On our trip up Platberg one couldn’t help seeing all the lichens about; especially prominent on rocks, but also on the plants, especially on the ouhout, Leucosidea sericea and on the old oaks where we old okes had lunch.
Lichens are fascinating. Coincidentally a day or two after we got back from Platberg, a post from a wonderful blog I follow Fossils and Other Living Things arrived, which got me reading and searching:
The term “Lichen” applies to a symbiotic relationship, a way of life, that has married algae and fungi. The algae uses its photosynthetic power to manufacture carbohydrates, most of which are absorbed by the fungi with which they reside. The fungi, in turn, provide the algae with essential moisture, shelter from harmful ultraviolet rays, and toxins that ward off predatory animals. There is no typical arrangement of algae and fungi in lichens. There’s huge, wonderful variety – diversity!
OK so far; What then, are “Algae” (singular alga – use a hard ‘g’)? No easy answer; no consensus answer. Ruth Kassinger in her book, Slime: How Algae Created Us, Plague Us, and Just Might Save Us (2019) notes that algae is a catchall term, a name for a group of diverse organisms; They are not plants, though they photosynthesize. Different algal taxa did not evolve from a common ancestor, and Kassinger describes three main groups: single-celled blue-green algae or cyanobacteria; single-celled microalgae; and multicellular macroalgae (the seaweeds).
And what are fungi? “Fungi” (singular fungus) include microorganisms such as yeasts and molds, as well as the more familiar mushrooms in a separate kingdom from the other well-known eukaryotic life kingdoms: plants and animals. Fungi evolved from a single common ancestor. They expand by growth or by emitting spores which float off and start new growth. Fungi are the principal decomposers in ecological systems.
In the past, mycology – the study of fungi – was regarded as a branch of botany, but it is now known fungi are genetically more closely related to us animals than to broccoli. Nice and humbling, innit?
So back to Lichens: Lichens are loosely divided into groups by the way they look: crustose lichens lie very flat against the substrate on which they are affixed; foliose lichens which are much more three-dimensional with lobed growths and some separation from the substrate on which they live ; fruticose lichens which can resemble miniature tumbleweeds and attach to a substrate at a single point.
The relationship in lichens is symbiotic between the alga and the fungus but, Tony Edger points out that it seems to favour the fungus. Quite frequently in this relationship, according to Brodo in Lichens of North America (2001), Irwin M. Brodo et al., the fungi is killing the enveloped algae. That is safely offset by the algae’s rate of reproduction, though.
Lichens engage in a variety of ways of reproducing. One approach is asexual: When a fragment detaches and blows away, if it lands in a similar location, it attaches and starts growing as a new individual. For other lichen species, asexual reproduction is more deliberate and complicated. These species create little balls (called soredia), each consisting of a single algae surrounded by fungi filaments. If these reproductive spheres are detached from the lichen and come to rest in a hospitable environment, a new lichen can grow. The fungi in most of these lichens produce spores which begin as sex cells (gametes) and then, after fusing with other sex cells, are released. But these spores, cast to the winds, will create a new lichen only if they happen to land on an alga of a specific, requisite species. Here the fungi appear to be playing against long odds. Producing huge quantities of spores helps improve their odds of success.
Lichens are found almost everywhere. As are old fossils . .
They are extremophiles, and are found from the poles to the tropics, from the intertidal zones to the peaks of mountains, and on every kind of surface from soil, rock, and tree bark to the backs of living insects! They are an evolutionary success story with around 14,000 different species covering some six percent of Earth’s surface. Those lichens that employ rock as a substrate make soil by engaging in a very slow process of eroding the rock into ever-smaller particles. Without lichens plants could not have invaded the land. Their anchoring filaments penetrate cracks in the rocks and, as the weather alternatively moistens the lichens, expanding their anchoring filaments, and then dries them out, the substrate is broken up. A slow process, sure, but rocks without lichens disintegrate even slower – maybe ten times slower!
Three more fun facts I found: 1. Lichens are affected profoundly by air quality. The diversity of species in a location is a gauge of how polluted its atmosphere is. 2. The long life-span and slow and regular growth rate of some lichens can be used to date events. One of its advantages is it can date the last 500 years, which most other dating techniques can’t. 3. Lichens are not related to moss. Moss are plants.
**(Even more fun fact: When searching for ‘dating by lichens’ I was shown lots of sites for ‘Mature dating’ and ‘Dating for over 60’s! and ‘Meet mature singles near you’!)**
The illustration below of the interior of a simplified foliose lichen, captures some of the essence of the lichen structure. By Tony Edger. So it seems all you see is fungus – the alga is inside the fungus structure.
Tony Edger’s fascinating blog Fossils and Other Living Things