One of the great discoveries in photobiology is the fact that red algae (Rhodophyta) can exist at the great depths they do because of the accessory pigment - phycoerythrin - they possess that allows them to harvest the faint blue light - of around wavelength 475 nm - reaching these great depths. The absorption maxima of phycoerythrin lie at 498nm and 565 nm, the former wavelength relatively abundant at depths greater than a hundred and fifteen feet but the latter wavelength virtually absent for it falls into the green region of the spectrum.
Indeed at deep depths all that exists is a pervasive blue. 498 nm light, that lies at the border separating blue light from green light - which latter starts at 500 nm - is, as mentioned, present at depth. Phycoerythrin can and does absorb at 475 nm but with only about 40% efficiency, something that regular Chlorophyll a - also present in red algae - which absorbs at 435 nm cannot do at all, but which Chlorophyll b can!!, but which the red algae do not possess!!.
Why did not the red algae then opt for Chlorophyll b which is better at absorbing around 475 nm? One can think of two reasons. The blue absorption peak of Chlorophyll b is too sharp and narrow, making it inefficient at harvesting the other blue wavelengths present at these great depths, while phycoerythrin has a far more flat and broad absorption band in the blue wavelengths - that lie between 420 and 500 nm - allowing it to harvest a broader range of blue wavelengths..
Secondly, herbivory by fishes would be disastrous for the red algae, for energy resources are poor at these depths and the algae would not be able to recover from such depredation. Red colour is camouflage gear for at such depths it appears black in colour, a strategy the fish living at these depths also employ, for many of them are red in colour.
What is truly fascinating is that while there ARE blue wavelengths around 435 nm that DO reach such depths, the Chlorophyll a present in red algae - and designed to capture precisely this wavelength - seems to be unable to harvest it!! - see the beautiful paper on the photosynthetic action spectrum of marine algae by F.T.Haxo - the reaction center dependent entirely on the light energy “fed” to it by the phycoerythrin in order to conduct photosynthesis!!. Clearly in the red algae the Chlorophyll a has become a vestigial structure!!, similar to our vestigial appendix - useful in rabbits but not in us!!!
In the photo above we have a specimen of a green alga, a red alga and a red alga that has been bleached white by the Sun.
Yarada Beach, Andhra Pradesh, March 1, 2020.
