Fossil algae predates the origin of land plants and modern animals

Palaeontologists have discovered an algae fossil in China, identifying it as a new genus and species called Protocodium sinense. The ancient fossil — 541m years old — predates the origin of land plants, giving scientists new insights into the early diversification of the plant kingdom.

And interestingly the fossil is the first and oldest green algae from this era to be preserved in three dimensions, which has enabled scientists to investigate its internal structure with unprecedented accuracy.

They found that Protocodium appears almost identical to its close relative the modern Codium – a type of green algae found today in many seas – which pushes back the point in history that green algae and land plants shared a common ancestor. 

The findings have been reported in a new study in BMC Biology.

Protocodium” belongs to a known lineage of green algae and has a surprisingly modern architecture, showing that these algae were already well diversified before the end of the Ediacaran period,” says co-author Cédric Aria, postdoctoral fellow in the Department of Ecology & Evolutionary Biology at the University of Toronto and based at the Royal Ontario Museum (ROM) in Canada.

“Its discovery touches the origin of the entire plant kingdom and puts a familiar name on the organisms that preceded the Cambrian explosion over half a billion years ago, when the world’s first modern ecosystems emerged.”

Read more: Early plants evolved from algae, fossils show

Green algae can photosynthesise, converting light and carbon dioxide into sugars and oxygen. They were therefore likely important foundations of Earth’s early ecosystems, and the study suggests green algae were already established in the world’s shallow waters, as carbon dioxide recyclers and oxygen producers, before the Cambrian explosion.

Algae fossil
Images of Protocodium sinense using electron microscopy (A) and X-ray computed tomography (B-D). Panels B-D reveal the internal structure. Credit: Shu Chai, Cédric Aria and Hong Hua

The Protocodium were discovered as part of the Gaojiashan biota, a significant group of exceptionally well-preserved fossils at the Dengying Formation in the southern Shaanxi Province in north-west China.

“We know that seaweed-like fossils are at least one billion-years-old,” says first author Dr Shu Chai, postdoctoral researcher at Northwest University in Xi’an in Shaanxi Province. “But until now, flat, grainy two-dimensional preservation has made it challenging to recognise more than general morphological structures.”

The whole fossil and its fine cellular details were preserved in three dimensions in a process called phosphatisation where the original organic material was replaced by phosphate, allowing researchers to use electron and X-ray microscopy to virtually slice the through the fossil to reveal its internal structures.

Protocodium fossils are spherical and small (only about half a millimetre wide), with a surface covered by many smaller domes. Inside, the complex, single cell contains thin strands called siphons surrounded by a uniform layer of bulb-shaped structures – a morphology that is typical of certain modern single-celled seaweeds from the genus Codium.

“It’s very telling that such an organism has remained practically unchanged over at least 540 million years,” says Aria. “By the Ediacaran, evolution had driven it towards a stable adaptive zone — it’s been comfortable there since, and more than that, quite successful. So much so, in fact, that nowadays Codium takes advantage of global trade to easily outcompete other algal species.”

Simplified phylogeny or tree of life showing the relation of protocodium and other green algae known as chlorophytes to other major lineages of plants. Cedric aria 850
Simplified phylogeny or tree of life showing the relation of Protocodium and other green algae, known as chlorophytes, to other major lineages of plants. Credit: Cédric Aria

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