I will end this short series of posts on the organisation of the major groups of algae with a look at the Xanthophyceae, or yellow-green algae. My old copy of West and Fritsch’s Treatise on British Freshwater Algae from 1927 includes this group of algae with the green algae, although we now know that, apart from a generally green appearance, these two groups of algae have very little in common. The big differences lie, however, in the types of details that are beyond the purview of the casual natural historian, so you may well find yourself flicking back and forth between “green algae” and “yellow-green algae” as you try to put a name on a specimen. The definitive test is to add some iodine to your sample, as the Xanthophyceae do not produce starch as a storage product, and so do not produce the characteristic blue-black colour in the cells. However, iodine is messy stuff and most of us will struggle along without for as long as possible.
The five orders of Xanthophyceae are shown in the table below. In contrast to the case for most algal groups where molecular studies have led to many revisions of traditional classifications, the Orders of the Xanthophyceae have proved to be quite robust when subjected to this type of scrutiny. Two of the Orders have siphonous organisation, though the form that this takes is very different in each (see “The pros and cons of cell walls” for more about siphonous lifestyles). Tribonematales is an Order of filamentous algae that can be difficult to differentiate from filamentous green algae, whilst the Mischococcales are easily confused with small Chlorophyceae.
The organisation of the Xanthophyceae into five orders. Organisation follows Algaebase. The image at the top of this post shows Tribonema smothering the surface of a pond in Norfolk (photo: Geoff Phillips).
That’s one of the mysteries of freshwater algae: to the lay observer, an organism such as Vaucheria looks very similar to Cladophora or another green alga. Yet they are distant relatives, belonging to different Kingdoms (Chromista and Plantae respectively). That means that they share the same genetic affinity to one another as they do to us, which is a staggering thought (see “Who do you think you are?”). What we are seeing is two organisms supremely well adapted to living in similar habitats, which means that natural selection has, gradually, shaped two quite distinct gene pools in quite different ways to arrive at the same end-point. Just as motor manufacturers have, in the hatchback, found a style of car that is well-adapted to urban living, so the rival algae manufacturing corporations (“Plantae Inc” and “Chromista plc”) have come up with two broadly similar models that are both well-adapted to life in lowland streams. Just as, in the case of hatchbacks, you can lift up the bonnet and see differences in the engine (petrol, diesel, hybrid, electric) but within the same basic shape, so many of the big differences in algal groups concern their internal machinery not outward appearances.
Reproductive structures growing from a filament of Vaucheria frigida (photo: Chris Carter)
Maistro, S., Broady, P.A., Andreoli, C. & Negrisolo, S. (2009). Phylogeny and taxonomy of Xanthophyceae (Stramenopiles, Chromalveolata). Protist 160: 412-426.
Links to posts describing representatives of the major groups of Xanthophyceae found in freshwaters. Only the most recent posts are included, but these should contain links to older posts (you can also use the WordPress search engine to find older posts).
|Botrydiales||Botryidium: The littoral ecology of Lough Down|
|Mischococcales||Watch this space …|
|Rhizochloridales||Watch this space …|
|Tribonemetales||Tribonema: Survival of the fittest (1)|
|Vaucheriales||Vaucheria: When the going gets tough …|
Some other highlights from this week:
Wrote this whilst listening to: Two Hands, by Big Thief
Cultural highlights: Jon Hopkins at the Sage. What Radio 3’s Ibiza night might sound like.
Currently reading: the last few pages of Bill Bryson’s The Body: A Guide for Occupants (454 pages) prior to starting Hilary Mantel’s The Mirror and The Light (904 pages)
Culinary highlight: fish pie. Spécialitié de la maison.