The microbe that rewrote biology (for a year)
In December 2010, NASA announced a discovery that promised to reshape our understanding of life. A team led by NASA astrobiology fellow Felisa Wolfe-Simon reported they had isolated a bacterium from the arsenic-rich sediments of Mono Lake. The bacterium, a Gammaproteobacteria strain named GFAJ-1, appeared to do the impossible. According to their paper published in Science, when starved of phosphorus—a necessary element for all known life—GFAJ-1 could substitute arsenic in its place, incorporating the toxic element directly into its DNA and other essential molecules. The claim was stunning. Arsenic sits just below phosphorus on the periodic table and is chemically similar, but it's typically a potent poison precisely because it disrupts cellular processes that depend on phosphorus. The existence of an organism that could build itself from arsenic suggested a "shadow biosphere" and expanded the possibilities for life on other worlds. However, the scientific community was immediately skeptical. Other researchers, Rosie Redfield of the University of British Columbia, publicly questioned the study's methods.
By 2012, two independent studies, also published in Science, definitively refuted the original claim. The new research demonstrated that GFAJ-1 is an arsenic-resistant, but still phosphorus-dependent, organism. It is exceptionally good at scavenging tiny amounts of phosphate from its environment, and while it can tolerate high arsenic concentrations, it does not build arsenic into its DNA. The original paper was formally retracted nearly 15 years later.
Life in a chemical soup
The "arsenic life" story was debunked, but Mono Lake remains a place of biological interest. It is among the oldest lakes in North America, estimated to be at least 760,000 years old. Because it has no outlet, evaporation has concentrated salts and minerals for millennia. The water is about 2.5 times saltier than the ocean and has a highly alkaline pH of around 10. Its arsenic concentration, derived from hydrothermal springs, is one of the highest in the world at about 200 μM.
Despite these seemingly lethal conditions, the lake supports a simple but productive ecosystem. The food web is based on microscopic algae which, in the spring, can turn the water green. This algae is grazed upon by trillions of endemic brine shrimp, Artemia monica, a species found nowhere else on Earth.
The other main resident is the alkali fly, Ephydra hians. These flies congregate in dense swarms along the shoreline. They have an adaptation for feeding and laying eggs: their bodies are covered in fine, wax-coated hairs that trap a bubble of air, allowing them to crawl underwater completely dry. They can remain submerged for up to 15 minutes, grazing on algae on the lake bottom.
These two invertebrates provide a food source for over a million migratory and nesting birds, including phalaropes and California gulls.
Towers of limestone
The features of Mono Lake are its tufa towers—porous spires of calcium carbonate, or limestone. These towers form entirely underwater. Calcium-rich freshwater from subterranean springs bubbles up into the carbonate-rich lake water. The resulting chemical reaction causes limestone to precipitate, slowly building the towers around the spring's opening over centuries. Some towers grew to heights of over 30 feet.
The towers are only visible today because the lake's level dropped dramatically. In 1941, the Los Angeles Department of Water and Power began diverting Mono Lake's freshwater tributary streams, causing the lake to shrink to half its volume and doubling its salinity. This exposed the underwater towers and threatened the entire ecosystem. Conservation efforts led by the Mono Lake Committee have since helped to stabilize the water level, protecting this unique environment.