Limestone provides the raw material for some of the world's most iconic buildings — from the Egyptian pyramids to Notre Dame and the Parthenon.

And while these remarkable structures have stood the test of time, climate change and pollution are today putting them at risk: studies suggest that since the industrial revolution, the rate of decay of limestone buildings has sped up significantly.

Yet scientists have found a solution in an unlikely corner. Recent research has shown that bacterial cultures could become key players in the colossal effort to protect the world's historic limestone buildings from environmental degradation.

Saving historic buildings

The soft stone, consisting mainly of calcium carbonate, is vulnerable to acidic forms of air pollution in urban environments, such as the nitrous oxide and sulphur dioxide produced when we burn fossil fuels. Mixed with rainwater, these gases can cause black crusts to form on the stone which over time leads to serious deterioration.

Climate change is also a threat. Scientists fear that hotter summers in Europe are accelerating limestone decay. Rising temperatures could lead to more evaporation of rainwater, which leaves behind salt crystals that lodge inside and crack the porous stone.

A process known as bacterial carbonatogenesis has been shown to successfully protect damaged limestone with few negative effects on the building or the wider environment.

Bacterial cultures — such as bacillus cereus, bacillus subtilis, and myxococcus xanthus — are grown in a laboratory and either injected into the stone or applied with spatulas to the degraded surface. A nutritional solution is then added to feed the bacteria and the process is repeated until the bacteria have produced enough calcium carbonate to heal the limestone.

Although the treatment itself is relatively new, the earth's limestone was formed, in part, through calcium carbonate-producing bacteria and the process occurs naturally in oceans, soils, and lakes.

The technique was pioneered by the French Ministry of Culture in the 1990s and since then has been used on monuments and buildings across France, Portugal, and Spain. Preliminary tests have also been carried out at the Mayan site of Copan in Honduras.

Professor Carlos Rodriguez Navarro, a scientist at the Department of Minerology and Petrology at the University of Granada in Spain, leads a research group studying the process of using bacteria that naturally produce calcium carbonate, the main ingredient of limestone, to repair and protect damaged stone.

The fact that the bacteria produce calcium carbonate ensures a perfect compatibility with the substance already present in the limestone, says Navarro. "This is not the case with other materials, which strongly differ from the calcium carbonate present in limestones."

Less toxic than conventional methods

Jean-François Loubiere, a now-retired scientist at the French Ministry of Culture's Historic Monuments Research Laboratory, was part of the first team to use the bacterial method on a church in the 1990s in the town of Thouars, in western France.

"It is much more ecological and kinder to the stone because it is a natural process," said Loubiere, comparing it to the most common synthetic conservation techniques that can harm the stone and release toxins into the environment.

Some traditional chemical treatments using polymers release toxic chemicals, known as volatile organic compounds, which can be harmful to the environment and to human health.

According to Navarro, applying synthetic products can also create a waterproof surface that plugs up the pores of the stone and accelerates salt crystallization. Because the bacterial process simply adds to the calcium carbonate naturally present, it allows it to 'breathe.'

When it comes to aesthetics, Navarro says biomineralization, the process of the bacteria producing minerals, also has benefits. It does not appear to significantly change the stone's color or produce the typical wet or shiny appearance common with synthetic treatments.

Loubiere believes that the technique would also be appropriate for repairing Notre Dame's limestone blocks, which suffered heat damage in the major fire in April 2019.

Competing with cheaper synthetic methods

However, according to Faisl Bousta, a microbiologist at the Historic Monuments Research Laboratory, the technique is unlikely to be used on the Parisian cathedral, which is still being restored.

While the technique is frequently employed throughout France, particularly in the private sector, it has struggled to compete with synthetic methods.

"It is more expensive than the alternatives, and it is more restrictive in terms of application," said Victor Soulie, the director of AMONIT, a French company that makes the biological solutions used in the process and that works closely with the Laboratory of Historic Monuments. It can take months to add enough layers for the treatment to work.

Navarro believes the need to have a laboratory to grow bacteria, which can be costly and complex, is off-putting for many conservators.

However, his team at the University of Granada hassuccessfully demonstrated  a way of bypassing the laboratory and using the bacteria already living on the stone. A nutritive solution is sprayed onto the limestone regularly over six days, feeding the indigenous bacteria. This approach has recently been used on monuments in Portugal. Not having to grow the cultures in a laboratory will save time and money.

With the impacts of climate change increasingly felt around the world, protecting monuments and historic buildings feels like a race against time for the team.

But Navarro is optimistic about the future use of this eco-friendly conservation solution.

"We strongly believe that this latter type of treatment will be more widely adopted over the coming years," Navarro said.