Black bioscum coats the Jefferson Memorial, as seen in this photo from August 2016. (Nikki Kahn/The Washington Post)

It’s not mold, soot, or automotive exhaust. Rather, officials say, it is biofilm, an unappetizing-looking smear of microscopic organisms splotched over the dome of the Jefferson Memorial.

The instantly noticeable black-and-gray stain does nothing for the appearance of the 74-year-old monument. Last week, as part of its search for a way to get rid of the biofilm, the National Park Service said it had started testing a process called laser ablation.

Laser ablation removes material from stone by beaming a laser on it. The stone absorbs energy and grows warm. As its temperature rises, the offensive gunk loosens its grip and evaporates.

Scaffolding has been erected on the eastern side of the dome, rising to near its top. It is visible to motorists and pedestrians in the area of the Tidal Basin in Southwest Washington and presumably from such vantage points as the White House.

Biofilm is not unique to the Jefferson Memorial or to Washington. But it thrives on the monument’s dome, the Park Service says, because rain has slowly pitted the stone, providing the ubiquitous organisms with an ideal environment in which to grow.

(National Park Service photo by Mathew John/Gray biofilm covers much of the marble dome of the Jefferson Memorial. National Park Service began a test last week of laser ablation as a way of getting it off. )

Biofilm contamination at the Jefferson was first spotted in 2006, the Park Service said.

But it has become progressively worse since then at the memorial, which honors the country’s third president and the principal author of the Declaration of Independence.

To first-time visitors, or to Washington-area residents who have not seen the memorial in a while, the dome might appear to be something of an eyesore, suggestive of gloom and decay or the after-effects of a major fire.

A variety of techniques have already been tested, the Park Service said, including chemical treatments designed to kill the offending microbes. But the challenge is one typically faced in such restorations: removing the stain while saving the stone.