Comment by adrian_b
1 year ago
Chlorophyll a, which is the pigment that actually uses solar energy to split water, absorbs red light and violet light. Thus its color is blue-green, as it can be seen in some lichens that have only symbiotic cyanobacteria.
This is most likely a historical accident, with no special meaning.
Most algae and plants have auxiliary pigments, which absorb other parts of the solar spectrum and then transfer the energy to chlorophyll a.
The land plants and the green algae use mostly chlorophyll b as auxiliary pigment, which absorbs light in a blue band adjacent to the violet band of chlorophyll a, and in a red band that is distinct and adjacent to the red band of chlorophyll a.
Thus the addition of chlorophyll b increases considerably the amount of captured energy.
The algae that are dominant in oceans, e.g. diatoms and brown algae, have more auxiliary pigments, so that many are dark brown, even close to black.
Unlike for marine algae, for land plants, capturing more solar energy is not desirable, because they already have difficulties in avoiding overheating and excessive loss of water. So the pigments used by them are good enough for their needs.
I don't think the mystery goes away when you consider the other photosynthetic pigments. chlorophyll-a, chlorophyll-b, lutein, B-carotene, zeaxanthin, lycopene... they're all active between 450 nm and 550 nm. And then chlorophyll-a and chlorophyll-b have secondary activity between 650 nm and 700 nm.
The the lack of photosynthetic activity between 550 and 650 is still suspicious. I've learned from other commenters here that my assumptions about the gap corresponding with peak solar energy weren't on solid ground, but there is a gap.
Perhaps a different way to frame the question is: why do the chlorophyll pigments have two peaks, while the others appear to have only one? Perhaps they have an evolutionary past which involves absorbtion from a star besides sol?
Most organic pigments have 2 or more absorption bands, but this is not always apparent for humans, because the second band is located in near ultraviolet or in near infrared.
Like I have said, in marine algae you can find a variety of colors, as already seen in their names, e.g. red algae, yellow algae, brown algae. Most of them have strong absorption for green light, due to the auxiliary pigments that they happen to use.
Only the green algae and their descendants, the land plants, do not absorb green light and as a consequence they are competitive with the other algae only in places with abundant light, i.e. in very shallow waters or on dry land, where they can get all the energy that they can use without damage, so they do not need better coverage of the solar spectrum.
The energy of the captured photons does not matter much, because even the red photons have enough energy.
The captured photons are not used for "photosynthesis", which is a misnomer whose origin lies in a time when the mechanism of "photosynthesis" was not known.
In oxygenic phototrophs, a part of the captured energy is used to oxidize some manganese atoms so strongly that they can oxidize the oxygen from water, converting it into free dioxygen. The hydrogen from water is bound into a reduced organic substance (NADPH), which will be used later as a reducing agent (without needing light) to make carbohydrates from carbon dioxide.
The rest of the energy of the captured photons is used to pump ions through the membrane of the chloroplasts. The energy stored in an ion gradient will be used later to power all organic syntheses.
The ionic pumps could work even with infrared photons, as they do in phototrophic bacteria that live under algae, so they have modified chlorophylls whose red absorption band is shifted into infrared, away from the red band that is removed from solar light by the algae sitting above. However, those bacteria cannot split water, the energy of infrared photons is too low for that (splitting water needs around 1.25 eV, while red photons have over 1.5 eV).
You're totally right, thanks for being patient with me. It should have been obvious, but I hadn't initially made this connection:
> Only the green algae and their descendants, the land plants, do not absorb green light
The earlier sources that I found had a bias towards land plants.
I'm much happier to accept as mere coincidence the fact that land plants evolved from a specific kind of algae (i.e. one that might have adapted for absorbtion of light at a certain depth and therefore shows a preference for absorbing blues over greens) and not some other kind which would've had different preferences.
Initially I thought that each of the green pigments I listed had evolved separately on land and it seemed rather spooky that they shied away from green. But that's likely not where they evolved, so now it's not spooky.