Stromatolites are layered accretionary structures formed in shallow water by the trapping, binding and cementation of sedimentary grains by biofilms of microorganisms, especially cyanobacteria (commonly known as blue-green algae). Stromatolites provide some of the most ancient records of life on Earth by fossil remains which date from more than 3.5 billion years ago. The widespread disappearance of stromatolites, the earliest visible manifestation of life on Earth, may have been driven by single-celled organisms called foraminifera. 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 in the Proceedings of the National Academy of Sciences.
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“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, lead author of the study.
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.
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 Foraminifera are a phylum or class of amoeboid protists. They are characterized both by their thin pseudopodia that form an external net for catching food, and they usually have an external shell, or test, made of various materials and constructed in diverse forms. Most forams are aquatic, primarily marine.
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.
Next, Bernhard, Edgcomb, and postdoctoral investigator Anna McIntyre-Wressnig created an experimental scenario that mimicked what might have happened a billion years ago.
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.
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.
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.
For more information see Enigma.
Stromatolites image via Wikipedia.
