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Protocodium sinense is the first and oldest green alga species from the Ediacaran period (635-539 million years ago) to be preserved in three dimensions, enabling paleontologists to investigate its internal structure with unprecedented accuracy.
Overall morphoanatomy of Protocodium sinense from the Dengying Formation of South China. Scale bars – 100 μm. Image credit: Chai et al., doi: 10.1186/s12915-022-01394-0.
“Protocodium sinense 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,” said Dr. Cédric Aria, a researcher at the University of Toronto and the Royal Ontario Museum.
“Its discovery touches the origin of the entire plant kingdom and puts a familiar name on the organisms that preceded the Cambrian explosion over half a billion years ago, when the world’s first modern ecosystems emerged.”
The microfossils of Protocodium sinense were found in the Dengying Formation in the southern Shaanxi Province, China.
The specimens are small spheres about 0.5 mm in diameter, like large grains of pollen, covered by a multitude of smaller domes.
Thanks to the 3D examination, Dr. Aria and colleagues determined the domed surface to be 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.
Detailed anatomy of Protocodium sinense and comparison with modern Codium fragile ssp. tomentosoides. Scale bars – 20 μm in (a), 10 μm in (b), 50 μm in (c), 20 μm in (d), 500 μm in (f). Image credit: Chai et al., doi: 10.1186/s12915-022-01394-0.
Apart from its smaller size, Protocodium sinense appears surprisingly identical to the modern Codium, a type of green algae found in many seas worldwide.
Certain types of this seaweed are notoriously invasive — such as Codium fragile subspecies tomentosoides — and spread along with commercially farmed shellfish.
From an evolutionary perspective, green algae like the ancient Protocodium sinense and land plants share a common ancestor that was thought to be about one billion to one billion and a half years old, but now likely older — the assignment of Protocodium sinense 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 has remained practically unchanged over at least 540 million years,” Dr. Aria said.
“By the Ediacaran, evolution had driven it towards a stable adaptive zone — it’s been comfortable there since, and more than that, quite successful.”
“So much so, in fact, that nowadays Codium takes advantage of global trade to easily outcompete other algal species.”
The discovery is reported in a paper in the journal BMC Biology.
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S. Chai et al. 2022. A stem group Codium alga from the latest Ediacaran of South China provides taxonomic insight into the early diversification of the plant kingdom. BMC Biol 20, 199; doi: 10.1186/s12915-022-01394-0
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