By Dr. Diana Bizecki Robson
Scientists have spent many decades arguing with each other about lichens. It’s a plant! No, it’s a fungus! No, it’s a fungus parasitizing an alga. No, it’s an alga parasitizing a fungus! Nobody’s parasitizing anybody; they’re living in harmony like a bunch of hippies in a commune! For a long time, no one understood what lichens truly were, and honestly I’m still not sure we do.
Above: Reindeer lichen (Cladonia mitis) can reproduce themselves sexually, asexually, and vegetatively when stepped on by the reindeer and caribou that eat them. © The Manitoba Museum
Back in the days before microscopes, the only organisms that people could see were creatures that moved (animals) and creatures that didn’t move (plants). Linneaus (1753) formalized this concept by creating a two-kingdom classification system when he published his Species Plantarum. Since lichens could not move, they were considered plants, albeit unusual ones as they were not always green.
Eventually, scientists began to realize that the natural world was much more complex than they could possibly have imagined. After microscopes were invented in the 17th century, scientists realized that there were lots of small creatures (microorganisms), that we can’t see with the naked eye. In 1866, a third kingdom, the Protista, which included these tiny, one-celled organisms, was proposed by Ernst Haeckel. However, it wasn’t until 1969 that Robert Whittaker’s five kingdom classification considered fungi to be separate from plants due to their absorptive mode of nutrition. After that paper was published, lichens were transferred from the Kingdom Plantae to the new Kingdom Fungi.
Above: A diverse community of lichens including pixie-cup (Cladonia) and dog lichen (Peltigera), can be found in Whiteshell Provincial Park, growing on rock outcrops. © The Manitoba Museum
In actuality, lichens cannot really be classified as anything because they are composite organisms containing species from as many as four different kingdoms! The name of the lichen is based on the scientific name for the main fungal host, referred to as the “mycobiont”. But, lichens also contain photosynthetic organisms called “photobionts”. The photobiont is usually a green alga (Kingdom Plantae) but sometimes a golden or brown alga (Kingdom Chromista), and/or cyanobacterium (Kingdom Bacteria). Some lichens, like the dog lichens (Peltigera spp.) have both green algae AND cyanobacteria in them. Functionally, the algae, and cyanobacteria if present, photosynthesize, producing sugar, which they share with the fungus. The fungus absorbs water (from rain and air) and dissolved minerals directly from the environment, and shares it with the other species. Cyanobacteria, if present, also take in nitrogen gas (N2) from the air, and turn it into a usable form of nitrogen, like NH3, through a process called nitrogen-fixation. Having multiple nutritional modes means that lichens can survive in barren environments like bare rock or sand, where none of the lichen partners could grow if they were alone.
Above: The Sand-loving Iceland Lichen (Cetraria arenaria) is one of the first organisms to colonize nutrient-poor bare sand. © The Manitoba Museum.
Recently, scientists also discovered cells of yeasts, bacteria, protists and even viruses inside some lichens thanks to DNA sequencing studies. The exact role that these species play in the symbiosis, or if they even have one, is not yet known (Hawksworth and Grubb, 2020). As a result, lichens have been most recently defined as “a self‐sustaining ecosystem formed by the interaction of an exhabitant fungus and an extracellular arrangement of one or more photosynthetic partners and an indeterminate number of other microscopic organisms” (Hawksworth and Grubb, 2020). Essentially, a lichen is an ecosystem, not an individual.
Lichen reproduction is quite diverse. The mycobiont in the lichen can reproduce sexually by producing “fruiting” bodies of various kinds (e.g. pycnidia, asci, apothecia etc.). These structures release small, single-celled spores which germinate into young fungi. However, to become lichens again, these tiny fungi must find a free-living photobiont in the environment to join with. A less risky strategy, which allows the lichen partners to disperse together, is to produce asexual propagules of various kinds (e.g. soredia, isidia, pycnidia etc.), usually at the lichen tips. These tiny clusters of fungal, algal and bacterial cells, once dispersed, often by wind, will immediately grow into new lichens. Lichens can also reproduce vegetatively. This often happens when animals, like caribou, step on dry lichens, breaking them into tiny pieces which can then disperse in the wind.
New research shows that humans (and other animals) are similar to lichens in that we are not alone. Our bodies contain trillions of microorganisms, like bacteria and yeast, that help us digest our food, support our immune system and even affect our brain development (Gilbert et al. 2012). Scientists estimate that of the 70 trillion cells in our bodies, less than half are human (Abbott, 2016). Without our symbiotes, we could not survive. So perhaps lichens are not so unusual after all; they were just the first organisms that showed us that co-operation between species is as viable a strategy for survival as competition is. As Gilbert, Sapp and Tauber (2012) note, “There have never been individuals. We are all lichens”.
Dr. Bizecki Robson has been the Curator of Botany at the Manitoba Museum for over 17 years. Though much of her research focuses on flowering plants and pollinators, she remains endlessly fascinated with fungi and lichens, and collects them whenever she can.
Abbott, A. 2016. Scientists bust myth that our bodies have more bacteria than human cells. Nature doi:10.1038/nature.2016.19136
Gilbert, S.F., J. Sapp, A.I. Tauber. 2012. A symbiotic view of life: we have never been individuals. The Quarterly Rev. Biol. 87: 325-341.
Haeckel, E. 1886. Generelle morphologie der organismen. Reimer, Berlin.
Hawksworth, D.L. and M. Grube. 2020. Lichens redefined as complex ecosystems. New Phytol. 227: 1281-1283.
Linnaeus, C. 1753. Species Plantarum. Stockholm: Impensis Laurentii Salvii.
Sheldrake, M. 2020. Entangled Life. Random House, New York.
Whittaker, R.H. 1969. New concepts of kingdoms of organisms. Science 163: 150-160.