New Scientist - USA (2013-06-08)

8 June 2013 | NewScientist | 45

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These photosynthetic

bacteria produce sulphur,

not oxygen

face-to-face with some of these early

photosynthetic microbes. In 2011, Martin

Brasier at the University of Oxford and

colleagues discovered fossils of individual

bacterial cells in rocks that formed 3.43 billion

years ago, in what is now western Australia

(Nature Geoscience, vol 4, p 698). “They

occurred in a well-lit intertidal or supratidal

setting,” says Brasier. The chemical make-up

of the rocks, along with the plentiful light,

strongly suggests that some of the cells

photosynthesised without producing oxygen.

It may seem surprising that anoxygenic

photosynthesis evolved so soon after life

itself – the earliest fossils we know of are only

slightly more ancient, at 3.49 billion years old.

But Nick Lane of University College London,

who studies life’s origins, thinks that once

cells capable of living on chemical energy had

evolved, it was not a huge step for them to

start exploiting light energy instead. “Really,

light just gets electrons flowing through the

same equipment,” he says.

For researchers like Lane, the mystery is

instead why it took so long for the oxygen-

producing form of photosynthesis to evolve.

It may not have emerged until around

2.4 billion years ago, perhaps a billion years

after anoxygenic photosynthesis appeared.

Given the advantages of oxygen-producing

photosynthesis, why the delay?

Photosynthesis has two main steps. In the

second, electrons are added to CO 2 to help

convert the molecule into sugars. The first

step is getting the electrons. They are stripped

from a source molecule and used to generate

an electrochemical gradient that powers the

second step.

The billion-year delay

In oxygenic photosynthesis, the source

molecule is water. Removing electrons splits

water molecules into hydrogen ions and

oxygen gas. The hydrogen ions and electrons

play a key role in turning CO 2 into sugars. The

oxygen, though, is an unneeded by-product.

In anoxygenic photosynthesis, different

molecules provide the electrons. One of the

most common donors is hydrogen sulphide.

Splitting it generates sulphur as a waste

product instead of oxygen. The advantage

of hydrogen sulphide is that it is very easy

to remove electrons from, or oxidise. It was

also common in the early ocean, but probably

got used up quickly in surface waters where

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