Photosynthesis: Shaping the planet

Earth 30 January 2013

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Stromatolites are fossilised deposits formed by ancient cyanobacteria

(Image: Mint Images/Rex Features)

Read more:Instant Expert 30: Photosynthesis

When photosynthetic organisms first began to generate oxygen, it marked the start of a transformation of our world. Oxygen provided access to a more efficient source of energy through respiration, a process that would ultimately allow multicellular animals to evolve.

Now, more than 2 billion years after these changes began, the world is transforming again. Our emissions are causing damaging climate change. So how will photosynthetic organisms adapt to a warmer planet, and what are the implications for our biosphere?

Past: First breath

When the first bacteria began to harness light energy some 3.4 billion years ago, Earth’s atmosphere was mainly composed of nitrogen and CO2. These anaerobic photosynthetic organisms relied on hydrogen, or organic or sulphur compounds as a source of electrons. Then, about 2.4 billion years ago, the “great oxygenation event” kicked off. Photosynthetic organisms evolved – probably ancestors of present-day cyanobacteria – that were capable of splitting water to produce oxygen. Now oxygen levels in the biosphere began to rise (see timeline).

The changes that occurred over the next 2 billion years resulted in the extinction of many anaerobic organisms – it would have been a dramatic and cataclysmic transformation. Yet the arrival of oxygen was not entirely bad news.

Ultraviolet radiation from the sun hit oxygen in the upper layers of the atmosphere, and the subsequent reaction created ozone (O3). The layer of it in the stratosphere filtered out this harmful ultraviolet light, which damages DNA, helping life spread out of the deep …

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