The bed of some parts of the River Ehen on my last visit had a distinctly pinkish-red hue, which is a sure sign that we are moving towards autumn and winter. This is Audouinella hermanii, which we have already met in earlier posts. Having written recently about a green alga that is actually red, and also described Lemanea, a red alga that is typically olive-green in colour, it was reassuring to finally encounter a red alga that is unequivocally red in colour.
A rock from the River Ehen, near Ennerdale Bridge, with a thick coating of Audouinella hermanii. Photographed in October 2013.
The red colour in Audouinella and other red algae is due to a pigment called phycoerythrin. This is a protein-based pigment which is also found in blue-green algae. By contrast, the carotenoids which gave Haematococcus its bright colour are lipid-based. The astaxanthin of Haematococcus is, as we saw on 6 October, a natural sunscreen but phycoerythrin and many carotenoids have a different role inside the algae.
Phycoerythrin appears red to us because it is reflecting red light whilst at the same time absorbing other wavelengths. Beccause blue light penetrates water to a greater depth than light of longer wavelengths, phycoerythrin allows red algae to absorb this and use the energy for photosynthesis. We can think of the phycoerythrin (and, indeed, many carotenoids) as “turbochargers” for the photosynthetic apparatus, allowing the algae that contain them to live in a wider range of habitats.
This argument works in coastal areas, where red algae can live at great depths where relatively little light penetrates but why should Audouinella be so common in the River Ehen? The same reasoning applies: the stretch of river where I found Audouinella was heavily shaded and this species is most common in autumn and winter when the sun’s rays are much weaker. Again, the phycoerythrin gives the normal photosynthetic apparatus a boost to help it make the most of the light that is available.
Cells of Gomphonema truncatum growing within the Audouinella colonies in the River Ehen near Ennerdale Bridge, 9 October 2013. The two left hand images are in “valve view” whilst those on the right are in “girdle view”. Scale bar: 10 micrometres (= 100th of a millimetre).
Whilst examining the Audouinella under my microscope, I noticed some clumps of diatom cells that looked like inverted Coke bottles. These are a species known as Gomphonema truncatum which grows on long mucilaginous stalks which branch to form bush-like colonies. Unlike Coke bottles, Gomphonema cells are not round, so the characteristic shape is only seen when peering down onto the flat surface of the cell. When viewed from the side, the cell is wedge-shaped. Diatomists refer to these two different views as “valve view” and “girdle view” respectively. Note, too, the characteristic yellow-brown appearance of the chloroplasts. This is due to the carotenoid fucoxanthin which, like Audouinella’s pigments, gives a boost to the photosynthetic machinery when natural light is in short supply.