To understand the pink, you first have to understand the salt.

Australia's pink lakes are hypersaline, meaning they contain salt concentrations far beyond that of the ocean. The lakes are about ten times saltier than the ocean, and some are even more extreme than that. The main species involved, Dunaliella salina, can survive in waters with up to 35 percent salt concentration by weight, compared to seawater which contains only around 3 percent.

Most life cannot tolerate these conditions at all. The salt draws water out of cells by osmosis faster than they can replace it. Fish, aquatic insects, most algae and bacteria: all eliminated. What remains is a highly selective environment that only the most salt-adapted organisms, called halophiles or extremophiles, can survive in.

These lakes are the remnants of ancient rivers that flowed across the landscape more than 15 million years ago. As those rivers dried up, pockets of water were left behind and partially evaporated over time, concentrating salt and creating the conditions that now sustain the microorganisms responsible for the colour.

The extreme salinity is not a problem for the organisms that thrive here. It is a competitive advantage. By tolerating conditions that exclude almost everything else, they have inherited an ecosystem largely to themselves.

Two main organisms are responsible for the pink in most of Australia's coloured lakes.

The first is Dunaliella salina, a single-celled green microalgae. Under normal conditions it is green, like most algae, because it uses chlorophyll to photosynthesise. But under the stress of extreme salinity and intense sunlight, Dunaliella salina produces large quantities of beta-carotene as a protective pigment. Beta-carotene is the same compound that makes carrots orange, gives flamingos their pink, and colours the flesh of salmon. In Dunaliella salina, it accumulates in such concentrations that the cell turns from green to a vivid orange-red, and when enough cells are present in the water, the lake turns pink.

The second is Salinibacter ruber, a rod-shaped bacterium that produces its own reddish pigments as part of its normal metabolism. A 2015 study of Lake Hillier by the Australian Genome Research Facility found Salinibacter ruber present in the lake's waters, contributing to its famously stable pink colour.

What is particularly interesting about Lake Hillier is that unlike other pink lakes whose colour fluctuates with outside temperature, its water maintains its pinkish hue year-round. The combination of organisms present, and their specific balance in that particular body of water, produces a colour that is remarkably consistent regardless of season or conditions.

Other lakes are more changeable. Lake Bumbunga in South Australia, for example, appears more intensely pink in spring than in winter due to higher concentrations of fresh water and more sunlight, and generally evaporates in the dry summer months leaving salt deposits rather than pink water.

The same pigment, different creatures

One of the most elegant things about the biology of the pink lakes is how it connects to colour science you might already know.

Beta-carotene, the pigment that Dunaliella salina produces under stress, is a carotenoid. Carotenoids are a large family of pigments produced by plants, algae, and bacteria, and they are responsible for an extraordinary range of colours in the natural world. The orange of a carrot. The red of a tomato. The yellow of a sunflower. The pink of a flamingo. The deep orange of a cooked lobster. All carotenoids, all the same molecular family, expressed in different concentrations and contexts.

Animals cannot produce carotenoids themselves. They must consume them. This is why flamingos, which eat Dunaliella salina and brine shrimp loaded with carotenoids, are pink: they are wearing a colour that originally came from algae. The pink lake and the pink flamingo are, in a very direct sense, variations of the same colour story.

In the pink lakes, the algae are not consuming the carotenoid but producing it, using it as a biological sunscreen to protect themselves from the intense Australian sunlight. Colour, here as everywhere in nature, is not decoration. It is function.

The formerly famous Pink Lake in Esperance, Western Australia, has been a milky white colour for several years now. The cause is thought to be changes in the local hydrology that altered the salt concentration and the balance of organisms in the water. Remove the right conditions and the biology shifts, and the colour goes with it.

Scientists have suggested that climate change could cause new lakes to start turning pink as conditions become more suitable for halophilic organisms, while others that are currently pink may dry out entirely as drought intensifies. The pink is not permanent. It is a snapshot of a particular ecological equilibrium, held in place by temperature, salinity, light, and the organisms that have adapted to exactly those conditions.

Australia's pink lakes are one of the most visually striking demonstrations of something that runs through all of colour science: colour in nature is almost never arbitrary.

Every vivid colour you encounter in the natural world is the output of a biological or physical process. It is information. It is the visible record of chemistry, adaptation, and environment expressed in wavelengths of light.

The pink of Lake Hillier is a single-celled organism's response to stress. The red of Blood Falls is ancient iron meeting oxygen for the first time in a million years. The blue of the sky is short wavelengths of sunlight scattering off atmospheric gas. The cyan, magenta, and yellow of a CMY Cube are wavelengths of light being filtered through transparent resin.

Different mechanisms, different scales, different corners of the world. But the same underlying question: what happens when light meets matter?

The answer is always colour.