Cyanobacteria and photosynthesis

The story

A trick that changed the world

Every green leaf you see is a small factory. It takes light from the Sun, carbon dioxide from the air and water from the soil, and turns them into sugars, which the plant feeds on, giving off oxygen. This process is called photosynthesis, from the Greek for “putting together with light”. But plants did not invent it: they inherited it from much smaller and much older living things, the cyanobacteria.

Cyanobacteria are bacteria that live in water, and for a long time they were called “blue-green algae”, because they are a colour between green and blue and can be seen as a film on the stones of a river or on the surface of a pond. They are microscopic, but they live in enormous colonies. They are thought to have been the first organisms able to do what is called oxygenic photosynthesis, the kind that splits water molecules and releases oxygen.

The discovery of another way of living

Before them there were already bacteria able to use light, but without producing oxygen: they used other substances, such as hydrogen sulphide, instead of water. Water, however, is everywhere, and splitting it takes a great deal of energy and a very sophisticated chemical machine. Scientists think that the switch to this mechanism was one of the most important events in evolution. Once the cyanobacteria managed it, they had an almost endless resource at their disposal: light, water and carbon dioxide are found everywhere.

We do not know exactly when it happened. The most likely traces go back about 2.7 billion years, and come from rocks of the Tumbiana Formation in Western Australia: microbial mats and stromatolites that left chemical signs of photosynthesis, although some scholars debate whether it produced oxygen. There are hints that it began earlier, perhaps as far back as 3.2 billion years ago, but they are disputed. That is why the date on the timeline is approximate.

Mats in the shallow seas

Cyanobacteria grew in shallow water near the coasts, where the light reached. They formed colourful mats of colonies, one on top of another. By day, in the light, the bacteria produced oxygen and moved upwards; at night, or when they were covered by sand, they climbed up again. In this way, little by little, their mats and the grains of sediment built up in layers and became stromatolites. These layered rocks are the oldest traces of life visible to the naked eye, and in a few places in the world, such as Shark Bay in Australia, living ones can still be found.

Cyanobacteria have not disappeared: they are everywhere. They live in oceans, lakes, soil, deserts and even in the rocks of Antarctica. One of them, Prochlorococcus, discovered only in 1986 by the American biologist Sallie Chisholm and her colleagues, is so small that its cells are less than a thousandth of a millimetre across; it is estimated to be the most numerous photosynthetic organism on Earth, and it produces a large part of the oxygen in the oceans.

Inside today's plants

There is one last chapter. More than a billion years ago, a complex cell swallowed a cyanobacterium without digesting it; the two began to live together, and over time that cyanobacterium became a part of the cell, called a chloroplast. All green algae and all plants have chloroplasts, and inside them there is still a small piece of DNA like that of cyanobacteria. So when you look at a wood, you are looking at descendants of that ancient partnership.

Today it is calculated that about half the oxygen we breathe comes from the plankton of the seas: microscopic algae and cyanobacteria. They are far more important than they look.