Australia's Ephemeral Pink Sea
Kati Thanda-Lake Eyre is a place of extremes. Located at Australia's lowest point, about 15 meters below sea level, it is the continent's largest lake—when it contains water. Most of the time, it is a vast, dry salt pan. The lake is an endorheic basin, meaning water flows in but only leaves through evaporation. This process has created a hypersaline environment where salt concentrations can be ten times that of seawater, a condition hostile to most forms of life.
Yet, life persists. The pink and red colors that appear when the lake has water are not from minerals, but from a community of extremophilic microorganisms. These are life forms adapted to environments that are typically lethal. The main organisms are a green alga called Dunaliella salina and several species of salt-loving archaea and bacteria, including Halobacterium salinarum and Salinibacter ruber. D. salina can survive in salt concentrations up to 35%, a level that would destroy the cells of most other organisms.
To cope with the immense osmotic pressure, D. salina produces high concentrations of glycerol within its cell. When stressed by high salinity and intense sunlight, it also produces massive amounts of a pigment called beta-carotene, the same compound that gives carrots their orange color. This carotenoid acts as a protective shield against UV radiation, tinting the alga, and the water, a shade of orange-red.
A Colorful Biological Cascade
The archaea, such as Halobacterium salinarum, contribute their own distinct color. These single-celled organisms possess a protein called bacteriorhodopsin in their cell membranes, which can make up nearly 50% of the cell's surface area. This protein is purple and is a light-driven proton pump, capturing solar energy to produce ATP, the cell's energy currency. Other bacteria, like Salinibacter ruber, produce a pigment called bacterioruberin, which is also spread throughout the cell. The combined effect of these different pigmented microbes creates the lake's pink, red, and orange hues.
The bloom is a rare event. Minor floods, causing a one-and-a-half-meter depth, happen roughly every three years, but the lake has filled to capacity only three or four times in the last 160 years. When floodwaters from Queensland's channel country arrive, they dilute the salt crust, triggering a biological explosion. Dormant brine shrimp eggs hatch by the million, feeding on the algae. The population of Dunaliella salina blooms first, followed by the archaea and bacteria that feed on the byproducts of the algae. This entire bright ecosystem becomes so expansive that the lake's color change is visible to satellites orbiting 36,000 kilometers above the Earth.