate. There were those, though, that set aside these prejudices and preconceptions and began to look at lichens more closely.
Lichens on rocks at Dowdy Lake, Colorado. Photo by D. Sillman
In 1877, Albert Frank, a German botanist, coined the term “symbiosis” to describe the relationship between the fungus and the alga in a lichen. The idea of symbiosis was quickly expanded to cover a spectrum of interactions between species from parasitism to mutualism. The cooperative relationship of a lichen’s mutualistic symbionts (and other types of mutualistic pairs that were subsequently described) was a powerful counterpoint against some of the more extreme interpretations of Darwin’s Theory of Natural Selection. Huxley’s description of life as “a gladiator show where the strongest, the swiftest, and the cunningest live to fight another day,” and Spencer’s and Alfred, Lord Tennyson’s “nature red in tooth and claw” were tempered by the reality of “fitness” often meaning cooperation!
Lichen cross section. Photo by Nefronus. Wikimedia Commons
Lichens cover 8% of the Earth’s surface. This is a greater area than is currently covered by Tropical Rainforests. Lichens are the vital pioneering colonizers of newly exposed or emerging rock surfaces. The physical (via hyphal growth into micro-crevices in the rock) and chemical (via acid production) breakdown of the rock generate the mineral matrix of soil. Lichens are the pioneers in almost all terrestrial successional sequences.
Lichens are an important part of desert crusts (see Signs of Summer 7, June 30, 2022). They can survive for long periods of time in dehydrated states (“suspended animation”). They can survive, to a degree, in space (although, the heat of re-entry is very tough on them!). Lichens are the principal food of arctic reindeer and other arctic grazers. Lichens are chemical factories that can synthesize more than a thousand chemicals not found in any other life forms.
Lichens first clearly show up in the fossil record about 400 million years ago. Lichens, according to DNA analysis, have evolved independently 9 to 12 times! Fungi and algae will spontaneously fuse together to make either a lichen or a fairly undifferentiated fungal/algal mass. The functional assemblage of the lichen symbionts occurs if each participant brings some physiological or structural features to the composite organism (often referred to as a “holobiont”) that facilitates the survival of mutualistic pair.
Lichen symbiosis. Photo by M. Grimm, Frontiers
Recent research has explored the nucleic acid composition of lichens. In addition to the DNA of the fungal and of the algal mutualists, these studies have revealed DNA from a myriad of bacteria and yeasts (which are single-celled fungi). The picture of a lichen, then, is more complex that we had assumed. It is not just a mutualistic symbiosis between a fungus and an alga, it is an entire micro-ecosystem of organisms! As one researcher put it, “lichens don’t have a microbiome, they ARE a microbiome!
So, what is an individual? Lichens are obviously not individuals: they are mini-ecosystems! But what about people? In an “individual” human there are, according to the most recent estimates, about 30 trillion “human” cells. In addition to those human cells, though, there are also estimated to be 39 trillion bacteria living in the internal and external microbiomes of the body. Add to this numbers maybe 4 trillion fungi (1% of bacteria microbiome), and you have a complex, diversly populated system.
So what is an individual?
An interesting aside: in a paper recently published in Nature, researchers state that a major factor causing a person to experience sustained SARS-CoV2 infections (i.e. “long COVID) is an imbalance in the fungal microbiome of the intestines. We are a multitude!