Lichens

A lichen is a composite of two organisms, a fungus and an alga, living together with both providing benefits.  Algae can survive on their own in saltwater or freshwater habitats.  They can also survive in land-based habitat as part of a lichen.  Fungi cannot produce their own food and can only survive by depending upon another organism for their nutrition.  Many fungi are decomposers, depending on absorbed nutrients from dead or dying plants, but some form relationships with other organisms such as algae, which can photosynthesize food for both partners. 

Lichens grow in a variety of habitats.  Lichens  cannot retain water, but they can absorb it from a standing or flowing source, including water vapor from the atmosphere.  When a lichen is dry, it is in a dormant state, and when wet, it is in a growing state.  Poikilohydry is the process of freely alternating between these two states.  Lichen also require clean, fresh air.  They absorb many of the compounds present in the atmosphere.  Nutrients needed include carbon dioxide, oxygen, and nitrogen.  Nitrogen in the atmosphere is in an unusable form.  It must first be “fixed” – changed into a form that lichens can absorb and utilize.  This is accomplished by various types of algae with this capability, such as cyanobacteria.  Algae use light in the presence of chlorophyll for photosynthesis to create food to feed both organisms.  The spectrum of light as well as the quantity of light received determine the color of lichen.  Every lichen needs a substrate, or anchor point, that provides a surface that does not move for the time it takes the lichen to form on it.

Lichens come in three distinct forms, adding color, texture, and beauty to their environment.  Foliose lichen appears as a cluster of leaflike shapes, with two distinctly different sides.  Fruticose lichen are hairlike or cuplike, with branches coming from a central core.  Crustose lichen form a crusty surface, often present in bright colors.  There are 20,000 known species of lichen that cover 6%-8% of the Earth’s surface.  More than 200 species have been documented in DuPage County.  Their lifespan is difficult to determine, but an arctic species called map lichen has been carbon dated to 8,600 years old.  They have also been taken to outer space and resumed normal growth after being returned to Earth.

Lichen use photosynthesis to convert carbon dioxide into oxygen in every environment.  However, their greatest threat is air pollution.  They absorb most compounds found in the atmosphere including heavy metals, carbon, sulfur, and other pollutants.  Scientists are able to measure each compound in lichen and in the surrounding habitat.  Using this data, the U.S. Forest Service is able to determine actions to recommend for mitigating air pollution. 

Lichens are a major food source for many animals living in areas where food is scarce in winter, most notably caribou, flying squirrels, and red-backed voles.  Some lichen have antibiotic properties under investigation for use in the pharmaceutical industry.  Wolf lichen has been used by Native Americans and settlers alike for both tea and poisoned arrowheads.  Many varieties are used to make dyes for clothing and baskets.  Having lichen growing around you signifies clean air and water.  They are easy to find and study during winter months when many low lying surfaces are no longer hidden from view.  Take along a field guide on your next walk to look for and enjoy lichens in their many forms.

A beginner’s guide to lichens in our area can be downloaded from the Chicago Field Museum by clicking on: Common Lichens of Chicagoland

More information on how the U.S. Forest Service studies lichens and air pollution can be found by clicking on: National Lichens & Air Quality Database and Clearinghouse