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12/09/2007

Children of the sun

Biochemist Gottfried Schatz follows light across time and space, from the Big Bang to the ocean floor.

In the beginning was light. The Big Bang, which created the universe roughly 14 billion years ago, was a radiant explosion of energy. Then, as the universe expanded and cooled, the light slowed and turned into invisible radio waves. After just several hundred thousand years, a darkness set in. This lasted 30 million years, during which time part of the radiation condensed to matter. This matter, in turn, conglomerated into gas clouds, and then into galaxies. Within these, the immense force of gravity compressed atomic nuclei with such force that they fused with each other and released formidable amounts of energy as light. The atomic fire of these first stars once more brought light to the universe.

Some stars found no stable orbits, and ultimately - together with the rest of the gas cloud - hurtled into a black hole in the centre of their galaxies. With this fatal plunge, these stars heated to such a degree that for a short time they shone brighter than all the billions of stars in the rest of their galaxy. Most of these gigantic but short-lived light sources died out roughly ten billion years ago. Nevertheless we can still see them today, because the rapid expansion of the universe after the big bang propelled them - together with their galaxies - so far into the outer reaches of the universe that their light is only reaching us now.

Many stars ultimately burned out. Others exploded, and in so doing created the matter for new stars. One of these is our sun, which first began shining some 4.5 billion years ago. Like some other stars, it catapulted matter into its surroundings as it formed, which then became the planets. On one of these planets, tiny clumps of matter formed ever more complex assemblies, which multiplied, moved and finally even developed consciousness and intelligence.

I am a late descendent of this nobility of highly ordered matter. My lineage is over 3.5 billion years old, and gives me every reason to be proud. Very early on, my ancestors "invented" the green solar collector chlorophyll, and in this way were able to feed themselves from light. To avoid dangerous ultra-violet light when the sunlight became too bright, they developed sensors for short-wave blue light, and so could see the world in colour. I carry three chemical offspring of this blue-sensitive sensor in my retina - they recognise blue, green and red. Since their colour spectra overlap and my brain compares their signals with each other, I can see not only three, but millions of different colours.

And yet I am almost blind, because what I sense as light is only a tiny fraction of all electro-magnetic waves. These range from 1,000-kilometre radio waves to the gamma rays of exploding stars measuring just billionths of a billionth of a metre. My eyes only recognise wavelengths between 400 and 700 billionths of a metre, and signal them to my brain as the colours of the rainbow - from blue-violet to deep red.

My sunlight-eating predecessors drastically changed the face of our planet. Since they fed on an inexhaustible source of energy, they grew over land and sea, releasing oxygen from water as they did so. This gas gradually accumulated in the atmosphere, which until then had been oxygen-free. Soon some living beings developed the ability to burn the remains of other cells with the help of this gas - they "invented" respiration. Our bodies and our nourishment are stored light energy - a faint reflection of the atomic fire in our sun.

Reverberations of this fire even glimmer in the depths of our oceans. These are by far the largest natural habitat on our planet, yet below a depth of 1,000 metres they are dark, cold and desolate, often scarce in oxygen. How do living things orient themselves in this limitless darkness? How do they find their prey - or their partners? And how do they recognise predators in time to flee? Much of this is still shrouded in mystery. Yet we do know that most deep sea creatures accomplish these tasks using light signals. Generally they create these in special light organs, in which nerve impulses trigger reactions between body substances and oxygen to create "cold" light - as glow worms do. Some fish even harbour light-producing bacteria in their lacrimal sacs, and switch these ingenious headlamps on and off by moving their skin.

Many ocean bacteria also create light. But unlike them, fish, cephalopods and crustaceans mostly emit their light in pulses which may carry information. The light emitted is almost always blue-green, since such light penetrates water particularly easily. For this reason, many deep-sea fishes make do with a single, blue-green sensitive visual pigment, conducting its signals highly amplified to the brain. While they can see no colours, they can locate short or weak light signals with a high degree of accuracy.

Yet a pulse of light can also alarm prey or lure predators. The black dragon fish, which lives in depths below 1,000 metres, avoids this danger by producing not only blue-green, but also deep red light, invisible for other animals. Using this private frequency it can communicate undisturbed with other members of its species, and prey on unsuspecting victims. Its red-light emitters first create blue-green light, and then modify it with the help of an additional pigment. Then this red light is further "purified" with a coloured lens. To perceive the deep red light of its fellows, the black dragon fish stores a variant of green chlorophyll in its eyes which effectively absorbs red light and somehow transmits its energy to the blue-green sensor of the retina. Since it cannot produce chlorophyll itself, it probably ingests it with its food supply - but we are still in the dark about the source. Whatever the answer may be, all light-generating substances as well as the oxygen they need for their reactions stem ultimately from trapped solar energy. The pulsing points of light in the depths of our oceans are distant offspring of the sunlight.

Much about us and the world is mysterious and dark. And the darkness of our prejudices is more threatening than that of the ocean depths. Our reasoning offers us light in this darkness. Its glow is but weak - and yet it is the most wondrous of all the sun's children.

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The article originally appeared in German in the Neue Zürcher Zeitung on August 23, 2007.

Gottfried Schatz, born in Austria in 1936, is professor emeritus at Basel University. A biochemist specialising in mitochondria, he has taught as visiting professor at Harvard and Stanford universities. In his younger years he was also violinist in several Austrian opera houses.

Translation: jab.
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