Whether Lichens Conceal Or Reveal Depends On What You’re Looking For
by Ron Tschida
Broadcast 9.15.2015 and 9.8 & 9.11.2020

Photo by Lotus Johnson, CC 2.0.

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Hiking along a ridgetop above the wind-rippled surface of Flathead Lake, I approached a rock outcrop.  From a distance of a couple hundred yards, the rock looked barren, if a bit weathered. But on closer inspection, I found the weathering to be actually a lichen covering the surface of this rock. This lichen was thin, not much thicker than a few notebook pages, and it was tightly attached to the rock surface. A mottled grayish-green color, it felt nubbly, like running your hand over crushed velour. Because of the lichen, I couldn’t easily determine the rock type or what its color or texture was. I’m just an amateur, but even professional geologists can be frustrated by the growth of lichens because they obscure so much valuable geologic information. But in fact, lichens, while masking the surface, can actually reveal what lies beneath that surface within the solid rock.

Lichens are a combination of fungi and algae living together in a symbiotic relationship. In this symbiosis, the fungi and algae benefit from each other. This evolutionary adaptation allows lichens to grow in some incredibly harsh environments: in deserts and in the Arctic, on barren tundra and on bare rock. Lichens grow in such rugged environments that some early naturalists thought they existed on nothing but air and sunlight.

Actually, the algae provide carbohydrates through photosynthesis, and the fungi provide a structure and also nutrients from the substrate, or surface, on which the lichen lives. Botanists studying lichens have found that they secrete acids that break down the chemicals in rock, and draw minerals such as iron and copper into the lichen. In one study, lichens growing on rocks containing an unusual amount of copper likewise showed an unusual concentration of that element, up to 10 times normal. It’s this ability of the fungi that’s important to geologists. The minerals that make up different rocks help determine which lichen species live there. Those minerals also color the lichen. Lichens growing on iron-rich rocks take on a rusty color, while in the presence of copper-rich rocks, lichens are often distinctively dark-green or bluish-green.

Lichens live a long time and the thin, crusty lichens that grow on rocks live particularly long. Some colonies are estimated to be more than 4,000 years old. In all that time, lichens are able to accumulate larger concentrations of minerals than shorter-lived plants. Using this knowledge, geologists can prospect for mineral-rich rock by mapping the lichen growths on the rock surface. The ability of this seemingly simple organism to accumulate minerals over long periods of time provides an opportunity for botanists and geologists to benefit from each other’s work. Sort of a symbiosis, you might say.

 


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