The widespread disappearance of stromatolites, the earliest visible manifestation of life on Earth, may have been driven by single-celled organisms called foraminifera. Stromatolites (“layered rocks”) are structures made of calcium carbonate and shaped by the actions of photosynthetic cyanobacteria and other microbes that trapped and bound grains of coastal sediment into fine layers. They showed up in great abundance along shorelines all over the world about 3.5 billion years ago.
The growing bacterial community secreted sticky compounds that bound the sediment grains around themselves, creating a mineral “microfabric” that accumulated to become massive formations. Stromatolites dominated the 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. All over the globe, over a period of millions of years, the layered formations that had been so abundant and diverse began to disappear. To paleontologists, their loss was almost as dramatic as the extinction of the dinosaurs millions of years later, although not as complete: Living stromatolites can still be found today, in limited and widely scattered locales, as if a few velociraptors still roamed in remote valleys.
While the extinction of the dinosaurs has largely been explained by the impact of a large meteorite, the crash of the stromatolites remains unsolved. “It’s one of the major questions in Earth history,” said WHOI microbial ecologist Virginia Edgcomb, a co-author on the paper.
Just as puzzling is the sudden appearance in the fossil record of different formations called thrombolites (“clotted stones”). Like stromatolites, thrombolites are produced through the action of microbes on sediment and minerals. Unlike stromatolites, they are clumpy, rather than finely layered.
It’s not known whether stromatolites became thrombolites, or whether thrombolites arose independently of the decline in strombolites. Hypotheses proposed to explain both include changes in ocean chemistry and the appearance of multicellular life forms that might have preyed on the microbes responsible for their structure.
Bernhard and Edgcomb thought foraminifera might have played a role. Foraminifera (or “forams,” for short) are protists, the kingdom that includes amoeba, ciliates, and other groups formerly referred to as “protozoa.” They are abundant in modern-day oceanic sediments, where they use numerous slender projections called pseudopods to engulf prey, to move, and to continually explore their immediate environment. Despite their known ability to disturb modern sediments, their possible role in the loss of stromatolites and appearance of thrombolites had never been considered.
The researchers examined modern stromatolites and thrombolites from Highborne Cay in the Bahamas for the presence of foraminifera. Using microscopic and rRNA sequencing techniques, they found forams in both kinds of structures. Thrombolites were home to a greater diversity of foraminifera and were especially rich in forams that secrete an organic sheath around themselves. These “thecate” foraminifera were probably the first kinds of forams to evolve, not long (in geologic terms) before stromatolites began to decline.
“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.”
Next, Bernhard, Edgcomb, and postdoctoral investigator Anna McIntyre-Wressnig created an experimental scenario that mimicked what might have happened a billion years ago.
“No one will ever be able to re-create the Proterozoic exactly, because life has evolved since then, but you do the best you can,” Edgcomb said.
They started with chunks of modern-day stromatolites collected at Highborne Cay, and seeded them with foraminifera found in modern-day thrombolites. Then they waited to see what effect, if any, the added forams had on the stromatolites. After about six months, the finely layered arrangement characteristic of stromatolites had changed to a jumbled arrangement more like that of thrombolites. Even their fine structure, as revealed by CAT scans, resembled that of thrombolites collected from the wild. “The forams obliterated the microfabric,” said Bernhard.
That result was intriguing, but it did not prove that the changes in the structure were due to the activities of the foraminifera. Just being brought into the lab might have caused the changes. But the researchers included a control in their experiment: They seeded foraminifera onto freshly-collected stromatolites as before, but also treated them with colchicine, a drug that prevented them from sending out pseudopods. “They’re held hostage,” said Bernhard. “They’re in there, but they can’t eat, they can’t move.”
After about six months, the foraminifera were still present and alive—but the rock’s structure had not become more clotted like a thrombolite. It was still layered. The researchers concluded that active foraminifera can reshape the fabric of stromatolites and could have instigated the loss of those formations and the appearance of thrombolites.
The image at the top of the page captures the Antarctic discovery of stromatolites in April of 2011 that could help scientists better understand the conditions under which the planet's primitive life-forms thrived. “It’s like going back to early Earth,” says Dawn Sumner, a geobiologist at the University of California, Davis, describing her explorations of the eerie depths of East Antarctica’s Lake Untersee where Sumner and her colleagues, led by Dale Andersen of the SETI Institute in Mountain View, Calif., discovered otherworldly mounds of Photosynthetic microbial stromatolites.
The stromatolites, built layer by layer by bacteria on the lake bottom, resemble similar structures that first appeared billions of years ago and remain in fossil form as one of the oldest widespread records of ancient life dating from 3 billion years ago or more, to understand how life got a foothold on Earth.
Lake Untersee is located at 71°20'S, 13°45'E in the Otto-von-Gruber-Gebirge (Gruber Mountains) of central Dronning Maud Land. [Download Google Earth .kmz file of Lake Untersee]. The lake is 563 meters above sea level, with an area of 11.4 square kilometers and is the largest surface lake in East Antarctica.
The purple-bluish mounds are composed of long, stringy cyanobacteria, ancient photosynthetic organisms. Similar to coral reef organisms, the bacteria takes decades to build each layer in Untersee’s icy waters, Sumner said, so the mounds may have taken thousands of years to accumulate.
Today, stromatolites are found in only a few spots in the ocean, including off the western coast of Australia and in the Bahamas. They they have also been found thriving in freshwater environments, such as super-salty lakes high in the Andes and in a few of Antarctica’s other freshwater lakes.
The findings, by scientists at Woods Hole Oceanographic Institution (WHOI); Massachusetts Institute of Technology; the University of Connecticut; Harvard Medical School; and Beth Israel Deaconess Medical Center, Boston, were published online the week of May 27 in the Proceedings of the National Academy of Sciences.
The Woods Hole Oceanographic Institution is a private, non-profit organization on Cape Cod, Mass., dedicated to marine research, engineering, and higher education. Established in 1930 on a recommendation from the National Academy of Sciences, its primary mission is to understand the oceans and their interaction with the Earth as a whole, and to communicate a basic understanding of the oceans’ role in the changing global environment.
The Daily Galaxy via Woods Hole Oceanographic Institution www.whoi.edu
Image credit: A calcareous foraminifer within a stromatolite from Highborne Cay, Bahamas. The hair-like structures are the foram’s pseudopods. Photo by Joan Bernhard, Woods Hole Oceanographic Institution