fingers. I dismissed it as deposited airborne grime.
Then some months ago one chair started getting lighter. On closer
examination, I found this was due to many bright white wiggly tracks
on the surface. Some creature was scraping off the dark gray surface.
It looked for all the world as if a snail were grazing on the chair.
But why would a snail eat plastic?
Garden snails are herbivores, grazing on plant material with a
radula, a muscular tongue covered with hundreds of tiny sharp teeth,
like a flexible rasp.
In tide pools, salt water snails cruise around feeding on mats of
algae. You have probably seen freshwater snails in fish tanks,
scraping off the coating of algae that accumulates on the glass.
If snails are eating my garden chairs, there must be something
edible in that rough grey surface. It's got to be small, and it's
most likely algae.
Where would algae come from? From the air. Resistant cells of a
green alga named pleurococcus are common airborne particles. They
fall as part of the constant rain of microscopic particles, along
with things like tire rubber, dust, pollen and mites.
If the surface where the cells land has the right combination of
light, moisture and temperature, the algae can grow.
Pleurococcus is responsible for the thin green covering, often
mistaken for moss, on the north sides of tree trunks and branches.
Algae can also grow on porous rocks like sandstone and limestone.
In Laguna Canyon they contribute to that grey coating on exposed
My brick patio in the canyon turns green and slippery every
winter. In moist climates, the algal coating is a year-round problem
on sandstone sculptures and building blocks.
But how can these tiny plants survive on bare rock exposed to the
air? Pleurococcus extracts its most important nutrients -- nitrogen
and carbon -- directly from the atmosphere. Water contributes
hydrogen and oxygen.
Airborne dust and particulates from smog are the source of other
The famous limestone caves in southern France (Lascaux among them)
were closed to visitors when a green algal coating threatened to
cover the graceful prehistoric wall paintings.
The art had survived for upwards of 30,000 years underground in
cold, dry conditions, but crowds of visitors brought moisture, light
and carbon dioxide in large enough quantities to enable algae to grow
on the cave walls.
Even in our semi-arid climate, coastal fogs enable pleurococcus to
grow on rocks and sidewalks as well as trunks and branches of coastal
sage shrubs and trees.
Next time you're out, look deep into a tangle of old woody
branches for a tell-tale green coating. Or just check out the nearest