Biologists pinpoint fossils of first photosynthesising bacteria

Photosynthesis first evolved in living organisms at least 1.75 billion years ago, according to a new study into ancient organisms by a team of Belgian biologists.

The updated timescale is the result of their study into fossilised cyanobacteria (sometimes referred to as ‘blue-green algae’) sourced from the McArthur Basin, which stretches from the northern fringe of Australia along the Arafura Sea and Gulf of Carpentaria.

The group from the University of Liège believe they’ve found in microfossils of Navifusa majensis the “oldest direct evidence” of specialised biological structures – thylakoid membranes – which are essential to oxygenic (oxygen-producing) photosynthesis.

A cyanobacteria
A photomicrograph of a modern-day cyanobacterium. Credit: N Nehring via Getty Images

Photosynthesis is the chemical process by which plants and some single-celled organisms create energy. In the cells of most photosynthesising organisms, carbon dioxide and water are converted using light energy into sugar and oxygen.

Typically, this occurs at sites in plant cells called chloroplasts. Within these structures are thylakoids, which house the green pigment chlorophyll that absorbs sunlight for use in photosynthesis.

Unlike plants, cyanobacteria don’t possess chloroplasts.

They do have thylakoids, which is why evolutionary biologists think these single-celled organisms were incorporated into complex plant cells as chloroplasts.

But not all cyanobacteria produce their energy using thylakoids – the genus Gloeobacter instead photosynthesises via light-capturing protein structures in their plasma membrane.

Understanding cyanobacteria evolution could help scientists understand a major change in Earth’s history.

Oxygen-producing photosynthesis is the likely cause of a major historic spike of oxygen in Earth’s atmosphere about 2.4 billion years ago – vital for the development of life on our planet. Scientists working across several disciplines have struggled to pinpoint the precise cause of this so-called Great Oxidation Event, however the Liège study published today in the journal Nature at least pushes the dial backwards.

Led by PhD student Catherine Demoulin and Dr Emmanuelle Javaux, the research team studied N. majensis microfossils obtained from rock formations in the Northwest Territories in Canada, the Democratic Republic of Congo and the McDermott Formation in the Northern Territory, Australia. Of these, the McDermott samples were the oldest – dating back 1.75 billion years – providing a new minimum timepoint for the emergence of thylakoid-containing cyanobacteria.

Aerial photo of stromatolites
Stromatolites formed by billion-year-old cyanobacteria at the Hamelin pools in Western Australia. Credit: Intst via istock/Getty Images Plus

In their study, they note the specimens were highly preserved, allowing the arrangement of thylakoid membranes to be microscopically observed.

“Thylakoids represent direct ultrastructural evidence for oxygenic [oxygen-producing] photosynthesis,” they write.

“The discovery of preserved thylakoids within N. majensis reported here provides direct evidence for a minimum age of about 1.75 Ga for the divergence between thylakoid-bearing and thylakoid-less cyanobacteria.

“By probing the older fossil record, it may also allow testing of the hypothesis that the emergence of thylakoid membranes may have contributed to the rise in oxygen around the GOE, and to the permanent oxygenation of the early Earth.”

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