The secret life of patio chairs

Sandstone rock outcrops are a striking feature of the Laguna Canyon

landscape. Besides the distinctive wind-worn pits, the yellow

sandstone often sports a grayish surface discoloration. It’s easily

overlooked, but it’s more interesting than you might think.

In my garden is a set of ancient white plastic furniture, the

surface long since turned rough and increasingly dark grey. The color

doesn’t rinse off, and when rubbed, it leaves a powdery substance on

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

sandstone surfaces.

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

nutrients.

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

garden furniture.