Air Pollution Assay Lab
Lichen is a symbiotic association (mutualistic) between a fungus and an algae or
cyanobacterium. The body of the lichen (thallus) is a combination of both
organisms. Thus, one would not be able to survive without the other. The fungalalgal partnership is capable of survival in environments that most other organisms
would find intolerable. Lichens occur from the Arctic to the Antarctic. In this
mutualism, the fungus is supplied with carbohydrates, nitrogen compounds, and
vitamins by the algae. The algae, in turn, is protected from high intensity light,
provided with water, minerals, and other organic nutrient and provided with water,
as well as given a place to live! The fungus is the dominant partner, so lichens are
characterized as a fungus.
Lab Objective: To determine how heavy metal air pollution effects lichen species
Crustose Lichens
Crustose lichens, as their name implies,
form a crust on the surface of the substrate on
which they are growing. This crust can be
quite thick and granular or actually embedded
within the substrate. In this latter case the
fruiting bodies still rise above the surface. In
many crustose lichens the surface of the
thallus breaks up into a cellular, crazy-paving
like pattern. Crustose lichens tend to grow out
from their edges and have their fruiting bodies in their centre. Crustose lichens are very
difficult to remove from their substrates.
Foliose Lichens
These have an upper and lower cortex.
They are generally raised to some extent
above the substrate but connected to it by
rhizines (specialised root-like hyphae).
They are easier to remove from their
substrate when collecting because of this.
Fruticose Lichens
Fruticose lichens are shrubby lichens. They
are attached to their substrate by a single
point and rise, or more usually, dangle from
this. Some foliose lichens can be stubby like
fruticose lichens, however, close examination
will reveal that the algal part exists only on
one side of the flattish thallus whereas in
fruticose lichens it exists as a ring around the
thallus, even when it is flattened as in
Ramalina sp.
Background Information
Lichen is extremely slow growing. They may only grow 1 mm to 1 cm per year, but
can live for thousands of years. If large numbers of lichens are found in an area, it
can be assumed that they have been undisturbed for a long time.
Lichen can withstand dramatic swings in temperature, drought, mineral shortages,
extremes in light, and other hostile habitats. Lichen go into a state of suspended
animation during dry times, essentially coming to life when the rains begin again.
The one thing they must have is clean air- they cannot tolerate polluted air of cities
and industrial areas.
Lichens are not rooted like plants; they receive nutrients and water mainly from the
atmosphere. They absorb the nutrients very efficiently, so they can accumulate large
amounts of toxins. There is not an outer layer to protect them; the organisms are
completely exposed 24 hours a day. If too many heavy metals (cadmium, copper,
lead, manganese, nickel, zinc, mercury, ) are absorbed, the lichen will die. These
metals are readily available from industry and vehicle exhaust. Simply, ecologists
look for lichen. The more delicate and abundant the lichen, the better the air quality
in the area. The less and more hardy the lichen found, the poorer the air quality.
Lichen fills a specific niche in nature. When attached to rocks, the lichen makes its
own soil by extracting nutrients. They provide food, shelter, and light for a number
of organisms. Lichen absorbs atmospheric nitrogen and makes it available to trees
and plants nearby. The forest uses nitrogen to create biomass. This means that
lichen are “forest fertilizers.” The industrial uses range from perfumes and
antibiotics to fixatives and medicines.
Materials
Lichen samples
test tubes
methylene blue solution
lithium chloride solution
manganese chloride solution
sodium chloride solution
blue colored glass cleaner
graduated cylinders
glass beakers
distilled water
test tube racks
Procedure
Sketching
Take your lichen samples and sketch each one carefully. Note their shape, size,
color, and morphology (physical make up)
Preparing Lichen Disks
1. Use a punch and punch out disks of lichen for your experiment.
2. Drop disks into methylene blue solution. Make sure they are completely
covered. Leave the disks for 30 minutes
3. Remove disks by straining through filter paper.
4. Rinse with distilled water.
Leaching of Methylene Blue
1. Choose one of the metal solutions, or distilled water.
2. Measure out 10 ml. of the solution
3. Add the four disks to the solution and allow them to soak for 15-20 minutes.
Gently shake the cylinders every 5 to 10 minutes.
4. Observe color of the solutions during the 30 minute leaching process. Record all
observations.
5. At the end of the time, shake the tubes well and compare the intensity of the blue
color of each solution to the distilled water (negative control) and the blue glass
cleaner (the positive control). Score the glass cleaner as ++++ and the distilled water
as -.
6. Evaluate the color intensity of each metal solution and assign them values
ranging from – to ++++
7. Record results in data table.
Solution
Lithium Chloride
Manganese Chloride
Sodium Chloride
Blue Glass cleaner
Distilled Water
Color Rating
*Explanation
What happened? It is known that fungal cell walls strongly bind metal cations
(positively charged atoms). Lichens trap metals by an ion (charged atom) exchange in
which sites on the cell wall release hydrogen ions in favor of metal cations.
Because lichens are the same shape (morphology) during the entire year, they are
constantly exposed to chemical changes in the air. In this exercise, the lichen binding
sites (where nutrient uptake happens) are saturated with the methylene blue molecules,
which have a positive charge but are more weakly held by the binding sites than the
cations are. The more blue that is leached from the lichen, the more metal cations have
attached themselves to the binding sites.
Conclusion Questions (this will take the place of your conclusion paragraphs):
1. Which metal solution seemed to have the greatest uptake into the lichen?
Using specific examples, list the metal solutions that had the greatest to least
effect on the lichen.
2. What was the result of our control group? What does this tell us about the
correlation between metal cation and hydrogen exchange at the binding
sites?
3. Because lichen are able to uptake nitrogen from the air and release it in
usable form to trees, what is the danger of binding sites being taken up by
metal cations?
4. Lichen is extremely tolerant to many atmospheric conditions, with one
exception. What is that exception?
5. Taking this exception into account, where might the most dangerous metal
exposures come from? (ex. Rain? Runoff? Industrial waste?)
6. Lichen grows very slowly. How can uptake of metals at binding sites slow
this growth even more?
7. We have looked at different types of lichen. Which types are most tolerant?
Which are least tolerant? Looking at their morphology (shape, size, etc.)
hypothesize why this may be so.
8. Why are lichen bioindicators? (Organisms that determine ecosystem health)
9. If we experimented with more sensitive lichen, would you expect the results
to be different? Explain and provide an example.