Rock 鈥渃himneys鈥� twice as tall as a person that tower above a lake in California may have been built, in part, by microorganisms. If that is confirmed, similar towers could give away the presence of life on other planets.
Mono Lake is extremely salty and highly alkaline. Despite these inhospitable conditions, it is home to a thriving ecosystem, including scuba-diving flies. In 2010 it came to worldwide attention after scientists claimed that bacteria from Mono Lake could incorporate arsenic into their DNA, in place of phosphorus. This remarkable claim was later debunked.
In places, towers of limestone rear above the surface of Mono Lake. One set of towers, which lies north of the lake itself, is known as 鈥淢ono City鈥�. There the towers are 3-4 metres tall and 1.5-3 metres across.
Advertisement
For many years, scientists studying Mono Lake鈥檚 towers have debated whether they formed by a purely chemical process or whether single-celled microorganisms played a role.
Towers made by bacteria
of the University of Aberdeen, UK and his colleagues to examine the towers more closely. They found that each tower was built of numerous cones or pipes made of calcite. Each pipe was 30鈥�60 centimetres in height and around 3 cm across, with a hollow centre about 1cm across.
The team found fossilised bacteria in the pipes. 鈥淲e鈥檙e not just talking about a few isolated cyanobacteria, but tens or hundreds of thousands of microbes,鈥� says Brasier.
Such bacteria often live in dense colonies, which are called 鈥�biofilms鈥� and look like mats of slime. The slime forms because the cells secrete a mix of goopy chemicals, and that explains how the cells could drive the formation of chimneys.
The process starts when water bubbles up through vents in the lake bed. Colonies of bacteria would grow around this water, forming tube-shaped mats. 鈥淭he few 20th-century observers [of Mono Lake] also saw biofilms of microbes growing around the rock-forming vents,鈥� says Brasier.
Chimneys of Mars
鈥淭he extracellular organic matter that cyanobacteria produce has been shown, by others, to influence formation of carbonate rocks in lakes and streams by trapping and binding calcium,鈥� says Brasier. When the bacteria die and the slime breaks down, the calcium crystallises into a mineral called calcite around the dead cells.
Over time, the tube-shaped mats of bacteria would give rise to tubes of calcite, which would grow ever higher.
鈥淲e are confident that this applies to the Mono Lake chimneys we studied, in the same way that it does to carbonate rocks of other environments, including hot-springs and volcanic crater lakes,鈥� says Brasier.
If the Mono Lake towers formed this way, it suggests that they would not form in the absence of microbial life 鈥� and that means they are a conspicuous marker of life. 鈥淲e think that metre-scale 鈥� so comparatively large and visible 鈥� chimney structures like these should be considered as high priority targets in the search for ancient and extra-terrestrial life,鈥� says Brasier.
For instance, if towers like those of Mono Lake could be found on Mars, it would imply that life once existed there 鈥� even if it has long since died out.
