The Reflecting Pool on the National Mall in Washington, D.C., has turned a bright pea green due to algal growth, a common occurrence in shallow, stagnant waters during the summer months as temperatures rise. Efforts to address this, including a $14 million repainting initiative commissioned without a competitive bidding process during the Trump administration, appear to have had little effect. Experts suggest that the darker shade used in the pool’s bottom coating may have inadvertently exacerbated the situation by absorbing more heat.

Algal blooms are becoming increasingly prevalent worldwide, driven by the combined effects of ocean warming from fossil fuel-driven climate change and elevated nutrient runoff caused by agricultural practices, deforestation, and urban pollution. Some blooms, referred to as harmful algal blooms (HABs), produce toxins that can adversely impact marine life, contaminate drinking water sources, and disrupt commercial fisheries and other industries.

The global escalation of algal blooms has had visible consequences, including extensive accumulations of sargassum seaweed along beaches in Florida and the Caribbean, as well as “green tides” of sea lettuce washing ashore in southern China. In March, the United Nations reported a marked increase in the geographic distribution, frequency, and severity of HABs, linked to mass die-offs of fish and marine mammals, as well as human health risks through consumption of tainted seafood or direct exposure to contaminated water.

Despite these challenges, algae have played an essential role in Earth's environmental history and current ecological functioning. Along with cyanobacteria, algae were instrumental in the Great Oxidation Event around 2.5 billion years ago, which introduced oxygen into the atmosphere and laid the groundwork for aerobic life. Today, marine microalgae such as phytoplankton contribute approximately half of the planet's oxygen production. Additionally, macroalgae—including about 12,000 species of seaweed—and coastal ecosystems like seagrasses, mangroves, and salt marshes provide another 20% of the world's oxygen supply.

At the University of California, Berkeley’s Jepson Herbaria, researchers maintain an extensive collection of preserved algae specimens that aid in molecular and genetic studies tracking oceanic changes. These investigations are particularly urgent as kelp forests, crucial macroalgal ecosystems composed of roughly 30 species, face rapid decline. A 2023 study published in Nature Communications estimated that kelp forests support a global economy valued at $500 billion annually. Their ecological role is vital, serving as habitat during critical life stages for species such as salmon, cod, rockfish, and herring, as well as supporting over 1,000 other marine species. Kelp also protects temperate coastlines from storms and erosion and is widely harvested for human use.

Kelp contains algin, a substance making up 40% of its cellular framework, which has been utilized since the 1920s as a stabilizer and emulsifier in food products, cosmetics, and pharmaceuticals. Algal derivatives are common ingredients in everyday items, including toothpaste, lipstick, shampoo, processed foods, and even heartburn medication.

Furthermore, research into sustainable agriculture has highlighted algae’s potential for environmental benefits. Incorporating small amounts of certain red algae species into cattle feed has shown promise in reducing methane emissions—a potent greenhouse gas—from livestock by 50 to 90%. This approach is already being implemented in countries such as Australia. In addition, innovative algae-based industries are emerging globally. For example, a biorefinery on the Black Sea coast in Turkey is producing jet fuel for Turkish Airlines and omega-3 supplements from microalgae, while utilizing carbon dioxide captured from a cement factory to reduce emissions.

Beyond biofuels and nutritional supplements, algae are being explored for their applications in bioplastics, construction materials, new pharmaceuticals, carbon dioxide sequestration, and medical treatments such as burn care.

While algal growth presents challenges in public spaces like the Reflecting Pool, the broader environmental and economic significance of algae underscores the need to appreciate their complex role in Earth's ecosystems and their potential in advancing sustainable solutions.