A caustic chemical bath
Lake Magadi is an extreme environment, a 100-square-kilometer saline, alkaline lake in the southern Kenyan Rift Valley. During the dry season, up to 80% of its surface is covered by a solid white crust of sodium carbonate minerals. The lake water is a dense brine with a pH that can reach 11.5, similar to ammonia solution. This intense alkalinity comes from a unique geological setting. The lake has no river outlets and is recharged primarily by saline hot springs. Underground water percolates through volcanic rocks, picking up sodium and carbonate ions. It is then heated and discharged into lagoons around the lake margins at temperatures up to 86°C (187°F). The intense equatorial sun causes rapid evaporation, concentrating the salts and precipitating vast quantities of a mineral called trona (sodium sesquicarbonate).
This process has been happening for millennia. The trona crust is up to 40 meters (130 feet) thick in places, representing an accumulation of at least 9,000 years. The lake sits in a basin containing more than 30 billion metric tons of this mineral. The distinctive pink and red colors of the brine are not from the minerals themselves, but from immense populations of salt-loving microorganisms. Halophilic archaea, a type of single-celled life, produce protective carotenoid pigments that give the water its dramatic hue, which is most pronounced in the dry season when salinity peaks.
Life in the extreme and a view of the past
Despite the hostile chemistry, life has found a way to thrive. The lake is an important feeding ground for Lesser Flamingos (Phoeniconaias minor), which flock here to consume microscopic blue-green algae, or cyanobacteria, that flourish in the brine. Their specialized beaks filter the algae from the water, and pigments in the algae are responsible for the birds' famous pink color.
The only fish in the entire lake is the Magadi tilapia (Alcolapia grahami), a small, endemic cichlid. This species is an extremophile. It lives in the slightly less alkaline lagoons near the hot spring inlets, tolerating water temperatures up to 42°C (108°F) and the caustic pH. It has unique physiological adaptations, including a fully functional urea cycle to excrete ammonia in an environment where it would normally be toxic.
Sedimentologists study Lake Magadi as an analogue for the "soda lakes" believed to have existed on prebiotic Earth. During the Archean Eon, before the oceans became dominated by sodium chloride, similar alkaline lakes may have been common. The chemical precipitation of sodium carbonates and silica-rich cherts at Magadi provides a modern model for how ancient rock formations were created. Studying the lake's evaporite deposits and the microbial life they entomb records the geochemistry and potential biology of our planet billions of years ago.