Lake Hillier, located on Middle Island in Western Australia, presents one of nature’s most striking visual phenomena: a body of water permanently colored a vibrant bubblegum pink. This small, saline lake, approximately 600 meters in length, is set against the deep blue of the Southern Ocean and the rich green of surrounding eucalyptus and paperbark forests, creating an almost surreal contrast. The lake’s permanent, bright hue has fascinated explorers and scientists since its first recorded sighting in 1802 by Matthew Flinders.
Geological Origin of the Lake Basin
The physical structure of Lake Hillier is intrinsically linked to the formation of Middle Island, the largest landmass within the Recherche Archipelago. The lake basin likely formed in a low-lying depression where seawater was originally trapped.
The isolation of this trapped water from the vast ocean is achieved by a narrow, natural barrier composed of sand and vegetation. This thin strip of land separates the hypersaline lake from the regular seawater of the Southern Ocean. This geological isolation allowed the water chemistry within the basin to evolve radically, setting the stage for the lake’s unique characteristics.
The Role of Extreme Salinity
The chemical environment within Lake Hillier is defined by its extreme hypersalinity, a prerequisite for the vivid pink coloration. The lake is a terminal system, meaning that water flows in but can only escape through evaporation, which concentrates dissolved salts over millennia.
This continuous evaporation, combined with low rainfall and limited freshwater inflow, results in salt concentrations that are comparable to or even exceed those of the Dead Sea. The salt concentration in the water is estimated to be around ten times higher than that of regular ocean water. This high salinity creates an environment inhospitable to most aquatic life forms, acting as a selective pressure that favors specific salt-loving organisms. The intense concentration of sodium chloride and other minerals is so high that a permanent salt crust forms around the shoreline.
The Biological Agents Causing the Pink Color
The cause of Lake Hillier’s permanent pink color is a thriving community of pigment-producing microorganisms, or polyextremophiles, adapted to the hypersaline conditions. The microalgae Dunaliella salina is a major contributor, known to accumulate high concentrations of the red-orange pigment beta-carotene when subjected to the stress of high salt and intense sunlight. This carotenoid is the same pigment found in carrots.
Recent extensive microbiome analysis indicates the pink hue is the result of a diverse microbial community, not just the algae. Halophilic archaea, such as those belonging to the class Halobacteria, and bacteria like Salinibacter ruber, are also abundant in the lake’s water and salt crusts. These organisms produce their own red-colored pigments, like bacterioruberin, which contribute a significant depth and permanence to the bright pink shade.