More than just an insignificant dot?

One of the more unusual habitats for freshwater algae that I’ve encountered recently is the surface of a “sonde” – a submersible instrument that measures water quality attributes (“sonde”, as far as I can tell, simply means “probe” in French, but seems to have slipped into general usage in the water quality monitoring fraternity).   Mel Lacan from Natural Resources Wales sent me some photographs of tiny colonies that she and a colleague had found on a sonde submerged in a small tributary of the lower Wye and photographed.   At first, I thought it was Heribaudiella fluviatilis, which forms similar dark-brown colonies and it took the eagle eyes of Dave John to spot that I had not just got the wrong genus, but the wrong phylum (Heribaudiella is one of just three representatives of the Phaeophyta, or brown algae, found in UK freshwaters.   These colonies are, in fact, formed from filaments of a Rhodophyta (red alga), probably Audouinella pygmaea.   We’ve seen another Audouinella species, A. hermanii, in the River Ehen (see “The schizophrenic life of red algae …” and references therein), but I had not seen this particular species before.   Whereas A. hermanii formed pink-tinged mats on the bed of a fast-flowing stream in winter, this population of A. pygmaea forms dark brown spots and was found just above the tidal limit in a tributary of the River Wye.

Colonies of Audouinella pygmaea on a sonde submerged in the Anghidi Fawr stream, May 2015 (The light brown objects are pupal cases of simuliidae flies). Scale bar: 1 centimetre. Photograph: Mel Lacan, Natural Resources Wales.

It is hard to be 100% sure about the identity of this population as it does not totally match the description in the Freshwater Algal Flora of the British Isles and, moreover, there are no reproductive organs present (so it could just be a “chantransia” stage – see comments in the earlier post). Some samples have been sent off to experts for their opinions.

Mel sent some of the colonies to Chris Carter and he has produced a series of spectacular photographs that illustrate the structure of these Audouinella colonies very clearly. The first pair show the linear, sparsely-branched filaments radiating out to form the hemispherical colonies that were growing on the sonde. The second pair shows the filaments at higher magnification and you can see the brownish-green colour of the cells very clearly. Along with chlorophyllsred algae contain phycobiliprotein pigments which lend the group their distinct colouration.   In most red algae, a red pigment called phycoerythrin dominates, giving the group their common name. However, a blue-colured variant called phycocyanin can also be abundant in some species, including A. pygmaea.   The description of A. pygmaea in the Flora refers to its blue-colour. Here, however, we have a brownish hue, presumably reflecting a mix of these two pigments along with the green chlorophyll. These accessory pigments helps red algae to make the most of the limited light available at great depths in the oceans.   It is probably also offers a competitive advantage to freshwater red algae, many of which are associated with shaded sections of streams.

A low-magnification view of an Audouinella pygmaea colony from the side (left) and from above (right).   Photograph: Chris Carter.

Medium- and high-power views of Audouinella pygmaea filaments from the submerged sonde from Anghidi Fawr stream, May 2015. Photographs: Chris Carter.

The evidence that demonstrates conclusively that this is a red alga can be seen in the image below. A gap in the cell walls often remains between daughter cells after cell division, which means that the protoplasm of the two cells is connected.   As the cells develop, these “pit connections” become filled by “pit plugs”, formed from protein and polysaccharide, which are just visible in the light microscope.   Not all red algae have pit plugs, but it is particularly characteristic of the class Floridophyceae, to which Audouinella belongs.

A high magnification view of filaments of Audouinella pygmaea from Anghidi Fawr stream, May 2015, with arrows indicating the pit connections between adjacent cells.

A few months ago, I wrote about the problems of knowing whether algae were genuinely rare or simply not very well recorded (see “A “red list” of endangered British diatoms?”). The Freshwater Algae Flora of the British Isles refers only to a single record of Audouinella pygmaea (River Deveron, Scotland), whilst my old copy of West and Fritsch also mentions a record at “Penyghent” in West Yorkshire.   Audouinella pygmaea is a good example of an alga that is probably under-recorded.   These tiny dark-brown spots are very easily overlooked by all but the most observant surveyors.   I suspect that it is more common than we think but that it is overlooked by both macrophyte surveyors (who are generally focussed on larger organisms) and diatom samplers (whose methods preclude the study of soft-bodied algae). A map of the current distribution of Audouinella pygmaea in the UK will, therefore, give an indication of where eagle-eyed surveyors are work than reveal any deep biogeographical truths.

Postscript: this post has been extensively re-written as closer observation by Dave John led us to change our initial conclusions about the identity of the alga. As Chris Carter pithily put it as we mulled over the issue, “anyone who does not make mistakes does not make anything”

References

John, D.M., Whitton, B.A. & Brock, A.J. (2011). The Freshwater Algal Flora of the British Isles. 2^nd Edition. Cambridge University Press, Cambridge.

West, G.S. & Fritsch, F.E. (1927). A Treatise on the British Freshwater Algae.   Cambridge University Press, Cambridge.