SOME TWO BILLION YEARS AGO THE CLOSEST THING TO LIVING REEFS ON OUR PLANET were widespread masses of stromatolites, mounds of cyanobacteria and sediment held together by calcium carbonate they secreted.
Before they arose to dominate the seas, the earth was a hellish place. Afterwards, it was a world on the road to the evolution of life as we know it.
Before stromatolites, the planet was largely a worldwide ocean, with landmasses no more than small expanses of barren rock. The atmosphere was thick with carbon dioxide, nitrogen, hydrogen, sulfide, methane and less than one percent of today’s free oxygen. The oceans were brown with dissolved iron, the sky pinkish orange with organic smog. The only life forms present were archaebacteria and true bacteria.
Stromatolites set in motion forces that changed that. The build-up of landmasses led to a world with shallow seas, providing habitats for photosynthetic cyanobacteria to sustain themselves by converting solar energy into food. Building up as stromatolite communities, they spread prodigiously.
Processing water and carbon dioxide for nutrients was their real function – but a byproduct of it was the release of free oxygen into the atmosphere. Taking place on a massive scale, their carbon-fixing created an oxygen-rich atmosphere that would make advanced life possible – and an ozone layer to shield that life from ultraviolet rays.
There are, to use a scientific term, zillions of species of bacteria – and of cyanobacteria. Some types of bacteria are photosynthetic, many aren’t, including many cyanobacteria. But many are, and cyanobacteria may have been among the first photosynthetic organisms on earth. Because their bluish coloration was so noticeable, cyanobacteria were popularly called blue-green algae for a long time. But today they’re now clearly recognized as a form of bacteria.
And cyanobacteria are everywhere – in tidal pools, on coral reefs, in soils, on the surfaces of rocks and trees, in hot springs and icy lakes underneath thick ice. The dramatic blue-green colors of thermal pools in the Yellowstone geyser fields are due to cyanobacteria. Like all bacteria, they’re tiny and, individually, virtually invisible to the naked eye. The largest are less than one-tenth of a millimeter in diameter and most are much smaller. But some aggregate to form long filaments, strands or thick mats that are visible. Not all are bluish-green. Most cyanobacteria in the oceans also have a reddish pigment that can be dominant, depending on the proportions of the pigments. Some “red tides” are caused by cyanobacteria.
First described in the late 19th Century, stromatolites (“living mattresses”) were recognized among fossil formations for many years – often resembling sliced cabbages or cross-sections of corrugated cardboard panels, some formations as much as a half-mile thick and hundreds of miles wide. It was the 1950s before their nature was understood, based on studies of modern living specimens in Western Australia and other sites.
Essentially, stromatolites built up from a succession of microbial mats that cover shallow-water sediments, secreting a gel-like coating that protects them from ultraviolet radiation – and that helps accumulate sediments into thick, ossified layers of dormant bacteria, chalk, sand, gypsum and other debris. Like coral reef structures, the living part is only millimeters thick and is built atop the matrices of earlier, now-deceased predecessors. Those uppermost layers themselves are striated, with communities of sulfur-secreting anaerobic microbes below the surface and dependent microbes that live on the bodily remains of the higher layers below them.
The oldest known stromatlite structures date back to 3.5 billion year ago, but they grew exponentially beginning some three billion years ago. At first the buildup of oxygen in the atmosphere was tentative – for many millions of years, it was being absorbed by living organisms, metal compounds and minerals and atmospheric gases.
Perhaps 2.2 billion years ago, as the bacteria adapted and the oxygen sinks began to be used up, oxygen began to accumulate in quantity. For one thing, the earth rusted: Massive amounts of the iron accumulated in the oceans were oxidized and precipitated to the bottom (taking with it the oceans’ brown soupiness). The results are the iron deposits that have been mined throughout human history. With no other chemical sink sufficient to absorb the ongoing surge of oxygen, the gas built up, first dissolved in water, then released into the atmosphere. With it, the earth’s pink skies turned to the more comfortable blue we know today.
In one way, this build-up of oxygen was a catastrophe at a microbial level – perhaps the first great extinction. For many organisms such as anaerobic microbes, exposure to oxygen can be toxic. Many kinds of bacteria were wiped out. But the microbes that evolved to resist oxygen exposure multiplied and cyanobacteria and other organisms adapted to a metabolic system that required rather than avoided oxygen respiration.
The growing abundance of oxygen set off a cascade of speciation with new forms of life and life cycles. Eventually, complex cell structures appeared, early on in the form of single-cell green algae, opening the way to multi-celled bodies with varying sizes, shapes, tissues and functions.
Modern-day stromatolites are still with us as living fossils, but in limited sites and numbers. As life forms diversified, stromatolite colonies couldn’t compete against grazing predators in the world they helped create. They survive today in only a few locations where the topography fosters high salinity or other conditions that potential predators avoid. An important site is Hamelin Pool, a hypersalinated body at Shark Bay on Australia’s western coast. Hamelin Pool is a Marine Nature Reserve, and Shark Bay a U.N. World Heritage Site. Other communities of living stromatolites are known to be located in the Bahamas and the Persian Gulf.
Principal Sources: Microcosmos, Lynn Margulis; The Book of Life – An Illustrated History of the Evolution of Life on Earth, Stephen J. Gould, et all; The Oceans, Ellen J. Prager, Sylvia A. Earle; University of California Museum of Paleontology, http://www.ucmp.berkeley.edu/help/timeform.html; Miller Museum of Geology, Queens University, Kingston, Ontario, http://geol.queensu.ca/museum.
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