WASHINGTON, D.C. — The water in the Lincoln Memorial Reflecting Pool turned green on June 16 due to an algal bloom. This event occurred as researchers reported an interaction between iron and hydrogen peroxide can cause a sudden algal die-off.

On June 25, researchers reported in the journal Science that this chemical reaction produces reactive molecules that burst cell membranes. The study focused on ferroptosis, a chain reaction involving ferrous iron and hydrogen peroxide known as the Fenton reaction. This reaction creates reactive oxygen species, also called free radicals, which break down fatty chains in cell membranes. Cyanobacteria, which are photosynthetic bacteria that can proliferate in fresh and saltwater, store excess iron as ferrous iron.

When hydrogen peroxide was introduced to M. aeruginosa in a laboratory setting, researchers observed an increase in the levels of iron and free radicals within the cells. These free radicals subsequently attacked lipids in the cell membranes of the cyanobacteria. Yi Tao, a phycologist at Tsinghua University in Shenzhen, China, stated, "Once released, these molecules tend to automatically aggregate." Truncated phospholipids then form vesicles that can spread chain reactions when they interact with neighboring cells.

Hydrogen peroxide typically forms in water when water and decomposing matter are exposed to sunlight. When large numbers of cyanobacteria die, other bacteria consume them, which can deplete oxygen in the water. This process can lead to the suffocation of fish and aquatic invertebrates. Additionally, dead cyanobacteria have the potential to release toxins into the water. Algal blooms are estimated to cost the United States $50 million annually.

The algal bloom in the Reflecting Pool appears to be composed primarily of Scenedesmus, which is a eukaryotic cell type. Lewis Molot, an aquatic scientist at York University in Toronto, noted that "Hydrogen peroxide concentrations used to control cyanobacteria in research trials have had a minimal effect on eukaryotic algae." Molot was not involved in the study published in Science. Markus Dengg, a biogeochemist at the Otago Regional Council in New Zealand, also did not participate in the study.

The dark blue paint used in the Reflecting Pool contributes to warmer water temperatures. The water that fills the Reflecting Pool originates from the Potomac River, which contains microorganisms and nutrients.