Saturday, February 26, 2011

Opposites Attract, or What’s Up with Lichens

What is it about lichens that’s so fascinating?? I caught “the bug” many years ago, but I guess unless you’re exposed, your immunity remains intact. Once you start noticing them, though, they can be hard to resist.

First, there’s the startling array of shapes and colors. That trait they have in common with other fungi so maybe it’s not lichens’ most unusual feature (those are fungi, above, by the way). Still, for everything from barely perceptible crusts to wacky forms worthy of a sci-fi movie, lichens are hard to beat.

Second, they’re pretty much everywhere. Once you start spotting them, you can find them in deserts or high mountains and everywhere in between, growing on rocks, old stumps, tree bark or twigs, mosses, soil, or substrates provided by humans. Some even live underwater.

And everywhen. Fossil lichens have now been reported as far back as 600 million years ago (in China, 2005), and it’s been proposed (though not widely accepted) that the famous Ediacaran biota of Australia may have represented fossilized lichens (Retallack, 1994; refuted by Waggoner, 1995). They’ve even been given credit for, very early on, building the oxygen atmosphere and making the planet fit for habitation.

Okay maybe that’s a stretch… and probably we should call them "lichen-like symbioses" rather than the lichens we know today. There is, however, an "unequivocal report" of lichen fossils from 400 million years ago, and that's not half bad.

Lichens are tough. Studying them in the harsh environment of the Arizona desert, where they’re lucky to get a few minutes to photosynthesize using dew at dawn before they dry out for another day, one develops a certain respect for these crusty critters. Imagine the temperature on a dark volcanic rock at midday of an Arizona summer... you'd be crustose too.

Lichens survive in the even harsher Namib and Negev deserts, where dew and fog are the only sources of water. We used to speculate that, if there was life on Mars, it was most likely in the form of lichens.

That latter hypothesis has since gotten a boost from actual research. As reported in New Scientist, 2005:

In an experiment led by Leopoldo Sancho from the Complutense University of Madrid, two species of lichen—Rhizocarpon geographicum and Xanthoria elegans—were sealed in a capsule and launched on a Russian Soyuz rocket on 31 May 2005.

Once in Earth orbit, the lid of the container opened and the samples were exposed to the space environment for nearly 15 days before the lid resealed and the capsule returned to Earth.

The lichens were subjected to the vacuum of space and to temperatures ranging from—20°C on the night side of the Earth, to 20°C on the sunlit side.

We just knew it!! When the lichens came home from their jaunt in space, and were returned to reasonable conditions, they came back to life and actively metabolized, just as if they hadn't been subjected to intense solar radiation and a huge range of temperatures, not to mention absolute vacuum. They have remarkable recuperative powers.

By the way, that temperature has got to be wrong; space is no where near that warm!! According to, "In Earth orbit, the temperature of objects in sunlight can rise to 120°C/250°F. The actual temperature in space is about 3°K (-270°C or three degrees Celsius above Absolute Zero)."

That's literally cool, but what about the attraction?

Maybe the neatest thing about these incredible organisms is that they're not one organism at all, but a combination. The lichen symbiosis is not unique, but has happened many times, and with many different partners. Now that fungi, and therefore lichens, are no longer considered plants but their own entire kingdom as are algae, the lichen combination unites members of two extremely distinct groups of organisms. (Ever think it's tough to get along with mates of our own species?? Consider the lichen.) The photobiont consists of algal or cyanobacterial cells immersed in the hyphal strands of the mycobiont, or fungal partner. Here's a great orange lichen; note the green layer of algal cells in the lower right of the photo, where the thallus has been cut.

Above: Xanthomendoza mendozae close-up. Photo by Chris Wagner, U.S. Forest Service. By the way, the Forest Service has a well-written section on lichens; worth checking out for more on the basics!

In some lichen species, the symbiosis gets really interesting. They may have a primary union with, say, Trebouxia, the most common green alga in lichens, but they also have a little cyanobacterial thing going on. Take this Peltigera for example. When wet, its bright green color tells us that most of the thallus contains green algal cells (Chlorophytes, eukaryotes). But see those darker bumps on the surface? Called cephalodia, those harbor cyanobacteria (Protists, kingdom Monera), adding a third kingdom to this particular symbiotic union.

In our neck of the woods (Pelts are creatures of the forest-floor), most Peltigera species are grayish or brown and never show green, even when wet. Those species, including the abundant Peltigera canina, have remained true to their original cyanobacterial commitment.

Back with more on these wonderful beings soon!

1 comment:

Mary said...

What a terrific overview of lichens!

I'm looking forward to your next post on them, which will no doubt further increase my growing likin' for lichens (ouch!).