An alkaline lake in California with bizarre limestone towers called tufa rising from the water. In 2010, NASA announced that bacteria here used arsenic in place of phosphorus. The claim was later debunked, but the lake is a location for extremophile research.
m01229 from USA, CC BY-SA 2.0
A unique chemical environment
Mono Lake is a terminal lake, meaning it has no outlet; water flows in but only leaves through evaporation. This process, occurring over at least 760,000 years, has concentrated salts and minerals, making the lake two to three times saltier than the ocean. Its water is also highly alkaline, with a pH of about 10, similar to household glass cleaner. The water feels slick to the touch and contains high levels of chlorides, carbonates, and sulfates.
The most visible features are the tufa towers—spires of porous limestone that grow entirely underwater. They form when calcium-rich freshwater from underground springs seeps up and mixes with the carbonate-rich lake water. This chemical reaction causes calcium carbonate (limestone) to precipitate, slowly building the towers over centuries. Some towers reach heights of over 30 feet. The towers became visible only after 1941, when the Los Angeles Department of Water and Power began diverting the streams that feed the lake. This caused the lake's level to drop by more than 45 feet, exposing the limestone formations that had grown beneath the surface.
The microbe at the center of a storm
In 2010, Mono lake entered the international spotlight. NASA-funded researchers announced the discovery of a bacterium, GFAJ-1, that appeared to substitute the toxic element arsenic for phosphorus in its fundamental biomolecules, including its DNA. This claim challenged the understanding that all known life requires six essential elements, including phosphorus. The rod-shaped bacterium belongs to the family Halomonadaceae and was isolated from the lake's arsenic-rich sediments.
The scientific community responded with immediate and intense skepticism. Critics argued that any DNA with an arsenate backbone would be too unstable to exist and that the original experiments were likely compromised by phosphate contamination. By 2012, independent studies confirmed these doubts. Researchers, including Rosie Redfield of the University of British Columbia, demonstrated that GFAJ-1 is an arsenic-resistant, phosphorus-dependent organism. It lives in the high-arsenic environment but does not incorporate the element into its DNA. The original 2010 paper was eventually retracted. Despite the debunking, the controversy spurred new research into the limits of life, making Mono Lake a place for studying extremophiles.
💡Fun Facts
The lake is one of the oldest in North America, with geologists estimating its age at a minimum of 760,000 years.
The high salinity and alkalinity make it impossible for fish to live in the lake. The ecosystem's foundation consists of algae, brine shrimp (*Artemia monica*), and alkali flies (*Ephydra hians*).
In 2019, scientists discovered eight new species of arsenic-resistant nematode worms living in the lake, one of which has three distinct sexes: male, female, and hermaphrodite.
Millions of migratory birds use the lake as an important feeding stopover, relying on the abundant shrimp and flies. At peak times, 1.5 to 1.8 million Eared Grebes have been counted on the lake.
The South Tufa Area is open 24 hours a day. The Mono Basin Scenic Area Visitor Center is generally open daily from 9:00 AM to 5:00 PM in the summer, with reduced hours or closures in winter.
Admission
Access to the South Tufa Area requires a $3 fee for adults; children under 16 are free. National Park passes are accepted.
Accessibility
The South Tufa Interpretive Trail is a 1-mile loop. The first 0.25 miles are accessible, consisting of hard-packed native soil. Vault toilets and picnic tables are available.
