Tracking the Path of Green Slime
Andrew Knoll of Harvard University calls cyanobacteria the "microbial heroes of Earth’s history."
Most life on Earth owes its existence to tiny organisms called cyanobacteria. These primitive bacteria gave us oxygen for the atmosphere and a protective ozone layer, and they led to the development of all the green plants in the world today.
"Cyanobacteria are the microbial heroes of Earth history," says Andrew Knoll, a professor of evolutionary biology at Harvard University. "They are the inventors of ‘green plant’ photosynthesis and the ultimate source of breathable air."
Of course, when cyanobacteria first appeared, the other life forms on Earth weren’t too happy about it. It was as though an ill-mannered cousin showed up with a big stinky cigar, blowing smoke right in their faces. Only in this case, the smoke was a poisonous gas known as oxygen. Untold numbers of organisms were wiped out as cyanobacteria released oxygen into the atmosphere. If they were lucky, the stressed organisms managed to hide away in places where oxygen couldn’t reach – deep down in anoxic mud or in the cracks and crevices of hydrothermal vents under the sea. The offspring of these survivors can still be found living in these places today.
Cyanobacteria, too, are still around today. They can be found everywhere from the surface of the oceans to underneath rocks in the desert. They can live in bright light or low light, in salt water and fresh, in extreme cold or heat, with oxygen or without it. In fact, cyanobacteria are so widespread that J. William Schopf, professor of paleobiology at UCLA, calls cyanobacteria, "evolution’s most successful ecologic generalists."
Not only can cyanobacteria live just about anywhere, but they’ve also managed to survive throughout much of Earth’s biotic history. Whatever ecological catastrophes fate has thrown at the Earth – be it another Ice Age, a large asteroid impact, or changes in the atmosphere – through it all cyanobacteria have survived.
"Like fantastic aliens of a class B movie," Schopf writes in his book, ‘Cradle of Life,’ "they’ve proven impossible to wipe out, surviving on and on as life around them has gone extinct."
And, even more extraordinarily, cyanobacteria appear to have survived relatively unchanged. Schopf says that they do not look appreciably different from the cyanobacteria of two billion years ago. How could cyanobacteria be so untouched by the processes of evolution, when in the same amount of time the rest of life evolved from a single celled organism to the vast range of forms we see today, including our own human species?
As it turns out, different organisms experience different rates of evolution. Some organisms, like insects, evolve quickly. That’s why pesticides stop being effective after a certain period of time – those that can tolerate the pesticide survive to breed, and before you know it all the offspring are immune. Others, like cyanobacteria, evolve more slowly.
Pictured above are two kinds cyanobacteria from the Bitter Springs chert of central Australia, a site dating to the Late Proterozoic, about 850 million years old. On top is a colonial chroococcalean form, and on the bottom is the filamentous Palaeolyngbya.
Another reason cyanobacteria have been able to get away with so little evolutionary change is because of their ability to live almost anywhere. Evolution is often propelled by the need to adapt to environmental change. In addition, cyanobacteria reproduce non-sexually. The vast numbers of possible genetic combinations that we see in sexually reproducing organisms just don’t occur with cyanobacteria.
Evolution hasn’t completely by-passed cyanobacteria. There is evidence that some modern cyanobacteria are more sophisticated than their ancestors, forming communities with a range of adaptations to maximize their share of the available light and nutrients. But because cyanobacteria fossils look so similar to modern cyanobacteria, this suggests that some forms of cyanobacteria have changed very little over the years. This makes tracking their history in the fossil record difficult. Scientists would dearly like to know, for example, when cyanobacteria first came on the scene.
In 1993, Schopf made headlines when he claimed to find the earliest known fossilized life. These structures, which he described as "cyanobacterium-like," were found in 3.5 billion-year-old chert (a type of silica rock) from Western Australia. Since his discovery, these ancient structures are often cited as the oldest fossilized life on Earth.
However, Schopf’s finding may not be as definite as the textbooks and news reports would have you believe. Some scientists contend that the shapes found by Schopf are not cyanobacteria, or even fossilized life forms at all. Martin Brasier of Oxford University leads the opposition against Schopf’s claims. Brasier says that rather than being biological fossils, these structures are chemical artifacts formed from hydrothermally-heated graphite. The debate rages on, and many scientists are conducting their own studies of the enigmatic structures in an effort to find the truth.
