A couple of weeks ago a discussion of alien biology asked if life was found on other planets would some of it be plants? A reasonable question, and there are reasons why biologists would expect photoautotrophs and why any such organisms of any size would have some similarities to plants, but the more intriguing question was would they be green?
Plants, or more correctly, photoautotrophs on other planets may well be green, but not necessarily. Different stars have different light quality just as incandescent and fluorescent bulbs each produce a different spectrum of light, and neither are the same as sunlight. Perhaps you’ve noticed that plant grow lights, light bulbs designed to produce a spectrum of light more like the sun, look sort of reddish to us. So the spectrum of light may be such that a non-green pigment would work fine. If there is life on a planet, it needs energy and the only three sources that make any sense are geothermal, geochemical, and light, and organisms using all three types of energy can be found on Earth. Geothermal and geochemical energy sources are quite site specific in comparison to light, and so photoautotrophs are common on the surface of our planet, although most of them are unicellular so not plant-like at all. The geochemical autotrophs are probably wide spread too deep within the Earth's crust (they're every where we look).
You probably realize that the “visible electromagnetic spectrum” is a very small portion, a narrow band of wavelengths, and perhaps you wonder why out of all those wavelengths plants just use the portion that we see. Is this some kind of cosmic coincidence or purposeful design? First, there isn’t just one visible spectrum; different organisms have sight adapted to seeing different wavelengths, e.g., bees and lots of other organisms see into the ultraviolet wavelengths and this is why flowers under UV light look different then when seen with just light visible to us. But in general no one would expect photoautotrophs to be using wavelengths of energy very much beyond the visible spectrum and the reasons are pretty simple. As you move up into the ultraviolet wavelengths they become energetic enough to damage many different organic molecules and lots of organisms have UV filters to prevent such damage. You wear sunblock to reduce damage from UV. At the other end of the spectrum infared wavelengths (heat) are beyond red, and except for those wavelengths closest to red, they are not energetic enough to be used by photoautotrophs although a geothermal autotroph is known to exist. Many organisms see this same range of wavelengths because these wavelengths are abundant enough and enegetic enough to be useful and not particularly harmful. So light sensing organs and photoautotrophy are adapted to using some of the same wavelengths, although not exactly the same. Any pigment that captures a useful amount of “light” energy could work, so photoautotrophs could be several colors: reddish-brown, blue-green, green, yellow-green, and that’s just here on Earth. An alien photoautotroph could be, but wouldn’t necessarily have to be, green. Green predominates because chlorophylls are more efficient than bacteriochlorophylls and work well under water, and since land plants have an aquatic ancestry, they acquired their green pigment from them.
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