Ancient Stromatolites May Have Been Edged Out By Foraminifera
May 29, 2013

Widespread Disappearance Of Stromatolites May Have Been Driven By Foraminifera

April Flowers for - Your Universe Online

Stromatolites, “layered rocks” made of calcium carbonate, were the earliest visible manifestation of life on Earth. A team of researchers, including scientists from the Woods Hole Oceanographic Institution (WHOI); Massachusetts Institute of Technology (MIT); the University of Connecticut; Harvard Medical School; and Beth Israel Deaconess Medical Center, suggest that the disappearance of stromatolites may have been driven by single-celled organisms called foraminifera.

Stromatolites are shaped by the actions of photosynthetic cyanobacteria and other microbes. The organisms trapped and bound grains of coastal sediment into fine layered structures, which showed up in great abundance along shorelines around the world about 3.5 billion years ago.

“Stromatolites were one of the earliest examples of the intimate connection between biology–living things–and geology–the structure of the Earth itself,” said WHOI geobiologist Joan Bernhard.

The bacterial community bound the sediments around themselves by secreting sticky compounds that created a mineral “microfabric.” These microfabrics accumulated to become massive formations that dominated the coastal scene for more than two billion years, until late in the Proterozoic Eon.

“Then, around 1 billion years ago, their diversity and their fossil abundance begin to take a nosedive,” said Bernhard. Over a period of millions of years, the stromatolites began to disappear all over the globe. This loss was almost as dramatic, in paleontological circles, as the extinction of the dinosaurs millions of years later. The stromatolite disappearance wasn´t as complete as the dinosaurs, however. Living examples can still be found in limited and widely scattered locales.

The impact of a large meteorite has explained the extinction of the dinosaurs, for the most part. The stromatolite disappearance, however, is unsolved. “It´s one of the major questions in Earth history,” said WHOI microbial ecologist Virginia Edgcomb.

Another mystery that is just as puzzling is the sudden appearance of different formations called thrombolites — “clotted stones” — in the fossil record. Thrombolites are formed by the action of microbes on sediment and minerals, just like stromatolites. Unlike stromatolites, however, thrombolites are clumpy rather than finely layered.

Scientists do not know whether stromatolites became thrombolites, or whether thrombolites arose independently. Theories proposed to explain both scenarios include changes in ocean chemistry and the appearance of multicellular life forms that might have preyed on the microbes responsible for their structure.

The WHOI scientists thought foraminifera, or forams for short, might have played a role. Forams are protists, a kingdom which includes amoeba, ciliates, and other groups formerly referred to as “protozoa.” In modern ocean sediments, where they use numerous slender projections called pseudopods to engulf prey, to move, and to continually explore their immediate environment, forams are abundant. Before this study, however, their role in the loss of stromatolites and appearance of thrombolites has never been considered, despite their known ability to disturb modern sediments.

Searching for the presences of forams, the researchers examined modern stromatolites and thrombolites from Highborne Cay in the Bahamas using microscopic and rRNA sequencing techniques. They found forams in both kinds of structures, but especially in the thrombolites, which were home to a great diversity of foraminifera. They were especially rich in forams that secrete an organic sheath around themselves, called “thecate” foraminifera. Thecates were probably the first forams to evolve, not long before stromatolites began to disappear.

“The timing of their appearance corresponds with the decline of layered stromatolites and the appearance of thrombolites in the fossil record,” said Edgcomb. “That lends support to the idea that it could have been forams that drove their evolution.”

The team created an experimental scenario that mimicked what might have occurred a billion years ago, starting with chunks of modern-day stromatolites collected at Highborne Cay. These were seeded with forams found in modern thrombolites, and the scientists waited to see what would happen.

The finely layered arrangements of stromatolites changed to a jumbled arrangement more like that of a thrombolite after about six months. CAT scans revealed that even their fine structure resembled thrombolites collected from the wild. “The forams obliterated the microfabric,” said Bernhard.

Although the researchers found this result intriguing, it was not proof that the changes were due to the activities of the forams, as just being brought into the lab might have caused the changes. The team included a control in their experiment by seeding freshly-collected stromatolites with forams, then treating them with colchicines, a drug that prevents them for sending out pseudopods. “They´re held hostage,” said Bernhard. “They´re in there, but they can´t eat, they can´t move.”

After six months, the control group of stromatolites still had forams present and alive, but the rock´s structure had not changed from finely layered to clotted. This led the scientists to conclude that active foraminifera can reshape the fabric of stromatolites, and might have instigated the loss of those formations and the appearance of thrombolites.

The findings of this study were published in the Proceedings of the National Academy of Sciences.