Black Slime Is Coating World Monuments - And That May Be Good

Biofilms are affecting edifices like Angkor Wat and the Jefferson Memorial, and debate is raging over whether they are helpful or harmful.

By careful design, the legion of snowy white monuments and edifices in Washington, D.C., were built to link the American experiment with its Greek and Roman muses. Crafted from pure, unstained marble, the city’s classical architecture is a visual representation of the nation’s pursuit of civic ideals and self-governance.

But in a relatively short time span, these modern temples of democracy have begun sporting blemishes. Dark, blotchy patches have shown up on virtually every stone surface in the city—few as obvious as the darkening dome of the Jefferson Memorial, which has gone from a fairly bright white to a mottled mess in only 10 years.

Similar coatings of black slime also mar Roman ruins in Italy, ancient temples in Egypt, and stone faces carved at Angkor Wat in Cambodia.

They’re unsightly, yes. But destructive?

The answer, at best, is unclear. But the white surfaces of the newer American monuments are a clean slate for observing how biofilms develop, grow, and spread and so are a valuable experimental research ground for helping clarify how biofilms may be damaging—or protecting—the world’s great historic stone monuments.

“This is something that becomes increasingly complicated the more we learn about it,” says Judy Jacob, senior conservator for the U.S. National Park Service. “It’s not that it’s happening faster than anyone thought but that no one really thought about a time line. Suddenly, we’re watching it happen.”

Stubborn Veneer

More like a sooty dust than an ooze, biofilms are entire ecosystems of bacteria, algae, and fungi all living together in a funky and super-stubborn veneer. No two are quite alike. The microbes collectively produce and hang out in a polymer-like gel that becomes tightly bound to the micro-topography of whatever surface they colonize. The bacteria produce protective pigments, and the gel itself may facilitate darkening by collecting particles from the air.

As abundant and omnipresent organisms, bacterial biofilms can form on any surface, at any time. From lining the human gut to covering the surface of Earth’s oceans, these coatings are everywhere.

Dutch scientist Antoni van Leeuwenhoek made the earliest known mention of biofilms in 1683, when he observed “animalcules” collected from his own teeth under his microscope. He noted in his written accounts that even after gargling and swishing with vinegar, the organisms seemed unfazed.

Van Leeuwenhoek’s observations underscore how difficult it is to remove biofilms. While power-washing, scraping, and dousing in chemicals known as biocides can reduce them for a time, it’s impossible to kill every last microbial member of these communities. Ultimately, survivors of each extermination go on to create new, stronger colonies, so discoloration will often recur with time.


The Bayon Temple at Angkor Wat in Cambodia is also covered in dark biofilm. Photograph by Jim Richardson, NG.

So why have biofilms gone on an apparent rampage on stone monuments only recently?

One leading theory, for the United States at least: cleaner air. Ironically, the stepwise enactment since the 1970s of the Clean Air Act may have reduced the acidity of rainwater to an extent that it no longer hinders the growth of these bacterial communities.

In addition, monuments like the Jefferson are simply reaching the point in their life spans when normal erosion takes a toll. When it was completed in 1943, the Jefferson’s marble had a satin-smooth finish. Over time, that surface has been subtly pitted, becoming a more attractive home for biofilms.

A class of biocides known as quaternary ammonium compounds, or quats, is one chemical treatment that temporarily diminishes biofilms and their visible effects. But Jacob questions whether a treatment approach that involves cyclical, continued applications of chemicals is a particularly desirable course of action. She’s also looking at what the environmental effects of long-term use of quats would be on the surrounding areas.

“The strategy we’re looking at now is not to kill these microorganisms but to disarm them,” says Federica Villa, a microbiologist at the University of Milan who has been studying D.C.’s biofilms with Montana State University’s Center for Biofilm Engineering.

“If you know the mechanisms behind the formation of biofilms, you can work with more elegant solutions instead of using, or abusing, biocides or antibiotics.”

Friend or Foe?

Meanwhile, there's intense debate among stone monument conservators and researchers: Do the biofilms actually harm the surfaces of the buildings? If so, how should they be remediated? And if not, and they’re actually somehow protecting the stone from the elements, would the damage caused by removing them be greater than the aesthetic value of a clean-looking structure?

Since studies of biofilms really began to ramp up in the 1970s, many researchers have assumed that the presence of bacterial biofilms and lichens necessarily equate to damage of the underlying stone due to chemical action or penetration by anchoring structures.

Evidence of a direct chemical or physical action is sparser, but it does exist: One study from Egypt published earlier this year found that increased humidity in the Osirion sarcophagus chamber in Abydos facilitated the growth of a variety of fungi that chemically weathered the stone.

By contrast, Villa has found that biofilms seem to impart a protective effect. She grows simplified cultures of these bacterial communities in special chambers where she can control and track growth conditions. Her work so far suggests that the films prevent excess water—a common enemy of monuments—from penetrating stone.


Biofilm-coated statues line the Canopus colonnade at Hadrian's Villa in Italy, which is a UNESCO World Heritage site. Photograph by Deagostini, Getty images. 

“We’re still struggling with the question of balancing aesthetics with this natural process,” says Catherine Dewey, chief of resource management for the National Mall and memorial parks in D.C. Dewey has been involved in testing other methods for removing or mitigating the appearance of the biofilms, including using high-intensity laser treatments.

Villa insists that knowing the identity of the players at each site, on a case-by-case basis, is critical to determining the dynamics at play. While primarily fungal communities may be deteriorating the sandstones at Abydos, the dominantly bacterial biofilms on the Jefferson or other worldwide sites may be protecting them from the vagaries of wind, rain, and sun. Simply put, too much variation exists from place to place to be able to make a blanket statement about the positive or negative impacts of biofilms.

“We want to know more than who is there, but what they are able to do,” Villa says. “We’re interested in knowing the functional capability of the biofilm. What kind of activity do they exhibit, what are their metabolic pathways? We’re trying to look at biofilm from a different perspective.”

Villa and her colleagues are also studying whether there are enough similarities between biofilms from regions around the world to harness their behavior. If so, perhaps they can unlock a way of using biofilms to protect monuments but without the black mess.

“Biofilm is a very common phenomenon,” Villa says. “So if we are able to understand them better, in the future, we may be able to modulate biofilms and perhaps even apply those ideas to other fields like agriculture, industry, and bioremediation.”

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