GRANADA, Spain, April 15 (UPI) -- Common soil bacteria could help protect ancient stone monuments and marble statues from corrosive pollution, scientists in Spain report.
The research team's new germ-based technique "mimics what nature has been doing for eons" and promises to be inexpensive, geologist Carlos Rodriguez-Navarro of the University of Granada in Spain said.
"We foresee practical application of the proposed bacterial treatment a few years from now," Rodriguez-Navarro told United Press International.
The more polluted the air gets worldwide, the more acid rain endangers not only health but also priceless art. The marble and limestone found in sculptures and landmarks worldwide are made of the same mineral found in seashells, coral, eggshells and chalk. This calcium carbonate is especially vulnerable to corrosion.
"Imagine the unfortunately common situation of a decaying Gothic or Renaissance portal of a European cathedral," Rodriquez-Navarro said. "The economy of many countries with a rich cultural heritage strongly depends on safeguarding this principal asset."
In the past, conservationists tried impregnating stone with acrylic or epoxy resins to strengthen the decayed, porous rock, "but they resulted in further damage -- peeling of treated layers, yellowing in some cases," Rodriguez-Navarro said. Scientists also considered washing stone with limewater, but this only forms a loose powder that's too superficial to protect anything.
In the past two decades, researchers suggested harnessing mineral-secreting germs as an environmentally friendly, low-maintenance art conservation workforce to harden rock. Most soil bacteria secrete calcium carbonate to help balance out internal body acidity.
Scientists previously tried using the common soil bacterium Bacillus cereus, which can cause vomiting or diarrhea if it contaminates food. Unfortunately, the calcium carbonate layer didn't cement the interior of the rock together, since it only penetrated a few microns deep -- approximately the width of a human blood cell.
Bacillus cereus also left behind a residue of organic remains that plugged the stone's pores. Stone needs to "breathe" to allow moisture that may get trapped inside to evaporate, Rodriquez-Navarro said. If water from rain or humidity builds up, it will dissolve the rock and lead to salt crystals that weaken and degrade the stone.
The researchers then tried using another common soil bacterium, a harmless germ known as Myxococcus xanthus, which can glide en masse over surfaces and penetrate deep into the stone's pores. Moreover, while Bacillus sometimes grew uncontrollably, stopping up pores, Myxococcus -- handled properly -- spontaneously kills itself once feeding stops without leaving behind spores.
Rodriquez-Navarro and a team of crystallographers and microbiologists experimented with a highly porous limestone often used in Granada's most outstanding yet crumbling landmarks. In findings appearing in the April issue of the journal Applied and Environmental Microbiology, the new cement their bacterial soup formed "is exceptionally hard, even harder than the original," Rodriquez-Navarro said.
The researchers speculate this new cement may contain organic material, just as the shells of abalone do.
"Such shells are remarkably strong and tough, even though the minerals they are made from are very brittle, because the tiny mineral crystals are stacked like bricks surrounded by a layer of organic 'mortar,'" materials scientist George Scherer at Princeton University in New Jersey explained. "If one little crystal cracks, the crack is arrested by the rubbery mortar layer, so the crack does not continue to run down the shell."
X-rays and scanning electron microscopy revealed the crystal layers created by the new technique were as porous as the rock below them, and developed fast, mostly within five to 10 days.
However, these networks only extended down a half-millimeter to a millimeter, which while hundreds of times deeper than what was seen with prior bacterial treatments, "is still not enough, I think," said Scherer, who did not participate in this research.
"Damage from acid rain, frost and salt crystallization often extends to a depth of several millimeters or even a few centimeters into the stone," he explained. "It remains to be seen whether the bacteria can be persuaded to do their work deeper inside the stone."
Still, Scherer said, "this work was done with great care and thoughtfulness. This work is typical of the intelligent and creative research done by Carlos and his group. He is one of the leading lights in this field."
The researchers are now in the process of implementing field-testing at selected buildings in Granada. Rodriquez-Navarro said his team also plans to see if the bacteria can be used to fill cracks in marble and protect non-chalky rocks such as granite and sandstone.
"This area is so new and promising that one can imagine a hundred directions to explore. For example, there must be many other potentially useful microbes," Scherer said, as well as optimized nutrient broths to grow the bacteria in. Additives might also influence the kinds and shapes of crystals the bacteria grow, he added.
(Reported by Charles Choi, UPI Science News, in New York.)
