What appears to be an oil slick contaminating the surface of this backwater pool along southern Spain’s Gaduares River is actually the result of a living, metabolizing colony of Leptothrix discophora. These chemotrophic bacteria don’t eat or photosynthesize; instead, they derive their energy from iron and manganese dissolved in the river water. The iridescent colors result from the bacteria’s oxidation of these heavy metals, a chemical reaction—not unlike the burning of charcoal in a grill—that releases energy the bacteria can use to fuel their cellular processes.

Leptothrix colonies tend to proliferate here in summer when stream levels drop, leaving relatively still pools of iron- and manganese-rich water. The bacteria can generate extensive films of oxidized metal across the water’s surface—this one covered some 20 square meters (215 square feet). When disturbed, by a fallen leaf or the motion of the underlying water, the film—a thin solid, not a liquid—will shatter.

Though Leptothrix colonies like this one exist in the wild, scientists are now working to harness the bacteria’s metal-oxidizing abilities to benefit human societies. Iron and manganese occur naturally in mineral deposits. Typically, they break down and are released into the environment slowly, but mining operations and development can speed up this process, contaminating streams, lakes, and groundwater. In Bangladesh, where some groundwater sources have toxic levels of iron and manganese, scientists have begun seeding water filters with Leptothrix bacteria. The bacteria oxidize the metals dissolved in the water and convert them to a solid that can be strained out to make the water safe to drink.

Sierra de Grazalema Natural Park, Spain

Andrés Miguel Domínguez

Born in Madrid, Andrés Domínguez has always been attracted to animal life, especially birds. He trained as a technical forest engineer but has been dedicated to nature photography since 1994. His work has been recognized in various international competitions, including BBC Wildlife Photographer of The Year, and his images have been published in several books and magazines, including National Geographic and Terre Sauvage.

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