Algal Bloom Die-Off Mechanism Revealed in Study, But Lincoln Memorial Reflecting Pool Remains Green

The Lincoln Memorial Reflecting Pool in Washington, D.C., turned a startling green in June due to an algal bloom, but new research published in Science on June 25 explains how such blooms might collapse naturally. The study identifies a chemical reaction involving iron and hydrogen peroxide that triggers cell death in cyanobacteria, though it may not save the pool’s current ecosystem.

How Cyanobacteria Die Off

Researchers led by phycologist Yi Tao of Tsinghua University in Shenzhen discovered that a process called ferroptosis—a chain reaction involving ferrous iron and hydrogen peroxide—can cause cyanobacteria to self-destruct. This reaction, known as the Fenton reaction, generates reactive oxygen species that damage cell membranes, leading to rupture. The team observed this mechanism in a 2024 cyanobacteria bloom of the genus Microcystis in Dianchi Lake, China.

The study found that cyanobacteria store excess iron as ferrous iron, which interacts with hydrogen peroxide to create free radicals. These radicals attack lipid molecules in cell membranes, causing them to disintegrate.

Why the Reflecting Pool’s Bloom Won’t Be Fixed

Despite the breakthrough, the Lincoln Memorial’s green water may not benefit from the findings. The bloom there appears to be dominated by Scenedesmus, a eukaryotic cell type that responds differently to hydrogen peroxide treatments, according to aquatic scientist Lewis Molot of York University. “Hydrogen peroxide concentrations used to control cyanobacteria in research trials have had a minimal effect on eukaryotic algae,” he said.

The pool’s recent paint job, which was supposed to be “American Flag blue,” may have made the water warmer, noted biogeochemist Markus Dengg of the Otago Regional Council in New Zealand. The paint may contain iron, which scientists might have expected to help set off reactions, but the water came in from the Potomac River, full of microorganisms and nutrients. It creates a system, Dengg says, “where the strongest species survives in the end.”

Algal Blooms: Costs and Consequences

Algal blooms are estimated to cost $50 million per year in the United States. Beyond aesthetic concerns, dying blooms deplete oxygen in water, suffocating aquatic life and releasing toxins.

The new study offers insights into natural die-offs but does not provide a quick fix for the Reflecting Pool. The results offer a “nice window” into how algal blooms might collapse, says Dengg. “But real-world ecosystems are far more complex.”

What’s Next for Algal Bloom Research?

Scientists are now exploring whether the Fenton reaction could be harnessed to manage blooms. However, the study’s authors caution against oversimplification.