Purple planet

Whoopsy. I wrote this a couple days ago and forgot I saved it as a draft. So, let's go back in time... to TUESDAY!

There's a new idea making the science news today: plants may be green to avoid competing with organisms that were already using that part of the spectrum. Basically, the sun doesn't put out an equal amount of reds, oranges, yellows, etc., and if you look at it in just one color at a time, it's brightest in green. But it's odd that plants are green, because that means all that abundant green light is getting reflected away, and not being used by the plant for photosynthesis. You see a lot of adaptation in plants to compete for a finite amount of sunlight, so it's interesting that using a more abundant band of light isn't one of them.

The idea presented in the article is that before plants hit it big, the dominant photosynthesizing organisms used a chemical reaction involving a different molecule than chlorophyll, called retinal. Retinal does basically the same thing as chlorophyll (converts sunlight into food), but absorbs green and reflects blue and red light. If retinal-using organisms were already entrenched, then other organisms would have a difficult time competing with it directly for green light, but organisms that could efficiently use other parts of the spectrum would have an advantage.

I like the concept, but I get the impression it's just speculation at this point, and I don't know if a published paper would address the obvious question: let's say this did happen, why don't modern plants absorb green light? Any chlorophyll/retinal throwdown would have been a seriously long time ago, a couple billion years, and without any real inter-kingdom competition for light (meaning, plants only have to compete with other plants for it, mostly), an efficient green-sensitive molecule should be a very useful adaptation. In fact, taking that reasoning as far as it goes, you'd expect to see black plants that reflect very little light (as long as you'd never actually seen plants before). So I'm inclined to think that either A. the second scientist the article mentions is on the money, and absorbing green light would just be an overload, and therefore not as useful as it sounds; or B. there's some physical obstacle preventing an efficient green-sensitive molecule. Or C., there's some other subtle selective pressure against non-green plants, beyond simply overheating, but I'm not really sure what else would affect all plants everywhere in the same way -- consider the environmental conditions of phytoplankton in Antarctic waters, compared to those of a Saguaro cactus. There's not much they'd have in common, besides the laws of physics. Certain atmospheric qualities, and that's about it.

Interesting.