Nature does not get much more prosaic than this: my garden fence covered with a fine, powdery green coating. For most of the year this is hidden behind the foliage of our apple and willow trees but as autumn gives way to winter, so the bare green slats became visible again. Last week, staring out of the window whilst completing my previous post, it occurred to me that, in the six years that I have been writing this blog, I have never made the short journey across the grass to look at one of the most common algae in the country.
I scraped a single-sided razor blade across the surface of one of the slats to harvest a small quantity of the damp, powdery film, and put a few specks under my microscope in order to take a closer look. What this revealed was lots of clumps of small near-spherical green cells. That, along with an ability to live in terrestrial habitats are about all the natural historian has to go on when trying to name this organism. My old copy of West and Fritsch suggests Pleurococcus naegelii, adding that “there is probably no other alga about which there has been so much confusion” whilst the latest guide to British algae would call it either Apatococcus lobatus or Desmococcus olivaceum– their descriptions are very similar. Desmococcus olivaceum has been described as “the commonest green alga in the world”, which is a bold claim. Certainly, green powdery coatings such as these are found in shaded locations in a great many places but is the singular “alga” really appropriate? Cells such as these offer so few visual clues that the microscopist is apt to latch onto a phrase such as “one of the commonest terrestrial algae” alongside a description that roughly matches the material, and considers it to be job done. These groups have also been referred to as “LRGT” (“little round green things”) – the phycological equivalent of the ornithologist’s “little brown jobs”. Recent molecular studies suggest that there is a lot of diversity within these powdery films, and this is almost certainly going to be very difficult to resolve with traditional methods. It looks as if we going to struggle with these “LRGT” for the foreseeable future.
The Class Trebouxiphyceae seems to have a particularly large number of LRGT. Some (such as the one I am describing in this post) are free-living and capable surviving desiccation, but this group also includes many of the algae that unite with fungi to form lichens, whilst others prefer to live in truly aquatic situations. But it is the fence-dwelling forms that are of interest to me today, and even if I cannot put the exact name onto my powdery film, I can perhaps offer some thoughts on why it thrives where it does.
Cells of Apatococcus lobatum(?) from a garden fence in County Durham. Scale bar: 10 micrometres (= 1/100thof a millimetre). The photograph at the top of the post shows the fence in my back garden from which it was collected.
Some of the other terrestrial (or semi-terrestrial) green algae that I’ve described in this blog are endowed with brightly coloured pigments that protect them from the damaging ultra violet rays in sunlight (see “Fake tans in the Yorkshire Dales” and “An encounter with a green alga that is red”). Apatococcusand Desmococcus, by contrast, do not come with preloaded sunscreens. They thrive, by contrast, in relatively shaded locations where the gradual accumulation of cells on the fence surface means that the outer cells take the primary ultra violet hit and, in the process, protect those cells underneath. There is also evidence of Apatococcus producing lots of stress compounds. These belong to a class of compounds call “polyols” – complicated alcohols. A lifestyle that involves single cells sitting on a damp fence indefinitely might seem like an evolutionary dead end. However, they have the last laugh as, unlike us, they are genetically adapted to produce their own booze when the going gets tough.
A further adaptation that has been observed is that the cells can switch between producing their own simple sugars via photosynthesis, and absorbing sugars and other organic compounds directly, a strategy known as “mixotrophy”. Walls and fences are challenging habitats for any organism so having the ability to mop up any spare fuel (leaking down from one of those outermost cells that took one for the team, perhaps?) might give the organism a slight competitive advantage over time.
Back when I was doing my Fine Art degree, I was using algae to explore the boundaries between abstraction and representational art. My thesis was that an image of an alga could be either representational or abstract depending on how much prior knowledge the viewer brought to the image. I used the Apatococcus (or is it Desmococcus) from my garden fence as subject matter for this exploration, creating a sextych (honestly, that’s the word for a painting on six panels) that juxtaposed the minimal outline of fence panels with microscopic views of the alga. The three fence panels offer the unprepossessing view that most people will walk past for their entire life without a thought, whilst the microscopic views give an insight into the hidden world even though the arrangement of shapes and colours will not match any of the schemata lodged in the memories and experiences of most of the viewers (see “Abstracting from reality …”).
Apatococcus. 2008 50 x 130 cm. Acrylic and photomicrograph on canvas.
Gustavs, L., Schumann, R., Karstens, U. & Lorenz, M. (2016). Mixotrophy in the terrestrial green alga Apatococcus lobatus(Trebouxiphyceae, Chlorophyta). Journal of Phycology52: 311-314.
Laundon, J.R. (1985). Desmococcus olivaceus– the name of the common subaerial green alga. Taxon 34: 671-672.
Lemieux, C., Otis, C. & Turmel, M. (2014). Chloroplast phylogenomic analysis resolves deep-level relationships within the green algal class Trebouxiphyceae. BMC Evolutionary Biology14: 211.