The poem “Trees Are Made of Thin Air” has a line that sounds like a mistake. The leaf takes in carbon dioxide, it says, uses light to stitch the carbon into sugar, and lets the leftover oxygen go. Then comes the twist: that oxygen, the poem insists, comes “from water, not the air”. Most people assume it must come from the carbon dioxide. After all, carbon dioxide has two oxygens, the plant keeps the carbon, and oxygen comes out. It looks like simple subtraction. It isn’t, and the story of how we found out is one of the neatest pieces of detective work in biology.
For a long time, scientists assumed the obvious: that a leaf splits carbon dioxide, keeps the carbon, and breathes out the oxygen. The first serious doubt came in the 1930s from a Dutch-American microbiologist named Cornelis van Niel, who was studying, of all things, purple bacteria living in sulphur springs. These bacteria photosynthesise too, using light to build sugar from carbon dioxide. But they don’t release oxygen. Instead, they take in hydrogen sulphide, the gas that smells of rotten eggs, and they release sulphur.
Van Niel saw a pattern. The bacteria need hydrogen to turn carbon dioxide into sugar, because sugar is made of carbon, hydrogen and oxygen. They get that hydrogen by splitting hydrogen sulphide, and the sulphur is the leftover. So what if green plants were doing exactly the same thing, but with water? Water is hydrogen and oxygen. Split it, use the hydrogen to build sugar, and the leftover would be oxygen. It was an elegant guess. But it was only a guess, until someone could follow an oxygen atom through a leaf and see where it went.
That became possible in 1941. A team of chemists in California got hold of oxygen-18, a rare, heavy form of oxygen with two extra neutrons in its nucleus. It behaves chemically just like ordinary oxygen, but it’s heavier, so a machine that sorts atoms by weight can tell the two apart. They gave plants water made with heavy oxygen, and ordinary carbon dioxide, and collected the gas the plants released. It contained heavy oxygen. Then they did the reverse: heavy carbon dioxide, ordinary water. This time, the oxygen released was ordinary. The oxygen had come from the water. Van Niel was right, and so is the poem.
We now know where in the leaf the splitting happens, and it is extraordinary. Inside the chloroplasts, light is captured by chlorophyll and its energy is passed, in a tiny fraction of a second, to a protein machine called photosystem II. At its heart sits a tiny cluster of four manganese atoms and one calcium atom, held in a cage of oxygen. Each time a particle of light arrives, the cluster gives up one electron. After four such hits, it has built up enough charge to tear apart two water molecules at once, releasing four hydrogen ions, four electrons, and one molecule of oxygen gas. The electrons and hydrogen go on, through a chain of other proteins, to power the building of sugar from carbon dioxide. The oxygen drifts out through the leaf’s pores into the air.
Splitting water is hard. Water is a very stable molecule, which is why it doesn’t fall apart when you boil it. No human-made catalyst does it as efficiently at room temperature, and chemists trying to make fuel from sunlight study this little cluster closely, hoping to copy it.
The invention is ancient. The first organisms to split water were bacteria, ancestors of today’s cyanobacteria, and when they began, more than two and a half billion years ago, the air contained almost no oxygen at all. Over hundreds of millions of years, the waste gas from their water-splitting built up, transforming the atmosphere and making it possible for animals like us to exist. Plants later took the same machinery into their own cells, and the chloroplast in a leaf is a descendant of one of those bacteria.
So the line in the poem is exactly right. When you breathe in, the oxygen filling your lungs was, not long ago, part of a water molecule, perhaps drawn up through a tree’s roots, perhaps in the sea, where tiny plankton make about half of the world’s oxygen. Every breath you take is made of split water.