One of the first things life did after it evolved -- after making lots more of itself and taking over the planet, of course -- was to invent the high rise. Fossils from 3.7 billion years ago, just under a billion years after Earth formed, tell us films of bacteria started trapping tiny particles of rock and welding them into finely layered stone pillows that plumped into microbial skyscrapers.
The resulting geological confections, today called stromatolites, are the first evidence we have for life on Earth. Once, they covered the seafloor of all the shallow seas on the planet and their fossils are abundant. But they were long thought to be extinct, because no one could find a living example. Where had they gone?
The prime suspects, unfortunately, were thugs like us: newfangled grazing animals, which rapidly decimated the defenseless neighborhoods of placid bacterial towers. But given the hordes of ravenous animal crawling all over Earth at present, how could they possibly survive now?
Scientists were thus probably delighted in 1956 by the discovery of the Hamelin Pool at Shark Bay, halfway up Australia’s western coast. In this special place where the salinity is twice that of normal seawater, stromatolites built of blue-green algae called cyanobacteria persist, like a dream of a planet half-remembered by its former self. This is only possible, of course, because the hypersaline water acts as a natural defense system, forbidding hungry, destructive animals from entering.
Although a few other environments on Earth have been found to host stromatolites since then, they remain indisputably rare, and presumably only possible where “exotic” chemistry prevents animals from wreaking havoc. Such places include super salty environments in the Hamelin Pool, at Storr’s Lake in the Bahamas, in a lake on the Kiritimati Atoll in the Central Pacific, and in a few freshwater environments with their own peculiar chemistry like British Columbia’s Pavilion Lake or the Ruidera Pools Natural Park in Spain.
So it was with great surprise that I read the other day that stromatolites have been discovered in Australia again, but this time on land. Touché, cyanobacteria. I did not expect that one.
In a paper published in November in the journal Scientific Reports, scientists from Tasmania report on yellow-green globular growths in the sand and gravel beds of alkaline spring mounds scattered among the wetland in the Giblin River Catchment of the Tasmanian Wilderness World Heritage Area.
There, cold springs releasing mildly alkaline water filled with calcium and bicarbonate permeate the soil for a short distance before dissipating in the acidic water of the surrounding bogs. The stromatolites are growing on wetted ground, but the top several centimeters of the towers where the bacteria grow are bathed in air. That is a highly unusual configuration, but the region does get 11 feet of rain a year. The largest stromatolites are around four inches across, but most are much smaller, and they have fine internal calcite layers.
The scientists examined the DNA of the bacteria in these mas and discovered they seem to be a unique team, unlike those seen in stromatolites anywhere else on Earth, including those in other freshwater and lake environments. This discovery, the scientists say, means that stromatolites may be more common than realized, because people have not been looking for them in freshwater springs. Perhaps if we do, we will discover that they are not so rare after all.
This particular geologic setup seems to permit stromatolites to form, they add, because of an Achilles heel in the construction of the major grazers’ mobile homes. Snails, of course, have calcium carbonate shells. And snails sure like to eat stromatolites. But in the alkaline waters of these springs, the carbonate in the water will anneal to the shell of a snail, causing snail shells to grow increasingly heavy and unwieldy. The snail’s mobile defense system rapidly becomes a fortress of death, as it slowly taxes its builder’s resources to the breaking point.
Snails both young and old seem to be affected. Few live snails of any age were seen, while dense piles of empty shells testify to the lethal effectiveness of the water, but probably still fail to warn even especially bright snails away. Some of the discarded shells were so encrusted that they were no longer recognizable as a snail’s.
Although an assortment of other small mostly shell-less predators are around – a flatworm, some roundworms and various other worms, some juvenile stoneflies, and miscellaneous small crustaceans – their feeding does not seem to be an impediment to the stromatolites.
These spring mounds remind me of the Greco-Roman idea that there are nymphs who tend every spring and pool, protecting the creatures that dwell there. The waters of these Tasmanian springs are the spirits that protect their odd, land-lubbing stromatolites, allowing them to exist in seemingly the most improbable of places.
Proemse, Bernadette C., Rolan S. Eberhard, Chris Sharples, John P. Bowman, Karen Richards, Michael Comfort, and Leon A. Barmuta. "Stromatolites on the rise in peat-bound karstic wetlands." Scientific Reports 7, no. 1 (2017): 15384.