On The Demise Of The Stromatolites

Beautifully coloured modern day stromatolites, relics of the Precambrian.

People often talk about Earth history in relation to a particular group of organisms: the Mesozoic is known as the Age of the Dinosaurs and the Cenozoic the Age of Mammals. If we extend this, then the Precambrian ought to be known as the Age of Stromatolites.

On a semi-barren planet shrouded in toxic oceans and volatile continents, the stromatolites were the only form of life. They built the first biological empire on the planet: giant, layered colonies of bacteria and sediment.

Collections of stromatolites could exist on a scale rivaling that of Ancient Rome with swaying pillars and mounds of slime anchored to the sea bed. They dominated the oceans for over 3 billion years, but towards the end of the Precambrian they went into decline. Yet from the Cambrian onwards, they still existed and are found in the fossil record, living in the shadows. Today, stromatolites can be found in the most inhospitable places on Earth only, from alkaline lakes and hyper-saline bays to the sub zero beds of Antarctic pools and the oxygen starved, oxide-laden streams.

'Stromatolites were one of the earliest examples of the intimate connection between biology -- living things -- and geology -- the structure of the Earth itself,' said Woods Hole Oceanograpic Institute (WHOI) geobiologist Joan Bernhard. Their decline was an event comparable to the extinction of the dinosaurs, a global event which affected the entirety of the Earth's biosphere. For years the causes of this semi-extinction were completely unknown.

Some put it down to the two ice ages which wracked the Precambrian Earth, others to massive geochemical changes, such as the appearance of oxygen in the atmosphere. Yet while these things had a impact on stromatolite populations, the bacterial colonies simply returned in force afterwards. Even together, this would not have been enough to spell the end for the first biological empire on Earth. Now, a study conducted by the Woods Hole Oceanographic Institute throws up a strong potential candidate for the driving force behind the stomatolites' decline.

The laminations within stromatolites. Thrombolites bare no such
structures and have a more clotted and clumpy texture.

The trigger for the study is rather interesting. The stomatolites' decline marked the appearance in the fossil record of another type of bacterial formation known as a thrombolite. Stromatolites are characterized by their very finely laminated layers of bacteria and sediment. Thrombolites on the other hand do not possess any form of layering. While both types of colony are dome-shaped, thrombolites have a clumpy structure rather than lamination.

Various hypotheses were put forward to explain this. Bernhard and fellow WHOI microbial ecologist Virginia Edgcomb suggested the appearance of predatory foraminifera in the oceans. New predators had already been put forward as the reason for the change from stromatolite to thrombolite. Bernhard and Edgcomb's theory, however, is the only one to have strong supporting evidence. Foraminifera, forams for short, are a group of marine single-celled protoctists which use pseudopodia to engulf their prey.

Despite their known ability to disturb modern sediments, their possible role in the loss of stromatolites and appearance of thrombolites had never been considered until now. The researchers sampled material from both types of modern day bacterial formations from Highbourne Cay in the Bahamas. Using microscopy and rRNA sequencing techniques, they examined the foram content of each type of formation.

The organic sheaths of thecate foraminifera. While they are highly
complex, each one is only home to a single cell.

The thrombolites were home to a greater concentration of forams than stromatolites. What is more, the majority of these microscopic inhabitants were what are known as thecate foraminifera, so-called because they secrete a protective sheath of organic matter 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.' The connection, of course, may have been arbitrary. To disprove this possibility, the researchers decided to recreate a Precambrian ocean environment. They seeded chunks of stromatolites with forams found in thrombolites.

After six months, the laminated structures had been almost entirely destroyed and had taken on a clotted texture. 'The forams obliterated the microfabric,' said Bernhard. As a control to make sure that the forams were the trigger for the clotting, they added micro-organisms to a stromatolite sample but then treated the sample with colchicine a drug that prevented them from sending out pseudopodia. "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 structure of the colony had not become more clotted like a thrombolite. It was still layered. There was no doubt that forams were the clotting trigger. From this, the researchers concluded that these micro-organisms may have been the cause of the decline of the stromatolites during the late Precambrian. The theory fits nicely with the problem in hand.

A foram embedded within a stromatolite. The small thread-like lines
 coming off of the body and its pseudopodia. The green colour is not
natural and comes from a dye used to enhance the microscope image.

Complex cells, or eukaryotes as they are properly known, evolved around two billion years ago; long before the decline of stromatolites. What is more, forams have been found in the late Precambrian fossil record, meaning that they were around and could have acted as the event trigger. The theory fits the bill. Now all it requires is evidence from the fossil record of an increase in oceanic forams and forams living within thrombolites.

Many Precambrian bacterial colonies are preserved down to the cellular level, meaning that forams living within the structures should be detectable. If their presence in the fossil record is concordant with the results of Bernard and Edgcomb's experiments, the theory will certainly become the foremost explanation for the decline in Precambrian stromatolites.