One other alga that we saw at Burn Head (the location near Whitbarrow Quarry) was tucked away in a shaded area close to where the spring bubbled out from the base of a limestone cliff. At first glance, this was barely recognisable as a plant, as it looked more like a splash of red paint on a rock. It is, however, a thin crust of red algal cells, called Hildenbrandia rivularis. This is the only freshwater representative of the genus, although other species can be found on the seashore. Under the microscope, you can see polygonal cells, though we are actually looking here at the top of a short stack of cells.
I generally associate Hildenbrandia with good ecological conditions although, as for Batrachospermum, there are exceptions, as I have seen it growing at quite high nutrient concentrations in chalk streams and, I daresay, it thrives in enriched waters elsewhere. Here it was growing in very shaded conditions, and I have also seen it growing under quite thick patches of moss, which must also have trapped much of the light. However, I often see it growing in shallow, well-lit places as well.
Hildenbrandia rivularis from Burn Head, southern Cumbria, May 2014.
So why are some red algae red and others not? The answer to this question lies in the pigments that they contain. Red algae, like Cyanobacteria, contain chlorophyll a (the common green pigment), plus two protein-based pigments, phycoerythrin (red) and phycocyanin (blue). The balance of these two pigments influences the final colour of the organism: those with more phycyoerythrin tend to be red; those with more phycocyanin have a blue-green or grey-green colour. Red-coloured algae have an advantage in deep water as it can absorb those wavelengths of light that penetrate furthest. Being able to absorb over a broader range of the light spectrum than would be possible if it just had green chlorophyll means that a plant is able to use the limited light more efficiently. Why we find some red-coloured algae in shallow, freshwater situations is a mystery. It may simply reflect the evolutionary history of the genera concerned. It may be significant that two of the reddest freshwater red algae (see also “At last … a red alga that really is red”) both come from genera that can be found in both freshwater and marine locations whereas the two olive-green genera we’ve met (Lemanea and Batrachospermum) are found exclusively in freshwaters.
Looking down on a crust of Hildenbrandia rivularis, showing the tops of the polygonal cells. Photograph: Chris Carter.