Steven E. F. Brown

While hiking at Fort Funston on San Francisco’s windswept Pacific coastline yesterday, I noticed what seemed to be a fuzzy orange lichen growing all over the trunks and branches of the gale-bent cypress trees atop the cliffs. From a distance, the vivid orange seemed to be consuming the trees like fire.

It turns out this furry orange carpet on the trees isn’t a lichen, strictly speaking — it’s Trentepholia, a free-living alga. Though Trentepholia is a genus of Green Algae, it contains a strong carotenoid pigment that is bright orange and masks the chlorophyll color of the plant.

Although Trentepholia isn’t a lichen, the alga is an important part of some lichens. That’s because lichen, one of the queerest of Nature’s life forms, is a “mini-ecosystem” made up of at least two organisms and sometimes more. Typically, a lichen consists of a fungus, which can’t photosynthesize, living together with a partner that can photosynthesize. Sometimes the photosynthetic partner is a cyanobacterium, but often it is a type of green alga like Trentepholia.

The symbiotic partners don’t just live next to each other, either — they fuse and grow together to form a unique lichen body called a thallus.

To keep things simple (and nothing about lichen is simple!), lichens are named, scientifically, by their mycobiont, or fungal halves. That’s because each lichen fungus is unique, while more than one type of lichen uses, say Trentepholia, or Collema, or Leptogium algae in its partnership. The same species of lichen, growing in different places, may have a different algal partner.

But enough about lichen — this stuff is algae!

A strange patch of orange in the wind-bent cypress trees caught my eye.

Though it looks at first glance like lichen, this is actually a species of Trentepholia, a free-living green alga that sometimes partners with fungi to form lichens.

Trentepholia grows on tree bark where there’s plenty of moisture in the air, such as along California’s coast.

A cypress cone with Trentepholia growth.