Another outcome of my visit to Ennerdale Water a couple of weeks ago in July (see “Life in the Deep Zone”) was some tiny green spheres in the sample I collected from one of the small streams flowing into the lake’s north-west corner. The stream was very short, little more than a seepage arising from a wet rush-dominated area of a field just twenty metres or so from the lake margin and, at the point which I sampled, there was a tangle of filamentous algae (Stigeoclonium, Mirsrospora and Mougeotia) as well as a distinct diatom-dominated film on exposed stones. The colonies looked like tiny peas in my sample tray but I suspect that they were attached to rocks or aquatic vegetation before I disrupted them. Under the microscope, these turned out to be colonies of the green alga Chaetophora pisiformis, a relative of Draparnaldia and Stigeoclonium, both of which I have written about before (see “The exception that proves the rule …” and “A day out in Weardale …”. Like those, Chaetophora has branched filaments but they differ in forming well-defined colonies that are visible to the naked eye.
The pictures below show the form of colonies very clearly. Chaetophora colonies are firm to the touch and cannot easily be squashed under a coverslip. I overcame this by using a cavity slide, and taking one of the smallest colonies that I could find in order to photograph it with as little damage as possible. Note how there is a very clear edge to the colony, whereas Draparnaldia and Stigeoclonium have a mass of filaments and mucilage but no obvious border between the “colony” and the surrounding environment. Draparnaldia sometimes forms discrete colonies (see “The exception that proves the rule …”) but these are much softer and more easily squashed onto a slide.
Top: colonies of Chaetophora pisiformis from a small stream flowing into Ennerdale Water, with a one cent coin for scale; bottom left: lower power (x40) view of a colony. The picture frame is about two millimetres across; bottom right: medium power (x100) view of the same colony.
Viewed at higher magnifications, the branches of the filaments are clear. They tend to be clustered towards the tops of the filaments and, in this case at least, end abruptly, rather than tapering to fine hairs. I explained in the posts mentioned above how these fine hairs are used by the algae as means of capturing the nutrients that they need. Chaetophora can form these hairs, but it does so less often, in my experience, than Draparnaldia and Stigeoclonium. There will be dead and decaying vegetation in the rush-dominated swamp from which the stream originates, and the enzymes that these algae produce will be able to harvest any phosphorus from organic particles that result from this decay. That’s the theory for Stigeoclonium at least, but I suspect that the colonies of Chaetophora are also highly efficient recycling units: the filaments are embedded in a firm mucilage that is far more than an inert polysaccharide gunk. Any phosphorus that is released from a filament will be far more likely to be hoovered up by another filament than to drift downstream whilst the phosphatase enzymes will also be on hand at the colony surface to savenge any stray nutrients from the seepage. These tight colonial forms are, in other words, fortresses of plenty in an otherwise inhospitable landscape: well adapted to nutrient-stressed situations and, as a paucity of nutrients is the natural condition of streams, the presence of these colonies is a good sign that this stream is in good condition.
Filaments of Chaetophora pisiformis from a small stream flowing into Ennerdale Water, July 2018. Scale bar: 20 micrometres (= 1/50th of a millimetre).
Whitton, B.A. (1988). Hairs in eukaryotic algae. pp. 446-480. In: Algae and the Aquatic Environment (edited by F.E. Round). Biopress, Bristol.