Why are plants green?

By Gary Raham
Nature Writer and Illustrator

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Comedian Bill Cosby once reminisced about dating a philosophy major. She asked bizarre questions like "Why is there air?" Some biologists, like Nancy Y. Kiang, who works for NASA's Goddard Institute for Space Studies, have been asking a question that might seem equally philosophical: Why are plants green?

In Kiang's case, she hopes the answer to this question may help astronomers spot life on other planets from the relative comfort of their mountaintop or orbital observatories.

So, why are plants green and not blue, orange or mauve? Are all plants green? Have they always been green? Might they be different colors on different planets?

Colors are energy, made manifest as light. Our perception of green foliage results from the mix of light photons that plants fail to capture with their chlorophyll pigments. Plants preferentially absorb high-energy blue photons and lower-energy, but high-volume red photons to build sugars from carbon dioxide and water during photosynthesis. Our cherished lawns and gardens reflect unabsorbed light in the green part of the spectrum back to our admiring eyes.

Because the sun produces all this dazzling light energy that runs the world, Kiang and others have scrutinized her stellar attributes. How does our sun compare with other stars that might be nurturing life-infested planets? Fortunately for us, our sun - designated as a yellow dwarf star - leads a long and boring (albeit incendiary) existence that has allowed life the time to evolve. It has been pouring out sunlight reliably for 4.6 billion years and can be expected to continue for another 5 or 6 billion years.

Bacteria first starting harvesting sunlight for energy 3.4 billion years ago. Bacterial pigments absorbed energy in the far-red and infrared (invisible to most animals) part of the spectrum and used sulfur and sulfate compounds to help build their cells. The reflected light from these deep-sea microorganisms would have looked distinctly purple.

By 2.7 billion years ago cyanobacteria - aka pond scum - began using chlorophylls and compounds called phycobilins to make sugars more or less like modern plants, churning out oxygen as a waste product that proved rather useful for animals sometime later.

Cyanobacteria (blue-green algae in my old text books) cemented sand grains together to form huge high-rise pillars in the ocean called stromatolites. While stromatolites still grow in a few places, these primitive algae now usually form much more modest tangles of filaments or greenish-blue slime balls in suitably pond scummy conditions. But 2.7 billion years ago they made earth a blue-green planet. Her oceans didn't harbor green algae until about 750 million years ago, and plants didn't venture onto dry land until about 475 million years ago.

You couldn't take green for granted until then.

Astronomers classify yellow dwarf stars like ours as G-type stars. A star's letter designation signifies temperature in degrees Kelvin. The complete range of star types (from hottest to coolest) goes in the sequence O B A F G K M, which astronomers keep straight by remembering the phrase "Oh, Be A Fine Girl: Kiss Me!" (Those isolated observatories can be lonely.) Our G star falls toward the cooler end of the sequence. O-, B- and A-type stars tend to burn themselves out pretty quickly, but the other star types could burn long and reliably enough to nurture life on planetary systems. What kind of colors might we expect to see reflected from plants circling hotter or cooler stars than ours?

Hot F stars pour out light richer in the energetic blue photons. Land plants on planets circling such a star might need to reflect more blue photons to avoid damaging their tissues. Thus plants might display a distinctly blue tint. However, depending on the exact spectrum of light produced, the planet's distance from the sun, the photosynthetic pigments evolved and the atmospheric composition, plants could reflect lots of red and yellow photons. Orange forests and lawns might be the norm for E.T.s living on F-system planets.

Most stars in our galaxy burn cooler than our sun and fall into the K and M categories. Planets in such solar systems would have to stick close to their parent star for adequate warmth and might be subject to more gravitational stresses. Cooler stars produce more low energy red light. To get enough photons to maintain photosynthesis, plants might have to absorb nearly the entire available spectrum. Plants might well appear clothed in somber shades of brown and black.

Plants and even microscopic photosynthesizers make their presence known at astronomical distances by more than the colors they reflect in visible light. Earth plants reflect heavily in the infrared, too. Scientists routinely map plant cover via satellite infrared imagery. Lots of atmospheric oxygen and ozone screams, "Life is here."

Free oxygen combines with iron ores and other minerals quickly unless replenished. In fact, banded iron formations on Earth are relics from the time when our planet first rusted during the early days of aerobic photosynthesis. Oxygen/methane combinations in an atmosphere of rising and falling concentrations of methane are hard to achieve except through metabolic activities. Nitrous oxide in the atmosphere results from plant decay. Nonliving sources like lightning produce only tiny amounts.

Cosby, an athlete as well as a comedian, told his girlfriend there was no question why there was air: to blow up basketballs and volleyballs, of course.

Kiang tells us plants on Earth are green because they are using as much of the rest of the sun's spectrum as they can to live long and prosper. While details of the photosynthetic story on other planets in the universe might vary, life processes will always give themselves away. A planet's living history glitters in the light it chooses to reflect.

Nancy Y. Kiang's article, "The Color of Plants on Other Worlds," appeared in the April 2008 issue of Scientific American.