Sunday, September 12, 2010

Why Aren't Plants Black? - A True Evolutionary Enigma...For Now!

Plants run the world, not humans.  You may not have ever really and truly thought about it that way, but it's true.  If they weren't around, neither would we.  The reason being, of course, is the entire food web in which we are a part is dependent on the solar powered compounds, like sugar, that plants make.  Plants are so stinking important that our vision is most adapted to seeing shades of green, hence why night vision goggles scopes are green tinted (How awesome is that, right?  See diagram to the right).  So, from that perspective answering the question of how plants work is just about one of the most important question biology can really answer.

But, there's a problem.

An enigma concerning plants.

They're green.

Apparently it is easy being green.

Have you ever thought about that?  Why the heck are plants green?  Think about what it means to be green.  Being green basically says, "I want all the color spectrum EXCEPT green wave lengths."  Why do plants throw away green light?  Why not use green light to harness more solar energy?  The enigma gets even deeper.

What's the most abundant wavelength of light at the surface of the Earth?  You guessed it!  Green!!! (I should say, I've seen/heard different things on this, though.)

Notice the peak at green.  Also, ever wondered why we see the visible light spectrum and not infrared or ultraviolet like some insects?  Well, it's the most abundant, that's why!
Why, why, WHY would plants reject, ditch, throw away, ignore the most abundant wavelength at sea level?!?!?

From all I can tell, no one really knows, but here are some possibilities:

1)  No particular reason.  Plants could be pink.  They could be orange, yellow, red, black or deep purple.  In fact, bio-astronomers don't look for green when looking for alien foliage.  To them, green is arbitrary.  Not only that, but the spectra that alien-Earths have could be quite different with a different star and atmosphere.  Back on Earth, though often underappreciated we have many red, purple and brown photosynthesizers typically in the form of different types of algae and cyanobacteria.  There have even been some recent discoveries of infrared harnessing algae.
2)  They just don't need green.  Blue and red (the spectral peaks from which they absorb) do quite nicely.  Evolution only does what it needs to.  It doesn't 'know' that doing things differently might be better.  (I should say this is hard for me to swallow.  4 billion years of evolution and mutations and this green handicap is still around for no reason?  There has to be some kind of advantage?  Right?!...right?...)

3)  Blue and red (which plants utilize most) are the peaks of energy and photon densities.  'Relative photon flux density' is how many photons strike per time interval in a given area and 'irradiance' is how much energy they pack per time interval and area.  So, perhaps it's not a coincidence that green gets ignored.  Notice the surprising dip in energy in irradiance for green wavelength light on the surface of Earth.

4)  Rejecting green acts like shades.  This isn't completely ridiculous considering how plants use a good number of complex molecules to dampen the intensity of light...but...why green?  Why?!

5)  Earth might have been purple early on since retinal (a purple molecule used in some photosynthesis) was most likely easier to make in the oxygen poor atmosphere of early Earth. The archaea of today demonstrate the functionality of this purple molecule quite well.  It gets interesting when you start to overlap the spectra of purple and green photosynthesis.  Consider the reason they're purple is that they reject blue and red, the very spectra that green plants eat up!  Perhaps, the purple tinted water of early Earth was conducive to a late coming niche filler like chlorophyll to take advantage of the bounty!  Could this be a coincidence?  Absolutely.  But!  It is interesting that what green plants don't use, retinal archaea do use and vice versa.

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