[Science news] - The origin of green seaweeds elucidated

 
 

Researchers from Ghent University and Meise Botanic Garden have elucidated, for the first time, the origins of green seaweeds. The early evolution of green seaweeds likely shaped early animal evolution by altering benthic ecosystems and furnishing novel ecological niches. The results were published today in the journal PNAS (Proceedings of the National Academy of Sciences).

Green algae emerged more than 1 billion years ago. At the beginning of their evolution, a split gave rise to two groups, one from which land plants later emerged, about 400 million years ago; the other gave rise to a wide diversity of green algae from freshwater and marine waters, including the green seaweeds (macroscopic marine algae) that play a prominent role in marine ecosystems.

Due to their antiquity, the evolutionary relationships between the different groups of green seaweeds are very difficult to resolve and have remained unknown for a long time. By comparing the genes of a large selection of green seaweeds, combined with data from fossils, researchers have been able to clarify these relationships and identify important evolutionary events that have taken place over geological time.

The results indicate that green seaweeds diversified about 650-750 million years ago, at the end of the Neoproterozoic. This geological period was affected by two major glaciations in which ice caps reached the equator. The ancestors of green seaweeds survived these extreme climatic conditions in isolated refuges at the bottom of shallow seas. Isolation of millions of years resulted in the independent evolution of different green algal groups. After these glaciations, temperatures rose (Ediacaran geological period) releasing and opening up the sea floor. In these conditions, green seaweeds were able to disperse and diversify.

An increased supply of nutrients and biotic interactions such as grazing by animals probably triggered the evolution towards macroscopic growth in the different groups of green algae through different strategies, such as multicellularity (e.g. Dictyosphaeria, see picture) and the evolution of giant cells (e.g. Caulerpa, see picture). In turn, the proliferation of green seaweeds caused fundamental changes in shallow marine ecosystems that likely played a key role in the evolution of marine animals.

 

Scientific article: Del Cortona A., Jackson C. J., Bucchini F., Van Bel M., D’hondt S., Škaloud P., Delwiche C. F., Knoll A. H., Raven J. A., Verbruggen H., Vandepoele K., De Clerck O. & Leliaert F. (2019) Neoproterozoic origin and multiple transitions to macroscopic growth in green seaweeds. PNAS: www.pnas.org/cgi/doi/10.1073/pnas.1910060117

 
   

The tropical green seaweed Dictyosphaeria, a multicellular alga composed of several giant cells. Photo: Frederik Leliaert


 
The strange world of the Ediacarian (630-542 million years ago), a period in which green seaweeds were able to diversify after the severe ice ages of the Cryogenian (Wikimedia Commons contributors, "File:Life in the Ediacaran sea.jpg" (accessed January 3, 2020) (CC BY-SA 2.0)  
The tropical green seaweed Caulerpa consists of a single giant cell.