Xunlai Yuan, a paleontologist with the Nanjing Institute of Geology and Palaeontology; Shuhai Xiao, assistant professor of geosciences at Virginia Tech; and Thomas N. Taylor, professor of ecology and evolutionary biology at the University of Kansas, report their finding in the May 13 issue of Science ("Lichen-Like Symbiosis 600 Million Years Ago").
Yuan, Xiao, and their collaborators have been exploring the Doushantuo Formation in South China for a decade and have co-authored numerous reports of fossil discoveries, including algae and animal embryos. Taylor, a member of the National Academy of Sciences, is a paleobotanist who has reported on fossil lichens in Scotland.
Lichen is a consortium of two organisms that collaborate to survive in a harsh environment, such as exposed rock. One partner, a cyanobacterium or a photosynthetic alga, or both, are able to form food from carbon dioxide, while the other partner, a fungus, provides moisture, nutrients, and protection for the consortium.
"When and where did they first learn the tricks to form this collaboration?" Xiao asked. "The earliest lichen fossils described by Professor Taylor were from non-marine deposits about 400 million years old, when plants began to massively colonize the land. But did cyanobacteria or other algae form similar relationships with fungi in the marine environment, perhaps long before the evolution of land plants?"
Present-day examples of such relationships in the sea are abundant. Now, there is an example from ancient ocean life.
At a site where abundant algae lived in a shallow sub-tidal environment about 600 m illion years ago, Yuan and Xiao found three specimens that have evidence of two partners in a familiar relationship. "Enlargements of thin-section photomicrographs of the tiny specimens -- each of which was less than a millimeter in size -- show fossils of coccoidal or spherical cells surrounded by a net of fine filaments," Yuan said, describing the new fossils.
The scientists interpret the coccoidal cells as being sheathed cyanobacteria or possibly green algae. "The filaments have reproductive characteristics that make us think they are fungi," Xiao said.
Taylor said, "Clearly, there are two kinds of organisms living together and, we believe, interacting in more than a chance association."
In modern lichens and in the 400-million-year-old Scotland fossils, the coccoidal cells provide the nutrients and the fungal filaments provide protection against dehydration. But in the marine environment, dehydration is not an issue and the 600-million-year-old rocks also contain many fossils of coccoidal cells that are not surrounded by filaments. "So it is a loose lichen-like association," said Xiao. "The organisms are not obliged to live together."
Now there is a new question. "We know that this symbiotic relationship was forged 600 million years ago or earlier. But, was it carried over to land, or did each organism invade land and forge a new relationship independent of the marine relationship? If the latter, then the 600-million-year-old relationship may not be the direct ancestor of the 400-million-year-old relationship," Xiao said.
Fungi and algae in modern lichens can easily marry and divorce, he said. "Given the ease with which the symbiotic relationship is formed, I wouldn't be surprised if the land-based relationships formed independently of the older marine relationships."
"In fact, studies of modern lichens demonstrate that the lichen symbiosis evolved many times," Taylor said.
"The ability to form a symbiotic relationship between fungi and algae may have evolved long before the colonization of land by land-based lichens and green plants, which also form symbiotic relationships with various fungi," Xiao said.
"The Doushantuo Formation opens a window into ancient marine life. There is a lot more remains to be learned from these rocks," Yuan added.