Friday, May 25, 2007

Green and Pleasant Land

OK, here's an example of Science Fiction biology.

Robert Sawyer's novel "Calculating God" is an interesting read, postulating that life has been designed to converge in such a way that when all known sentients meet, it is possible for them to Take the Next Step together.

It's part of the plot that life everywhere is going to be similar, because it's been designed to be compatible. So in one way, it's not surprising to hear the disembodied voice of the book say at one point, "The plants were all green - chlorophyll . . . was the best chemical for its job no matter what world you were on." But in another, very different way, it makes me mad.

Although I've only been on one world so far, I can confidently state:
- A large percentage of plants (by number) aren't green.
- Much of plant-kind (by biomass) isn't green.
- Chlorophyll is just fabulous, but it often uses a helper of another color to do "its job" well.
- Some things photosynthesize without chlorophyll.

Photosynthesis is the process by which light energy is converted into chemical energy. Light is absorbed by a pigment, often chlorophyll, and the energy of the photon is used to create a high-energy compound called ATP. The major type of reaction splits water to produce oxygen as a waste product. (Lucky for us; we wouldn't be here if the early plants hadn't filled the nice reducing atmosphere with corrosive waste oxygen.) Chlorophyll is best at absorbing red light and blue light. Green light, in the middle of the visible spectrum, isn't well-absorbed. Since green light is reflected off the plant, a plant using chlorophyll appears green to the eye.

Chlorophyll is a complex chemical related to hemoglobin. There is, as Sawyer implies above, a limited number of ways that a reactive metal ion can be shackled in such a way that the important chemical reactions of life can take place on it repeatedly and reversibly. In hemoglobin it's an iron ion in a porphyrin cage. In chlorophyll, it's a magnesium ion. It isn't the only pigment to capture light, though.

Everyone, surely, except the disembodied voice of the book mentioned above, must have travelled down a lane where many of the bushes were Copper Beech, with their striking dark red leaves. Or at least have moved along the tide line on a beach and picked up the long brown belt-like fronds of kelp or the slippery dark-brown tangles of bladderwrack. Or eaten the red-black sheet used to wrap sushi, called nori in Japanese and laver in English – it's a red seaweed called Porphyra.

The different colors come from Accessory Pigments. These are highly-colored compounds that absorb different wavelengths of light from chlorophyll, allowing the plant to use more of the light falling on it. They are "accessory" because they absorb the light but pass the energy on to chlorophyll for the reaction to be completed. There are carotenoids, the bright orange pigment that gives the carrot its familiar color. There are fucoxanthins, the brown pigments in the brown algae and diatoms. Xanthophyll is a bright yellow accessory pigment sometimes used as food coloring. Peridinin. Phycoerythrin, the red in red algae. Phycocyanin, the blue in Cyanobacteria. Cyanobacteria use phycobilproteins – protein-based pigments - to capture energy and pass it on to chlorophyll. Copper beech has its dark anthocyanin pigment.

Between them, the accessory pigments cover a lot of the light that chlorophyll cannot use, and conversely, they provide a bright counterpoint to all the green.

Some things don't use chlorophyll at all, even as a back-end on differently-colored light-absorbing antenna. A non-chlorophyll photosynthetic pigment is Bacteriorhodopsin, a pigment made up of proteins rather than porphyrins. (As the name implies, it's related to rhodopsin, the pigment in our eyes which reacts chemically and triggers a nerve impulse when light of a certain wavelength falls on it.) The reaction does not produce oxygen. It's particularly good at using the otherwise wasted green light. It's found in the Archaea, which are prokaryotes (like bacteria) that have strange eukaryotic (like us) features. Archaea get all the bum gigs in town, living in high salt, high temperature, high pressure, low-oxygen, high-methane areas that no one else wants. They are the sort of things that if one was so inclined, one would say, "Ooh, these would do ok on the surface of some weird planet!"

Photosynthetic life, then, is not always green. It doesn't always use chlorophyll. It doesn't even have to split water and produce oxygen. Bacteriorhodopsin is one example, and there are many related compounds here on Earth, although most of them are proton-pumps that are used in sensing light and for phototaxis rather than energy generation. On other planets, where salinity, temperature and incident light may be different, there would be strong pressure to use and further develop non-chlorophyll pigments like bacteriorhodopsin.

If Earth life is that variable, what is the likelihood that other planets will look green from space? NASA recently published an article on non-green plants on other planets. They're interested because a non-green plant cover might be missed in a survey that was looking for Earth-like planets, so they are developing a way to predict what color plants might be on an alien world. "Not all stars have the same distribution of light colors as our Sun. Study scientists say they now realize that photosynthesis on extrasolar planets will not necessarily look the same as on Earth," says the article.

NASA, even life on Earth does not necessarly look the 'same' as life on Earth. It's all part of life's wonderful diversity. Truth is stranger than speculation.

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