Purple Earth hypothesis

> Yes, but why?

Scientific writing style is not always very good at highlighting the unknowns. "We don't know this" doesn't make very convincingly looking text, so people tend to avoid admitting it up front.

But you are, of course, correct to ask.

Like another comments said, this is an open question.

One theory is, that while the algae floating in water were absorbing broad spectrum, the algae growing attached at the bottom of the water evolved to chlorophyll to capture whatever was left at the edges of the spectrum. And then later land-based plants would have evolved from the water plants that were already attaching themselves to the bottom. But then why are also the current ocean-floating algae green now?

http://hyperphysics.phy-astr.gsu.edu/hbase/Biology/imgbio/pl...

Another theory is that a perfectly-absorbing leaf would somehow absorb too much energy and get overheated, and that it was better to absorb only part of the available light.

None of these theories are fully convincing, so the question remains open.

While all the phototrophs that are able to split water and produce free oxygen use chlorophyll a, which absorbs only red light and violet light, resulting in a blue-green color, which can be seen as such in some lichens and cyanobacteria, most of them have some accessory pigments, which absorb other parts of the solar spectrum, and then transfer the energy to chlorophyll a.

The green algae, which live only in shallow waters, and the terrestrial plants use as accessory pigment only chlorophyll b, which absorbs a different band of red light than chlorophyll a and also blue light, resulting in a green color.

This is enough for green algae and land plants, because where they live there is abundant light. For land plants the problem is that they have too much light, not too little, with the exception of those which grow under the shadow of trees.

On the other hand, most marine algae use accessory pigments that absorb much more of the solar spectrum, so that the color of chlorophyll is no longer visible and they have overall colors like red, yellow or brown, even very dark brown. This enables such algae to live down to greater depths in the water, where there is less solar light.

So there are a lot of living beings that make very efficient use of light.

Moreover, under water there are many places where practically all light is captured, by multiple layers of algae and bacteria, each layer absorbing some part of the solar spectrum. Even the near infrared light is absorbed by a bottom layer of bacteria, which do not produce oxygen, because the energy of infrared photons is insufficient to split water.

I asked this same question last week and got some good answers: https://news.ycombinator.com/item?id=44630224

Three things that stood out to me:

- Terrestrial plants evolved from green algae. There are other colors of photosynthetic algae. The advantage of absorbing red and blue may have been about avoiding competition with nearby red algae (which absorbs green), and about how blue light penetrates more deeply.

- The trick is not capturing as much energy as possible, but rather capturing and routing energy to the reaction centers such that it neither overshoots nor undershoots the energy necessary for the reaction. This works better with two absorption peaks instead of one (more here: https://www.quantamagazine.org/why-are-plants-green-to-reduc...). The absorption peaks of chlorophyll are both adequately distant from one another, and also more or less centered on the visible (i.e. most energetic) spectrum emitted by the sun.

- The idea that the green-yellow region of the visible spectrum is most energetic is, if not a misconception, at least more complex that it seems (https://www.oceanopticsbook.info/view/light-and-radiometry/l...).

As far as I understand it, this is a still debated question. One theory is it's about evaporating water: Plausible photomolecular effect leading to water evaporation exceeding the thermal limit (https://www.pnas.org/doi/10.1073/pnas.2312751120).

There are black plants though! And they're studied for the same kind of questions. E.g. The Functional Significance of Black-Pigmented Leaves: Photosynthesis, Photoprotection and Productivity in Ophiopogon planiscapus ‘Nigrescens’ (https://pmc.ncbi.nlm.nih.gov/articles/PMC3691134/)

A few different threads based on my limited online research:

1. Absorbing all spectrum of light would provide more energy than the organisms can handle. They need gas to run the engine, and all spectrum would provide jet fuel.

2. Current predominant species of plants evolved from the undergrowth. Original plants would absorb only green, so the undergrowth evolved to absorb the other spectrums because that’s what was left. After a few planet scale extinction events where the sunlight was scarce, being able to absorb a wider spectrum became a successful evolutionary trait and became the predominant one.

3. There are species of fungi that use melanin to absorb radiation for energy source and appear black.

Most of the green reflectance is an illusion because the human retina amplifies green light as much as 25× more than red (think of a green 5mw laser vs red).

On the other hand, leaves do absorb green light: nearly 75% is absorbed, in contrast to 85% of non-green light. See https://www.researchgate.net/figure/Reflectance-spectra-of-g...

This is actually even more puzzling than it seems, because the green part of the spectrum is the most energetic.

I've read elsewhere that photosynthesis is partially limited by dealing with free radicals, and at peak light flux, many plants would be damaged by the monoatomic oxygen species that light-capturing complexes would create. Hence pigments that reflect some green light.

Article:

https://www.quantamagazine.org/why-are-plants-green-to-reduc...

> I wonder why no plant evolved to use both and make the more even efficient use of light. These plats would appear dark, maybe almost black.

Many varieties of seaweed would seem to meet the description. Although I'm not sure that any of them are naturally anything like black without processing. Certainly some of them are brown, though.

Fun fact, early hand written lyrics were "purple haze, Jesus saves...". It was a recollection of a dream where he was walking under water. The connection to acid is more so by interpretation of the audience.

Chlorophyll reflects green light, meaning it doesn't use these frequencies.

Who knows, maybe that's why the retinal photosynthesis evolved first though.

In the rest of the niches in the entire domain of life it is the case that many different strategies were tried simultaneously, usually with a sole predominating outcome.

No that's just a pigment. They still contain chlorophyll.

Often you'll find leaves in full sun are redder, because they need less chlorophyll to operate at full efficiency. Leaves more in shade may be darker, as they require more chlorophyll (meaning light is absorbed across most of the visual spectrum by the pigment and chlorophyll together)

I remember vaguely reading about this in the 1970s, maybe Scientific American? Cannot find that article now though.

Something similar happened with me for another theory's article. I knew it existed before the article said. But I only had a primary source to prove it. Since it's not a secondary source I couldn't fix the article, so I put it on the talk page. Now the talk page has been wiped and the article is still wrong about the origin.