Another controversial topic in the debate over cyanobacteria is the existence of Archaean stromatolites. These layered rock formations are formed by cyanobacteria. The bacteria live in a colonial layer called a "microbial mat." When too many minerals and sediments became trapped in the sticky mat, sunlight can no longer penetrate and photosynthesis becomes impossible. The cyanobacteria then migrate up, creating a new mat layer on top of the old. This process occurs again and again, creating multiple sediment layers over time.
The existence of stromatolites dating back to nearly 3.5 billion years ago suggests that cyanobacteria were hard at work during the Archaean era. But not all stromatolites are formed by cyanobacteria; natural geological processes can build similar structures. Some have argued, therefore, that the ancient rock structures were formed by chemical precipitation or by the deformation of soft sediments.
At the time of this writing, microfossils from the 2.2 billion-year-old Gunflint Chert – found in the Great Lakes region of the United States – are the earliest uncontested evidence for cyanobacteria.
"The origins of cyanobacteria are still mysterious," says Roger Summons, a professor of biogeochemistry and geobiology at MIT. "However, because the genomes and the biochemistry of today’s cyanobacteria preserve some kind of evolutionary record, we can learn more about their earlier forms."
Understanding when cyanobacteria first appeared would not only help answer many questions about early life, it also would help pin down when oxygen began to be an important part of the environment. But even if cyanobacteria did form the Archaean stromatolites, they might not have been producing oxygen. In order to develop oxygen-producing photosynthesis, cyanobacteria had to undergo a series of evolutionary steps.
A modern-day photosynthetic cell undergoes two simultaneous reactions, both of which rely on a separate kind of protein. Photosystem I protein molecules use the trapped energy in sunlight to convert carbon dioxide into carbon and oxygen. This provides food in the form of carbohydrates, lipids, proteins and nucleic acids – the building blocks of life. Photosystem II protein molecules use light energy to split water into hydrogen and oxygen for plant respiration.
But the first photosynthetic organisms didn’t produce oxygen. The most ancient photosynthetic bacterial species are purple and green bacteria. Purple bacteria and green, non-sulfur bacteria rely on Photosystem II for energy, while green sulfur bacteria use
|Purple sulfur bacteria. The gold particles in the cells are globules of elemental sulfur.|
Photosystem I. Cyanobacteria, algae and plants use both Photosystem I and II, and it is generally believed that the two Photosystems arose from a single evolutionary ancestor. However, another possibility is that there may have been some gene swapping between the two photosynthetic groups. Called "lateral gene transfer," this type of gene sharing may have been common in life’s early days. Many believe that this could have been the means by which cyanobacteria gained access to the genes necessary for both Photosystems.
Another technique of early evolution is that cells could absorb other cells in an act of symbiosis. Rather than digest the absorbed cell as food, it would become a part of the devouring cell’s inner machinery. Cyanobacteria, it is thought, was absorbed by an early eukaryotic cell (a cell with a nucleus). The absorbed cyanobacteria became a chloroplast, the structure that is responsible for photosynthesis in modern plants.
"Cyanobacteria evolved further to be the chloroplasts of other photosynthetic organisms, particularly algae and the green plants," says Summons. "Thus, cyanobacteria may be just ‘green slime’ to some, but they are the foundations of the ecosystems of all complex life."
Schopf and his colleagues are intensely studying the 3.5 billion-year-old structures from the Australia chert. They are currently doing laser-Raman spectroscopy, atomic force microscopy, and ion microprobe carbon isotopic analyses to try to find out whether the structures are cyanobacteria.
Summons, meanwhile, is conducting studies of the molecular biomarkers of modern cyanobacteria with his colleague Kai Hinrichs at the Woods Hole Oceanographic Institution (WHOI). By working with biologists who are experts on cyanobacterial occurrence, culturing, and genetics – such as John Waterbury (WHOI), Penny Chisholm (MIT), and Linda Jahnke (NASA) – they hope to develop molecular signatures for cyanobacterial productivity in the oceans. Summons believes these studies will lead to new methods for recognizing cyanobacteria in the sedimentary record, and perhaps trace them back to their earliest ancestors.