Three-dimensional fossil algae over 541 million years old reveal modern-looking ancestors of the plant kingdom

Reconstruction of a cross-section through Protocodium sinense, with resemblance to modern Codium. Credit: Shu Chai

Paleontologists have identified a new genus and species of algae called Protocodium sinense, which predates the origins of land plants and modern 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 from this era to be preserved in three dimensions, allowing the researchers to examine its internal structure and identify the new specimen with unprecedented accuracy. The study was published today in BMC Biologyopening a window to a world of evolutionary puzzles that scientists are just beginning to unravel.

“Protocodium belongs to a well-known line of green algae and has a surprisingly modern architecture, showing that these algae were already well diversified before the end of the Ediacaran period,” said co-author Cédric Aria, postdoctoral fellow in the Department of Ecology & Evolutionary Biology at the Faculty of Arts and Sciences at the University of Toronto and located at the Royal Ontario Museum (ROM). “The discovery touches on the origins of the entire plant kingdom and gives a familiar name to the organisms that preceded the Cambrian explosion more than half a billion years ago, when the world’s first modern ecosystems emerged.”

541-million-year-old 3D fossil algae reveal modern-looking ancestors of the plant kingdom

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

The newly discovered Protocodium fossils were found by a team led by Hong Hua, a professor of geology, and including Shu Chai, a postdoctoral researcher, both of Northwest University, Xi’an, China. It is part of the Gaojiashan biota, the name given to a significant group of exceptionally well-preserved fossils, in the Dengying Formation in southern Shaanxi province. Over the past 20 years, this geological formation has yielded important fossil species documenting the end of the Ediacaran period 541 million years ago.

Organisms and their parts that do not absorb minerals originally – unlike shells or bones – require exceptional conditions to be preserved. In this case, the entire fossils and their fine cellular details have been preserved in three dimensions by the replacement of the original organic material with phosphate. This conservation method allowed the researchers to use various electron and X-ray microscopy techniques to virtually cut the fossil, reveal its internal structure with precision, and eventually identify it as a close relative of the modern Codium algae, a type of seaweed.

Protocodium fossils are tiny spheres half a millimeter wide, like large pollen grains, covered by a multitude of smaller domes. Thanks to the 3D study, the researchers determined that the dome-shaped surface is part of a complex, single cell that contains thin strands called siphons. This morphology is typical of certain modern single-celled seaweeds that contain many nuclei.

541-million-year-old 3D fossil algae reveal modern-looking ancestors of the plant kingdom

Simplified phylogeny or tree of life showing the relationship of Protocodium and other green algae known as chlorophytes to other major genera of plants. Credit: Cédric Aria

The discovery of Protocodium would prompt caution in identifying generic globular Ediacaran fossils, and may imply that organisms such as Codium are in fact much older and widespread. Doushantuo’s famous fossil embryos, also from China and preserved in 3D, have been at the center of debates about the deep origins of certain animal groups. Specific stages of some of these animal-like embryos look like the single-celled Protocodium on the outside, but 3D slicing reveals how they’re made up of many cells. On the other hand, there are also numerous 2D circular fossils of uncertain algae or other affinity known from the Ediacaran and older periods, but in less detail.

“We know that seaweed-like fossils are at least a billion years old,” said Chai, the study’s lead author. “But until now, flat, granular two-dimensional preservation has made it challenging to recognize more than general morphological structures.”

541-million-year-old 3D fossil algae reveal modern-looking ancestors of the plant kingdom

Combined view of the outer surface of Protocodium sinense (left), view through the outer layer consisting of utricles (center, stained) and central siphons (right, stained). Credit: Cédric Aria

Green algae are photosynthetic organisms, meaning they convert light and carbon dioxide into sugars and oxygen. They were therefore likely important foundations of Earth’s early ecosystems, and the study suggests that green algae were established in the world’s shallow waters as carbon dioxide recyclers and oxygen producers even before the Cambrian explosion.

Aside from its smaller size, Protocodium appears surprisingly identical to modern Codium, a type of green algae found in many seas around the world. Certain species of this seaweed are notoriously invasive, such as Codium fragile subspecies tomentosoides, which are called “dead man’s fingers” due to their appearance and spread along with commercially farmed shellfish. From an evolutionary perspective, green algae like the ancient Protocodium and land plants share a common ancestor thought to be about a billion to one and a half billion years old, but probably older now — the assignment of Protocodium so close to a modern group pushes the history of the whole plant kingdom back in time.

“It’s telling that such an organism has remained practically unchanged for at least 540 million years,” says Aria. “At the Ediacaran, evolution had propelled it into a stable adaptive zone — it has been comfortable there ever since, and more than that, quite successful. So much so that today Codium is taking advantage of global trade to easily outperform other algae species.”

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More information:
A strain group of Codium alga from southern China’s newest Ediacaran provides taxonomic insight into the early diversification of the plant kingdom, BMC Biology (2022). DOI: 10.1186/s12915-022-01394-0

Provided by the University of Toronto

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