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Why Are Plants Green?

March 1, 2016

The dominance of green chlorophyll on Earth may have come after a more purple period.



The northern portion of Utah’s Great Salt Lake has a reddish-purple tinge from tiny salt-loving microbes that live there. (NASA)


Why are plants green? I’m Bob Hirshon and this is Science Update.

A Science Update fan asks why plants are green and if plants on other worlds might be a different color. University of Maryland microbiologist Shil DasSarma wonders the same thing. He says that long ago, purple organisms called halophilic archaea may have ruled the oceans. They use a pigment called retinal that captures green light and reflect blues and reds.

SHIL DASSARMA (University of Maryland):

Retinal is much simpler than chlorophyll and it can be produced in a world without any molecular oxygen.


He speculates that green algae evolved later, in the purple shadow of halophilic archaea. Other changes in earth’s atmosphere caused the decline of those purple organisms, so green algae and plants dominate today.  But scientists looking for life on other worlds are hunting for the telltale purple of retinal as well as the green of chlorophyll. I’m Bob Hirshon, for AAAS, the science society.