June 8, 2011

Ancient Microorganisms Had Protective Armor: MIT

Geologists at MIT and Harvard have discovered fossils along the Alaska-Canada border that reveal protective plates for microscopic organisms. 

Phoebe Cohen, a postdoc in MIT's Department of Earth, Atmospheric and Planetary Sciences, and Francis Macdonald, an assistant professor of geology at Harvard University, spent two weeks chiseling out rock samples during the summer of 2007 in a remote mountain range in the Yukon.

They brought the rocks back to Cambridge and made a surprising discovery: The ancient carbonate contained hundreds of remarkably well-preserved fossils resembling tiny, shield-like plates.

Cohen, a Harvard PhD student at the time, says single-celled organisms may have produced the plates as armor, in a process called biomineralization.

Today, many organisms have evolved the ability to produce mineral structures.  For instance, mollusks generate shells, while mammals and birds form bone.  The 700-million-year-old fossils discovered by Cohen and Macdonald may be the oldest evidence yet of biomineralization, the scientists said.

In 1979, researchers from the University of Alaska identified strange fossils in the Yukon region "” not in carbonate, but in sections of glasslike rock known as chert.   However, it was unclear at the time just how old the fossils were.   During the current expedition, Cohen and Macdonald found the same fossils for the first time in carbonate.

In the lab, they were able to date the rock, and the fossils, to between 717 million and 812 million years old, placing them in the middle of the Neoproterozoic era.   This time period was one in which single-celled organisms likely flourished just prior to the first "Snowball Earth" event, in which vast sheets of ice likely covered the Earth.

Cohen says these microorganisms were likely killed off in the global freeze.  However, the fossils they left behind may give researchers clues to the complexity of life just before the planet froze over.

"These fossils "¦ fit in with a huge diversity of other single-celled eukaryotic organisms that seem to have evolved approximately at the same time," Cohen said.

To get a better look at the fossils, Cohen and Macdonald dissolved rock samples in weak acid, and mounted the residue on stubs.  Then they created high-resolution, three-dimensional images of the microscopic fossils using scanning electron microscopy.

The images revealed plates, each about 20 microns wide, arranged in a honeycomb pattern, with teeth-like spines jutting out and rimming the perimeter.

It's unclear which modern organisms might be related to these fossils, but on close examination, Cohen observed similarities between the ancient fossil patterns and those formed by modern-day coccolithophores "” spherical, single-celled algae found in enormous blooms throughout the ocean.

These tiny organisms create mineralized plates within their vacuoles, ultimately extruding the plates to the surface to form protective coverings.

The researchers believe the tiny fossil plates they observed may have formed via a similar process, although it remains a mystery why simple organisms evolved such a complex process.

"It takes a lot of effort, energy and just sheer biomass to create these [plates]," Cohen said.

"One of the big questions is: Why are these organisms making these intricate, detailed, morphologically complex structures?"

One theory is that spines and plates may help small organisms stay afloat. Today, coccolithophores are found within the photic zone of the ocean, at depths where light can reach.  Maintaining a "sweet spot" within this zone enables plankton to grow and thrive "” an advantage their ancient counterparts may also have evolved. 

The plates may also have served as armor. Hard mineral coverings may have put off predators for two reasons: protective shields simply make it harder to get at an organism, and a plate's mineral composition may have made the organisms less appealing to nutrient-seeking predators.

Cohen hopes the results will spur more researchers to investigate other such early signs of complex life, in rocks of the same time period from around the world.

"These fossils are really small and hard to find," she said.

"And maybe the fossils are there, we just have to look for them."

The findings are reported this week in the journal Geology. 


Image Caption: An image of the microfossil Characodictyon taken with a scanning electron microscope. This fossil was extracted using very weak acid from a carbonate rock. It's about 20 microns long - one-fifth the width of a human hair. Credit: Cohen/Macdonald


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