Ancient tiny fossils from Australia may carry evidence of great power: the ability to make oxygen through photosynthesis.
The fossilized bacteria, dating from 1.73 billion to 1.78 billion years ago, are chock-full of structures that resemble those where oxygen-producing photosynthesis takes place in most modern cyanobacteria and in plants. Called thylakoid membranes, the structures are the oldest ever found, researchers report January 3 in Nature. The finding pushes back the evidence of thylakoids in cyanobacteria by 1.2 billion years.
Cyanobacteria’s invention of photosynthesis is responsible for the oxygen in Earth’s atmosphere. “So they’re a big deal,” says Woodward Fischer, a geobiologist at Caltech who was not involved finding the thylakoid membranes. And “this is the kind of information that I thought we were not going to be able to pull out of fossils,” he says.
Most fossils preserve mineralized tissues such as bone or shells, but bacteria don’t contain such mineral structures. These fossils are “just compressions of carbon” squished into mud, Fischer says. To find the bacteria preserved is impressive enough, but the new fossils reveal complex structures inside the microscopic bacteria. “It suggests this kind of future where we might be able to pull more information, more cell biology and morphological detail out of these minuscule fossils,” he says.
Researchers already had indirect evidence from genetics and chemical studies that cyanobacteria had developed thylakoids by the time these fossilized bacteria lived, says Patricia Sanchez-Baracaldo, an evolutionary microbiologist at the University of Bristol in England (SN: 9/8/15). Still, exactly when the structures evolved is hotly debated (SN: 3/2/17). So it’s exciting to see fossil evidence of such old thylakoids, says Sanchez-Baracaldo, who was not involved in the work. “Any evidence that you have from that time period is important because the fossil record is really very sparse.”
Some researchers think that thylakoids may have evolved before the Great Oxidation Event around 2.4 billion years ago (SN: 12/11/19). Prior to that event, there were whiffs of oxygen here and there in the atmosphere, but it took the concentrated action of photosynthetic bacteria to send Earth’s oxygen levels skyrocketing. Stacks of thylakoids within cyanobacteria may have multiplied the bacteria’s oxygen production.
During the period when the now-fossilized cyanobacteria lived, oxygen levels in Earth’s atmosphere had plummeted again to a fraction of today’s levels, Sanchez-Baracaldo says. The fossils hint that there may have been small pockets where oxygen was abundant and could have fostered the evolution of the ancestors of plants and animals.
Many rocks that might harbor such fossils have been compressed and “cooked” destroying delicate intracellular structures like thylakoids, says Emmanuelle Javaux, an astrobiologist at the University of Liège in Belgium.
“We didn’t know that they could be preserved in such old microfossils,” she says. But she has no doubt that the dark lines stacked through tiny sausage-shaped cells represent thylakoids. “It cannot be something else, actually. This arrangement is very unique to cyanobacteria with thylakoids,” she says.
Javaux and colleagues found the oldest thylakoid-like structures in microfossils in shale from Australia. The structures were also present in about 1 billion-year-old fossils from Canada, but not in 1 billion-year-old ones from Congo. The rocks from Congo experienced slightly higher temperatures than the others, which may have destroyed fossil evidence of thylakoids. Or maybe those fossils are cyanobacteria that never evolved the structures or are another type of microbe entirely.
The researchers can’t tell from the fossils whether the Australian and Canadian cyanobacteria are direct ancestors of living species, Javaux says, but they are almost certainly cousins. The team hopes to investigate even older rocks from before the Great Oxidation Event for even more ancient evidence of thylakoids.
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