As well as the diatom growths, the bed of the River Browney was also covered with skeins of a green-coloured alga which, when I removed it, had a soft, felt-like texture. This is Vaucheria, a very common constituent of enriched rivers in Britain. Under the microscope, the filaments can be seen to be long tubes (think of sausage skins) within which there are numerous tiny green chloroplasts. This is not the first bright green alga that I have written about in this blog but appearances, in the world of algae, can often be deceptive.
We often see the evolutionary history of life on earth portrayed as a tree whose branches, representing each of the main groups of organisms, diverge again and again, culminating in “twigs” representing each species. Following any “twig” back towards the “trunk” links to successively larger groupings of organisms. So, for example, the species to which we belong, Homo sapiens, has no very close relatives, but is linked at the next level (family) to apes such as the chimpanzee. Humans and chimps are, in turn, linked to the primates (order) such as gibbons and lemurs, which are part the mammals (class), including elephants and lions, which is part of the chordata (phylum) which links us to fish and reptiles. Finally, the chordate belong to the Animalia (Kingdom) which links us to flies and slugs, and Animalia is part of the Domain Eukaryophyta, which links us to the rest of life except for bacteria.
We can use this analogy to express the relationship between any two organisms in terms of the number of steps along the tree before we find a common relative. If we compared Vaucheria to Bulbochaete, which we met on 16 August, another bright green growth from the bed of a river, surprisingly we have to take eight steps (equivalent to comparing humans with plants!). By contrast, the distance between Bulbochaete and Spirogyra is six steps (humans v fish) and between Vaucheria and Melosira (4 September) is a mere four steps (humans v gibbons).
The message is that the affinities amongst the algae are often not best discerned through seemingly obvious characteristics such as colour and shape but through biochemical composition, similarities in reproduction and the life cycle and in the structure of the DNA. The other message is that the umbrella term “algae”, usually bit-players in any consideration of life on earth, embraces as much diversity as a typical zoo. This is too easily forgotten, at least in part because they lack the televisual qualities that lend themselves to wildlife documentaries. Unfortunately, in the process, we also often lose sight of the importance of algae in ecosystems.
There is no universally agreed system of higher classification (see post of 11 August). For this exercise I used the tree of life project (www.tolweb.org) as the basis for the classification of animals and Algaebase (www.algaebase.org) for the algae.