Fossil algae, dating to 541 million years ago, offer new insights into the roots of the plant kingdom

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

Discovered at a site in China, the 541-million-year-old fossil is the first and oldest green algae from that time to be preserved in three dimensions, allowing researchers to study its internal structure and identify the new specimen with unprecedented precision.

The study was published this week in BMC Biologyopening a window into a world of evolutionary puzzles that scientists are only just beginning to solve.

Reconstruction of a cross-section through Protocodium sinense, showing resemblance to modern Codium (illustration by Shu Chai)

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”, explains the co-author Cedric Ariapostdoctoral researcher in the Department of Ecology and Evolutionary Biology, Faculty of Arts and Science, University of Toronto, based at the Royal Ontario Museum (ROM).

“His discovery touches on the origin of the entire plant kingdom and gives 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 emerged. .”

The newly discovered Protocodium fossils were found by a team led by Hong Hua, professor of geology, and postdoctoral researcher Shu Chai – both from Northwestern University in Xi’an, China. It is part of the Gaojiashan Biota, the name given to a large group of exceptionally well-preserved fossils in the Dengying Formation in 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.

Simplified phylogeny or tree of life showing the relationship of Protocodium and other green algae, called chlorophytes, to other major plant lineages (illustration by Cédric Aria)

Organisms and their parts that do not absorb minerals originally – unlike shells or bones – require exceptional conditions to be preserved. In this case, the whole fossils and their fine cellular detail have been preserved in three dimensions by replacing the original organic matter with phosphate. This mode of preservation allowed researchers to use various electron and X-ray microscopy techniques to virtually cut up the fossil, unveil its internal structure with precision, and ultimately identify it as a close relative of the modern fossil. Codium seaweed, a type of seaweed.

Protocodium the fossils are small spheres half a millimeter wide, the size of large grains of pollen, covered with a multitude of smaller domes. Through 3D examination, the researchers determined that the domed surface was part of a complex single cell containing thin strands called siphons. This morphology is typical of some modern unicellular algae which contain many nuclei.

The discovery of Protocodium would call for caution when identifying generic spherical Ediacaran fossils and may imply that organisms like Codium are actually much older and more widespread. The famous Doushantuo fossil embryos, also from China and preserved in 3D, have been at the heart of debates about the deep origin of certain groups of animals. Specific stages of some of these animal-like embryos resemble the single-celled Protocodium on the outside, but the 3D cutout reveals how they are made up of many cells. On the other hand, numerous 2D round fossils of uncertain algae or other affinities are also known from the Ediacaran and earlier periods, but in less detail.

“We know that algae-like fossils are at least a billion years old,” says Chai, the study‘s first author. “But until now, flat, grainy two-dimensional preservation has made it difficult to recognize other than general morphological structures.”

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

Green algae are photosynthetic organisms, which means 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 already established in the world’s shallow waters as CO2 recyclers and oxygen producers before the Cambrian explosion.

Besides its small size, Protocodium appears strikingly identical to the modern Codium, a type of green algae found in many seas around the world. Some types of these algae are notoriously invasive – such as Codium fragile subspecies tomentosoides, nicknamed “dead man’s fingers” for its appearance and which spreads with commercially farmed shellfish. From an evolutionary point of view, green algae like the ancient Protocodium and land plants share a common ancestor thought to be around a billion to a billion and a half years old – but is now probably older since the assignment of Protocodium so close to a modern group pushes back in time the history of the entire plant kingdom.

“It’s very telling that such an organism remained virtually unchanged for at least 540 million years,” says Aria. “By the Ediacaran, evolution had pushed it into a stable adaptive zone – it’s been comfortable there ever since and, more than that, quite successful – so much so, in fact, that today Codium takes advantage of world trade to easily outcompete other algae species.

Financial support for research and fieldwork came from the National Natural Science Foundation of China and the National Key Research and Development Program. Aria’s postdoctoral fellowship is funded by the Polk Milstein Family Trust Fund (ROM) and the Natural Sciences and Engineering Research Council of Canada

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