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Photosynthesis the Plankton Way

Photosynthesis is usually associated with plants. But the unusual single-cell algae called dinoflagellates capture the sunís photon energy and pass it along the road to photosynthesis with nearly 100 percent efficiency.

Imagine a technology that could capture energy from the sun, then transfer it with nearly perfect efficiency to a system that converts it to chemical energy. Humans surely canít do this, but the obscure dinoflagellate Amphidinium carterae can.

A team of researchers from Germany and Australia described in the June 21 issue of Science how x-ray crystallography reveals the process by which this organism taps into the sunís power.

If asked to designate the single most important biochemical pathway, most biologists would without hesitation cite photosynthesis. Simply put, nearly all life on the planet depends upon it.

A few years ago, environmental scientists publicly pondered life on Earth without the sun. They envisioned a "nuclear winter" resulting from an event so explosive that dust and debris thrown into the atmosphere would block the sun for a year or two. Whether caused by a volcanic eruption, a meteor crashing to Earth or a nuclear holocaust, the blotting out of sunlight would immediately drop global temperatures and usher in agricultural chaos, with light at perhaps a tenth of its normal intensity. As organisms that use photosynthesis to convert solar energy to chemical energy died, food webs would topple.

Textbooks give an oversimplified definition of photosynthesis: "Carbon dioxide plus water yields oxygen and glucose." As any biology student preparing for an exam knows, photosynthesis is actually a complex set of chemical reactions, centered on pigment molecules that absorb different wavelengths of light.

The ability of living things to capture the sunís energy lies in pigment molecules, which absorb light of specific wavelengths. Certain activated pigments, such as that of the molecule chlorophyll, boost certain electrons to higher energy levels when they absorb photon energy, and these boosted electrons enter the pathway that stores the energy in accessible biochemical forms. Accessory pigments sometimes initially absorb photon energy and then pass it to activated pigments.

Photosynthesis is best studied in plants, which use mostly chlorophyll pigments that absorb wavelengths of 600 to 700 nm and 400 to 500 nm. Chlorophylls absorb in the red and orange and blue and violet, and transmit green, giving plants their color. Accessory pigments generally absorb at wavelengths beyond what chlorophyll can handle, although they may absorb some of the same wavelengths, too.

Key to animal color

Familiar accessory pigments are the carotenoids, which absorb wavelengths between 460 and 550 nm, producing yellow, orange and red colors. Carotenoids provide the distinctive colors of carrots, tomatoes, bananas and squash. Animals do not produce carotenoids but owe some of their more interesting hues to these plant pigments. Colorful fishes, frogs and corals, flamingo feathers and egg yolks, a squidís ink and the bright red color of a boiling lobster arise from carotenoids.

Accessory pigments are particularly predominant in organisms other than plants that can photosynthesize, such as cyanobacteria (blue-green algae), certain bacteria and dinoflagellates. This is no surprise, because relying on different pigments enables these organisms to use wavelengths that a plantís chlorophyll cannot absorb.

Consider Rhodopseudomonas acidophila, a bacterium that lives beneath algae in murky ponds. Researchers from the University of Glasgow described photosynthesis in this bacterium in the April 6, 1995, issue of Nature. Its photosynthetic machinery is a cluster of doughnut-shaped molecular assemblies that capture stray light rays that penetrate the pondís surface, transferring the photons in mere trillionths of a second. The bacteriumís pigments absorb light of 800- to 850-nm wavelengths, which plant chlorophyll ignores.

Enter dinoflagellates

The current work dissecting photosynthesis in a dinoflagellate is important for two reasons. First, it analyzes an organism that lies somewhere between bacteria and plants in complexity. A dinoflagellate is a type of organism called a protist. It is single-celled, but its structure is more like that of an animal or plant than that of a bacterium. Dinoflagellates form plankton, and certain types of them are responsible for periodic "red tides." Secondly, the new work reveals how a predominantly carotenoid-based photosynthetic system works. Previous studies have examined organisms that rely mostly on chlorophyll.

Wolfram Welte and his colleagues at the University of Konstanz in Germany and Roger Hiller and his co-workers at Macquarie University in New South Wales, Australia, used x-ray crystallography to visualize the structure of the molecular complex that the dinoflagellate uses to capture solar energy, generically called an "antenna." Researchers already knew that the organisms have a predominant carotenoid, called peridinin, that absorbs blue-green wavelengths of 470 to 550 nm. This overlaps but extends beyond chlorophyllís range.

Spectroscopy studies had shown that peridinin associates with chlorophyll and protein in a structure called PCP, standing for peridinin-chlorophyll-protein. The x-ray crystallography image revealed that a molecule of the protein forms a roughly boat-shaped structure that contains precisely two chlorophyll molecules and eight peridinins, plus a fatty molecule that may enable the protein to maintain its shape.

It is this precise packing, with the two types of pigments so close, that enables the peridinin to snatch solar energy and in an instant pass it to chlorophyll. The chlorophyll absorbs the energy and releases an electron, and the process of photosynthesis is on its way.

By Dr. Ricki Lewis
Contributing Editor

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