WASHINGTON (AP) — What's darkening the formerly white rotunda of the Thomas Jefferson Memorial?
Is it dirt? Is it mold?
No, it's biofilm!
The National Park Service said in a news release Wednesday that biofilm, colony of microscopic organisms, has become increasingly pronounced at the memorial.
Officials say biofilm is not unique to the Jefferson Memorial; it was successfully treated at the D.C. War Memorial in 2011.
Gay Vietzke, superintendent of National Mall and Memorial Parks, said the increase presence of biofilm is a new challenge throughout the mall.
"We are continuing to study biofilm and research treatment methods, and look forward to restoring the dome to its original luster while ensuring its long-term preservation," Vietzke said in a statement.
Officials are testing several techniques to treat the biofilm, with an eye toward doing the least damage to the soft marble of the Jefferson Memorial and making it safe for the environment and visitors.
The film is actually a "multicultural" community of organisms living in the relatively harsh environment of the sun-blasted stone, Federica Villa, a microbiologist who has been studying the memorial, told The Washington Post.
The organisms — algae, bacteria and fungi — produce the black pigment to protect themselves from solar radiation, she explained. And Park Service experts note that it is not dirt or mold as some might suspect.
Officials aren't sure whether any of the efforts can prevent the organisms from coming back. They also don't know whether the biofilm is damaging, or perhaps even protecting, the stone.
"To be honest, we have a lot of work to do," Villa said.
The Park Service said that the marble blocks that make up the memorial were smooth when first hoisted into position but years of rain have slowly eroded the marble, pitting surfaces and creating a "perfect environment for a biofilm."
The statement added the biofilm first became noticeable at the memorial in 2006 and now is increasingly pronounced.
No timeline has been set for treatment of the dome, Park Service officials said.