The National Park Service has started a 15-month restoration project on the Jefferson Memorial. It includes using specialized lasers to remove a microbial colony that has been thriving on the monument stones for years.

WASHINGTON — Parts of the Thomas Jefferson Memorial will be closed to the public as the National Park Service begins a 15-month restoration project.

The park service plans to restore the roofs, repair stone and clean marble at the monument. The project includes the use of specialized lasers to remove black biofilm, a colony of microscopic organisms including algae, fungi and bacteria.

The biofilm is visible on the upper parts of the memorial. It first became noticeable in 2006, the park service said, and continued to grow in the past few years.

The park service has been studying the growth since 2014 to find the best way to remove the film without damaging the stone or harming the environment.

Biofilm can be found throughout D.C. It grows on tombstones at the Congressional Cemetery and has also been found on monuments around the world, the park service said in 2016.

The project will also involve replacing the upper and lower roofs that circle the dome to keep water from seeping in. Deteriorated waterproofing will be replaced underneath the marble covering the portico. Roof drains, downspouts and gutters will also be upgraded.

The east side of the memorial will be closed for construction staging, but visitors will be able to access the front of the memorial, including the chamber with the Thomas Jefferson statue. The front steps, accessible route, restrooms and the elevator will remain open as well.

The restoration is expected to be complete by May 2020.