The simple enjoyment of an orange, green alga
I've mentioned these orange cushions in passing a couple of times but the organism responsible for them is worth a little more attention. While it's an alga rather than a plant, in the evolutionary tree of life it's in the branch that splits off just below the green plants - so not too distantly related.
It's called Trentepohlia and you can find it on rocks or wood, in pretty much any part of the world. In Victoria (Australia), drive from Halls Gap to the Zumsteins, through the Grampians/Gariwerd National Park, and you'll see a lot of it on the roadside at slightly higher elevations.
Here on a rock wall in a village in the West Pinines, an hour or so from Durham in northern England, it thrives. There are about 40 species and this one is probably Trentepohlia aurea, a common species in these parts (although it may be applied more commonly than it actually occurs given the lack of recent taxonomic work on this group - studies by Mike Guiry and colleagues are the exception).
Although it is orange in colour, it is classified as a green alga. And if you look under a microscope you'll see that it is fundamentally green overlaid with an orange pigment, called haematochrome. In this picture the green dominates but sometimes it's all orange, orange, orange...
It's thought that the haematochrome helps protect the alga in the very exposed places it finds to live - out of water, on walls, often in full sun.
Sometimes it might be confused with various red or orange lichens such as those found on coastal rocks in Australia and elsewhere. It might also look like a clapped out moss I guess. A quick peak under the microscope will confirm it's identity but generally if in the field it is orange in colour and velvety in texture it's likely to a Trentepohlia. (Although be aware that this alga also finds it's way into the algal-fungal symbiosis we call a lichen.)
The UK wall featured here is said to be the densest and most extensive population of this alga in England. I have no way of testing that but other patches I saw were certainly smaller and the local community is justly proud of its orange wall. The habitat must suit it, and the wall would be old enough for this alga to have grown to such an extent (Trentepohlia is often described as growing 'very slowly' and I would expect it would take many decades or centuries to achieve this result).
Interestingly other walls nearby support mosses and lichens, due to either microclimate, or age and disturbance I suppose.
None of these growths will particularly harm the wall and when Trentepohlia grows on tree trunks it doesn't cause any damage. Another genus of algae in the same family (Trentepohliaceae) called Cephaleuros is less popular because it can become parasitic on tropical crops such as cocoa and tea, leading to extensive foliage damage.
Trentepohlia, like most algae, is simply there to be enjoyed.
Images: from my trip to Durham (skyline below...) in July last year, except for the microscope picture which I have copied from the University of Wisconsin Madison, Botany 330, Algae webpages.