The algae in the Lincoln Memorial Reflecting Pool bloomed seemingly overnight. Algal blooms also collapse suddenly, and for at least one type of algae scientists have now found a clue as to why: A chemical reaction between iron and hydrogen peroxide that explodes their cells.
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The newly painted Lincoln Memorial Reflecting Pool in Washington, D.C. was supposed to be “American Flag blue.” Instead, on June 16, the water turned an un-American shade of sickly green as an algal bloom burst through the water. While experts keep trying to eradicate the unsightly bloom, a research team has revealed new insights into how mass algal demise might happen naturally.
A sudden algal die-off could be caused by an interaction between iron and hydrogen peroxide, researchers report June 25 in Science. The chemical reaction produces reactive molecules that burst cell membranes apart. Death could spread from a single cell to collapse an algal bloom if conditions are right. While the results could explain why at least some algal blooms die off, the findings can’t turn the Reflecting Pool blue. The situation there may be too far gone
The study focuses on cyanobacteria (sometimes called “blue-green algae,” the organisms are actually bacteria). In the right environment, millions of these photosynthesic microbes can proliferate in fresh and saltwater, though they are less likely in moving water. While the sight causes consternation, it’s their deaths that are dangerous. The organisms die off en masse, and other bacteria gobbling their corpses use up oxygen in the water. Fish and aquatic invertebrates suffocate. The dead cyanobacteria can also release toxins into the water, threatening beach days and drinking water. Algal blooms are estimated to cost $50 million per year in the United States (not including the Reflecting Pool).
Why the cyanobacteria all die off at once has been a mystery. In the new study, researchers looked at ferroptosis, a chain reaction involving a chemical form of iron called ferrous iron. It interacts with hydrogen peroxide in a process called the Fenton reaction. The chemical transformations produce reactive oxygen species, also called free radicals, which chop up long fatty chains that make up cell membranes. Riddled with holes, the cell bursts apart.
Cells need iron, and cyanobacteria in particular need a lot of it. They store their excess as ferrous iron. All that’s needed for the Fenton reaction is hydrogen peroxide, and there’s plenty in the water all around the bloom, formed when water and decomposing matter is exposed to sunlight.
Phycologist Yi Tao and colleagues observed an algal bloom of the cyanobacteria genus Microcystis in Dianchi Lake in China’s Yunnan province in 2024. Right before the bloom ended, the responsible cyanobacteria had high levels of oxidation—an increase in free, reactive electrons—and three times the usual amount of ferrous iron. When hydrogen peroxide was given to one of the bloom’s species, M. aeruginosa, in the lab, the levels of iron and free radicals in the cells skyrocketed. Those free radicals attacked lipids in the cell membranes.
The freshly burst cells still weren’t done. The torn-up lipids, called truncated phospholipids, spread to nearby cells, tearing their membranes apart. The truncated phospholipids are “unstable,” says Tao, of Tsinghua University in Shenzhen, China. “Once released, these molecules tend to automatically aggregate,” he says. They form little bubbles called vesicles, which “spread chain reactions when interacting with neighboring cells.”
The results offer a “nice window” into how algal blooms might collapse, says Markus Dengg, a biogeochemist and science communicator at the Otago Regional Council in New Zealand who was not involved in the study. He’d like to see the if ferroptosis can take down a whole bloom in a lake where many other biological and geochemical processes are taking place.
Unfortunately, the new knowledge may not help the Reflecting Pool, says Lewis Molot, an aquatic scientist at York University in Toronto who was not involved with the work. While the bloom there might have started as a collection of cyanobacteria like the Microcystis in this study, it now appears to be dominated by Scenedesmus, which is a eukaryotic cell type. “Hydrogen peroxide concentrations used to control cyanobacteria in research trials have had a minimal effect on eukaryotic algae,” he says.
The dark blue paint not only makes the water in the pool warmer, Dengg says, but may contain iron, which scientists might have expected to help set off reactions. But the water then came in from the Potomac River, full of microorganisms and nutrients. It almost creates an experimental system, Dengg says, “where the strongest species survives in the end.”
