Fossil algae with modern features


View of different layers of the primeval alga Protocodium sinense. © Cedric Aria

In southern China, paleontologists have found three-dimensional fossils of a 541-million-year-old green alga. Detailed investigations of the external and internal structure revealed that the newly discovered Precambrian algal genus belongs to a known phylum of modern algae and bears striking resemblance to specimens alive today. The results suggest that the diversification of algae and the ancestors of land plants began earlier than previously thought. They also shed new light on other fossil finds from the epoch.

Most of today's animal and plant phyla arose in the Cambrian, which began around 541 million years ago. But long before the so-called Cambrian Species Explosion, the oceans were teeming with life. Fossils in the Dengying Formation in the southern Chinese province of Shaanxi testify to the epoch before the Cambrian, the Ediacarian, which began 635 million years ago. This site offers exceptional conditions that help to preserve organisms that, unlike shells and bones, do not absorb minerals. In these organisms, the original organic material was replaced by phosphate over the course of millions of years, thus preserving its original structure.

Protocodium sinense
Protocodium sinense under the electron microscope (A) and under X-ray tomography. © Shu Chai, Cédric Aria and Hong Hua

Get three-dimensional

A research team led by Shu Chai from Northwest University in Xi'an, China, has now described such an exceptionally well-preserved fossil from the Ediacarian: a spherical, unicellular alga with a diameter of around half a millimeter. While all previously known algal fossils from this period survive only as two-dimensional casts, the newly discovered fossil exhibits a well-preserved three-dimensional structure.

With the help of X-ray and electron microscopy, Shu Chai and his colleagues made the inner and outer structures of the fossil visible - and found that the green alga, which is more than 541 million years old, bears a surprising resemblance to the Codium algae that are widespread today. These are found in many seas worldwide and are spreading invasively into new habitats. They benefit from global trade and spread, for example, in tow of commercially farmed mussels.

Modern structure

The researchers named the newly discovered fossil genus Protocodium, in reference to its modern relatives, and named the species Protocodium sinense after its Chinese locality. "Protocodium belongs to a well-known phylum of green algae and exhibits a surprisingly modern architecture, showing that these algae were already well diversified before the end of the Ediacarian period," says Chai's colleague Cédric Aria.

The fossil algae are present as small globules that have a multitude of small domes on the outside. The 3D studies revealed that each sphere's multi-domed surface is part of a single complex cell containing multiple nuclei and criss-crossed by thin strands called siphons. This morphology is also considered typical of modern unicellular algae. "It is very revealing that such an organism remained virtually unchanged for at least 540 million years," says Aria. "By the Ediacarian evolution had driven it into a stable zone of adaptation where it has been comfortable ever since, and quite successfully so."

Older than thought

According to the researchers, the discovery is important for dating the origins of the entire plant kingdom. "We know that algal-like fossils are at least a billion years old," says Chai. This temporal assignment may now have to be shifted further into the past. Because if an alga that can be assigned to a modern phylum already existed more than 541 million years ago, it is likely that the last common ancestor of algae and land plants existed much earlier.

Knowledge of the distribution and diversification of green algae in the Ediacarian can also help to better identify other fossils that are only preserved in two dimensions. "The details of the three-dimensional morphology invite one to reassess the identification of other Ediacarian algae to better distinguish between unicellularity and multicellularity," the researchers write. "They also suggest that unicellular Codium-like forms may be much older and more widespread."

Source: Shu Chai (Northwest University, Xi'an, China) et al., BMC Biology, doi: 10.1186/s12915-022-01394-0

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