Fossil algae, dating back 541 million years, provide new insights into the roots of the plant kingdom

Paleontologists have identified a new species and a new species of algae called Protocodium sinensis It 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, the 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 Posted this week in BMC BiologyOpening a window into a world of evolutionary mysteries that scientists are just beginning to solve.

Cross-section reconstruction with Protocodium sinense, showing similarity to modern Codium (illustration by Shu Chai)

Protochodium It belongs to a well-known lineage of green algae and has a surprisingly modern architecture, which indicates that this algae was already well diversified before the end of the Ediacaran period,” says co-author Cedric Aryais a postdoctoral researcher in the Department of Ecology and Evolutionary Biology at the University of Toronto in the College of Arts and Sciences and is based at the Royal Ontario Museum (ROM).

“His discovery touches on the origin of the entire plant kingdom and puts a familiar name on the organisms that predated the Cambrian explosion more than half a billion years ago, when the world’s first modern ecosystems appeared.”

newly discovered Protochodium The fossils were found by a team led by Hong Hua, a professor of geology, and postdoctoral researcher Xu Zhai – both from Northwest 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. In the past 20 years, this geological formation has produced important fossil species documenting the end of the Ediacaran period 541 million years ago.

Simplified lineage or tree of life showing the relationship of Protocodium and other green algae, known as chlorophytes, with other major plant lineages (illustration by Cedric Arya)

Organisms and their parts that do not originally absorb minerals – unlike shells or bones – require exceptional conditions for their preservation. 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 close to modern. codium Moss, a type of seaweed.

Protochodium Fossils are small balls half a millimeter wide, the size of large grains of pollen, covered with a large number of small 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.

Discovery Protochodium May warrant caution when identifying general Ediacaran spherical fossils and may imply that organisms such as codium It is actually much older and prevalent. 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 are similar to unicellular Protochodium From the outside, but the 3D slicing reveals how it is made up of so many cells. On the other hand, several round, 2D fossils of unconfirmed 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.”

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

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

Despite its small size, Protochodium It looks surprisingly consistent with 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 codium Fragile subspecies of tomentosoides, dubbed “dead man’s toes” for their appearance and prevalence alongside commercially farmed oysters. From an evolutionary perspective, green algae like ancient Protochodium Terrestrial plants share a common ancestor that was thought to be between 1 and 1.5 billion years old – but is now likely even older since the designation Protochodium So close to a modern collection that it pushes back in time 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 him into a stable adaptive zone – he’s been comfortable there ever since, and more than that, he’s been very successful – much, in fact, nowadays. codium It takes advantage of global trade to easily outgrow other algae species.”

Funding support for research and field work came from the National Natural Science Foundation of China and the National Research and Development Programme. The Aria Postdoctoral Fellowship is funded by the Polk Milstein Family Trust Fund (ROM) and the Natural Sciences and Engineering Research Council of Canada.