Lichens on My Mailbox
A lichen consists of 2 different plants, a fungus and an alga (plural is algae) so closely associated that they appear as one organism. Because the fungus totally envelopes the algal cells, the appearance of the lichen is largely that of the fungus except for the green color which is derived from the chlorophyll pigment present within the algal cells. There are some 15,000 species of lichens and 400 genera.
Is the association parasitic or symbiotic? This is a fairly difficult topic of study. The strongest argument for the former is the presence of fungal extensions that penetrate the algal cells. Counter arguments recognize that the fungus can absorb and retain water for both species while the alga synthesizes carbohydrates via photosynthesis that benefits both. Note that interactions may vary with species and, or environmental conditions.
The arrangement is quite successful; lichens are cosmopolitan, found from Arctic tundra to equatorial forest and able to persist in environments too harsh for plants of any other kind. Many species grow without soil in full sun attached to rocks, bark, or cliffs and a few species are endolithic- that is growing within limestone rocks in their vegetative state!
Lichens also are able to colonize human structures made of stone, wood or metal. Take for instance my mail box which has been situated exactly in the same location for over ten years. Comparing the north-facing side of the box (actually 28 degrees east of north) with the south-west facing side reveals a striking difference- there are many lichens on the north side and none on the south! The main reason appears to be attributable to differences in sun exposure. Overhead branches appear by eye to be sparsely and evenly distributed on both sides of the mail box so I assume there are not differences in nutrient supply from rainwater drip or “through-fall” and from stemflow which leaches or dissolves small mineral quantities from tree bark. Note that, remarkedly, some lichens have the ability to thrive with access only to nutrients and water available from the atmosphere-e.g. on rocks found in full sunlight all day without trees or other vegetation present as sources of supplemental nutrients. One last thing- lichens are extraordinarily sensitive to air pollutants such as heavy metals, sulphur dioxide and ozone. If the mail box spent 10 years in a polluted environment, the two sides of the box would look identical-just like the south-facing side.