The invariable mark of wisdom is to see the miraculous in the common” (Ralph Waldo Emerson)
If you have ever noticed a common, rust-colored fuzz growing on damp walls or old headstones, you might be surprised to learn what it actually is. Believe it or not, it is a land-dwelling green alga called Trentepohlia aurea (tren-te-poh-lee-a). This particular photo was taken by the drainage canal near the Viewing Point in Kilmore Quay.
Even though it looks bright orange or yellow, it is legally a green alga. The green color comes from its cells, which use chloroplasts to turn sunlight into energy (photosynthesis). However, that green is completely hidden by a bright orange pigment called carotene. If you look closely at these organisms—especially under a microscope—they are incredibly beautiful.
There are at least 40 different species of Trentepohlia around the world. While most prefer warm, tropical climates, two species are native to Ireland: Trentepholia aurea and Trentepholia iolithus (most famous in Kerry and Galway for its dramatic, deep-blood-red or purple-brown colour).
Green algae are the ancient ancestors of every land plant on Earth. This means everything from simple mosses to massive Eucalyptus trees and prairie grasses evolved from them.
It is incredibly easy to walk right past these bright orange streaks without noticing them. But if you do spot them, don’t confuse them with standard lichen. While they look a lot like orange lichen, they are actually free-living green algae. Because it forms dense, velvety mats on trees and rocks, it serves as a vital food source. A whole microscopic world of invertebrates—including snails, slugs, mites, and tiny insects—grazes on these algae to survive. It is also a living climate indicator: scientists actually use Trentepohlia to track climate change. Because it thrives strictly in high humidity and mild temperatures, researchers have noticed these algae steadily creeping northward into new areas as winters become warmer and wetter.
