3D fossil algae over 541 million years old reveal a modern-looking lineage of the plant kingdom

Reconstruction of cross-section with Protocodium sinense, showing similarity with modern Codium. Credit: Shu Chai

Paleontologists have identified a new genus and new species of algae called Protocodium sinense that predates the origin of modern land plants and animals and provides new insight into the early diversification of the plant kingdom.

Discovered at a site in China, this 541-million-year-old fossil is the first and oldest green alga of this age to be preserved in three dimensions, enabling researchers to investigate its internal structure and identify the new specimen with unprecedented accuracy. The study was published today in BMC BiologyOpening a window into a world of evolutionary mysteries that scientists are just beginning to solve.

“Protocodium belongs to a well-known strain of green algae And it has surprisingly modern architecture, showing that these algae were already well diversified before the end of the Ediacaran period, says co-author Cedric Arya, a postdoctoral fellow in the Department of Ecology and Evolutionary Biology in the University of Toronto’s College of Arts and Sciences and is based in Royal Ontario Museum (ROM). Its discovery touches the origin of the whole plant kingdom It puts a familiar name to the organisms that predated the Cambrian explosion more than half a billion years ago, when the world’s first modern ecosystems appeared. “

3D fossil algae 541 million years old reveals a modern-looking lineage of the plant kingdom

Protocodium sinense images using electron microscopy (A) and X-ray tomography (BD). BD panels reveal the internal structure. Credit: Xu Chai, Cedric Arya and Hong Hua

The newly discovered Protocodium fossils were found by a team led by Honghua, professor of geology, including Xu Zhai, a postdoctoral researcher, both from Northwest University, Xi’an, China. It is part of the Gaojiashan flora, the name given to a large group of exceptionally well-preserved fossils, in the Dinging Formation in southern Shaanxi Province. In the past 20 years, this geological formation has produced important fossil species documenting the end of the Ediacaran period 541 million years ago.

Organisms and their parts that do not originally absorb minerals – unlike shells or bones – require exceptional conditions to be preserved. In this case, the entire fossil and its fine cellular detail were preserved in three dimensions due to the replacement of the original organic matter by phosphate. This mode of preservation allowed researchers to use electron microscopy and X-ray techniques to roughly slice the fossil, revealing its precise internal structure and eventually identifying it as a relative of modern Codium, a type of seaweed.

The fossils of the protocodium are small balls half a millimeter wide, like large grains of pollen, covered with a large number of smaller domes. Thanks to the 3D scan, the researchers determined the domed surface to be part of a complex single cell containing thin filaments called a siphon. This shape is typical of some modern unicellular seaweeds that have many nuclei.

3D fossil algae 541 million years old reveals a modern-looking lineage of the plant kingdom

Simplified lineage or tree of life showing the relationship of Protocodium and other green algae, known as chlorophytes, with other major plant lineages. Credit: Cedric Arya

The discovery of Protocodium may require caution when identifying general Ediacaran spherical fossils, and may indicate that organisms such as Codium are in fact much older and widespread. The famous Doushantuo fossil embryos, also from China and preserved in 3D, have been at the center of discussions about the deep origin of some groups of animals. The specific stages of some of these animal-like embryos resemble unicellular Protocodium on the outside, but the 3D slicing reveals how they consist of so many cells. On the other hand, several 2D circular fossils of uncertain algae or other affinities are also known from the Ediacaran and earlier periods, but in less detail.

“We know that the seagrass-like fossils are at least a billion years old,” says Chai, the study’s first author. “But so far, the flat, granular two-dimensional preservation has made it difficult to identify more than general morphological structures.”

3D fossil algae 541 million years old reveals a modern-looking lineage of the plant kingdom

Collage image showing the outer surface of Protocodium sinense (left), view through the outer layer made up of centrioles (middle, coloured) and central siphons (right, coloured). Credit: Cedric Arya

Green algae are photosynthetic organisms, which means they convert light and carbon dioxide into sugars and oxygen. So potentially important foundations for Earth’s early ecosystems, the study indicates that green algae did indeed originate in the world’s shallow waters as recyclers of carbon dioxide and oxygen producers before the Cambrian explosion.

Aside from its smaller size, Protocodium looks surprisingly identical to modern Codium, a type of green algae found in many seas around the world. Certain types of this seaweed are notorious for being invasive – such as the fragile subspecies Codium tomentosoides, dubbed “dead man’s fingers” because of their appearance, and are common alongside commercially farmed oysters. From an evolutionary perspective, green algae Like ancient Protocodium, terrestrial plants share a common ancestor that was thought to be 1 billion to 1.5 billion years old, but is now likely to be even older—the designation of Protocodium very close to a modern group that pushes back the history of the entire plant kingdom.

“It is critical that such an organism has remained practically unchanged for at least 540 million years,” Arya says. “By Ediacaran, evolution has pushed it toward a stable adaptive zone—it has been resting there ever since, and more than that, it has been so successful. So much so that Codium nowadays takes advantage of global trade to easily outcompete other algal species.”

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more information:
A Codium alga stem variety from the newest Ediacaran in southern China provides a taxonomic overview of the early diversification of the plant kingdom, BMC Biology (2022). DOI: 10.1186 / s12915-022-01394-0

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