That crusty patch of plant-like material growing on a rock behind your house is called a
lichen. But it is also so much more. Lichens are composite organisms formed by the association of a fungus (the
mycobiont) and an alga and/or cyanobacterium (the
photobiont). Together, they form an organism with physiological properties quite unlike that of either of the equivalent free-living constituents. For one, lichens are surprisingly tolerant to a range of environmental parameters, allowing them to live in some of the most extreme conditions on the planet. And yet, they serve as “canaries in a coal mine” when it comes to pollution, due to their sensitivity to many environmental contaminants. The unique symbiotic relationship exhibited in lichens reveals itself in a variety of shapes and sizes, including hair-like (filamentous), crust-like (crustose), leaf-like (foliose), and powder-like (leprose), among others. The photobiont provides sugars produced by the reduction of carbon dioxide using sunlight, while the mycobiont provides nutrients and a hydrous environment for the bacteria or algae. Apart, many of the companion organisms cannot survive. Food for thought: How does one classify a symbiotic organism made up of a fungus and an alga and/or bacterium?
Thanks for highlighting this group of interesting "organisms"!
ReplyDeleteYour title reminds of a riddle I first heard in a botany class long ago: "What happened when Fred the Fungus met Alice the Alga? They took a 'lichen' to each other. But now their marriage is on the rocks." (Gotta love biology humor.)
Readers in the Rhode Island area might want to check out Don Flenniken's "Rhode Island’s Foliose and Fruticose Lichens with Emphasis on the Nature Conservancy Preserves and Related Lands" available as a PDF from the Rhode Island Natural History Survey at http://www.rinhs.org/webpubs/lichens/index.html. (I had a problem loading the PDF recently, but I hope it will be fixed soon.)
I also wanted to point out two books on lichens that you might enjoy. The first, a short volume by William Purvis, is part of the "Smithsonian's Natural World Series" and is called simply "Lichens". The second is weightier tome by
by Brodo, Sharnoff and Sharnoff entitled "Lichens of North America".
I noticed you called the algal partner in the relationship the "photobiont". As a phycologist (biologist who studies algae), I wanted to point out a synonym: "phycobiont".
There have been a number of interesting experiments where biologists have been able to separate the algal and fungal partners of a lichen and grow them independently in pure cultures. The isolated partners typically look nothing like the composite "organism", but the complete lichen can typically be recreated ("resynthesized") if the pure cultures are reintroduced.
Since the partners of a lichen may be grown independently, it is interesting to contemplate the nature of the symbiotic interaction between them: is it mutualism, commensalism, or parasitism. In some cases, the answer seems a bit unclear.
Thanks for these insights and references (and humor)! It's always great to hear perspectives from the scientists who study this topic themselves!
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