A previously unknown type of stromatolite – rare and ancient biological structures that first evolved around 3.7 billion years ago – has been found living in freshwater wetlands in the Australian island state of Tasmania.
The discovery, by a team led by Bernadette Proemse from the University of Tasmania and published in the journal Scientific Reports, significantly extends the range and circumstances in which stromatolites are known to live.
Stromatolites comprise layers of biochemical material that gradually accrete from a combination of trapped sediments and mats of microbes, particularly cyanobacteria.
Stubborn survivors of the Eoarchean Era, stromatolites are extremely rare today. They are found mainly in shallow marine environments or hyper-saline lakes.
The best-known examples are located at Shark Bay in Western Australia. Others have been found in Chile and Brazil, Mexico and the Bahamas.
Despite the association of other examples with salty sea water and lakes, the Tasmanian stromatolites exist in mildly alkaline spring water surrounded by peaty soil. They comprise not only cyanobacteria, but also other microbes including alphaproteobacteria and an unusually high number of photosynthesising chloroflexi – a combination, say the researchers, that makes them unique.
Also unusual is the fact that some of the stromatolites extend several centimetres above the water level.
“Cool-temperate freshwater wetlands are not a conventional stromatolite niche,” the researchers write, “suggesting that stromatolites may be more common than previously thought.”
Whatever abundance stromatolites achieved during their first couple of billion years of existence was sharply curtailed around 600 million years ago when multicellular organisms started to emerge. Immobile and edible, the stromatolites were both easy to out-compete and consume.
Perhaps significantly, Proemse and her colleagues report that in the area of the UNESCO-listed Tasmanian Wilderness World Heritage Area where they made their finds, complex invertebrates are uncommon. Snails, known stromatolite predators, are extremely scarce, because the peaty soil leads them to accumulate debilitating levels of carbonate in their shells.