The Grand Prismatic Lake of the Yellowstone National Park is special due to two main reasons: first, the fact that it is one of the largest hot springs in the world, and more importantly, the several brilliant colours that its waters exhibit. Though many simply dismiss it as a photographic effect, the lake is indeed rainbow coloured, showcasing the colours red, orange, yellow and a deep blue, and thus it is called “prismatic”.
Like any other hot spring, the water at the centre of the lake is hot, and gradually cools down towards the edges. Due to the extremely high temperatures at the centre, it is assumed that almost no form of aquatic life exists in the region, leading to the blue colour that is normally associated with water due to the wavelength of light reflected by it.
However, as we approach the cooler yellow-green region (around 74ºC), certain organisms like Synechococcuss (a type of cyanobacteria) can be observed in the waters. These organisms depend on photosynthesis for nutrition and thus, contain photosynthesis pigments like chlorophyll. However, Synechochoccuss actually thrive in temperatures around 65ºC, and the harsh sun rays in the elevated National Park only make it more difficult for the bacteria to survive. A neat balance between photosynthesis pigments is thus required, and the chlorophyll in the bacteria has been surpassed by a carotenoid (photosynthesis pigments that are a different shade of colour than green, like red or yellow; carrots are orange in colour due to the presence of carotenoids) that makes it appear yellowish in colour. These carotenoids protect the bacteria from the harsh sunlight, and thus, allow it to survive in temperatures higher than usual.
The outer orange band is cooler than the yellow one, with average temperatures of around 65ºC, and many more microbes including Synechococcuss thrive here. A new family called the chloroflexi bacteria also appears, which makes use of slightly different types and concentrations of carotenoids and chlorophyll, leading to a net orange colour. Here too, the carotenoids are used to protect the life forms from harsh temperatures.
The red colour band is the outermost, and also with the coolest temperatures (around 55ºC). Several microbial species exist here, and the mix of their carotenoids leads to the red colour at the shores.
Since the presence of these carotenoids is dependent on temperature, the bands often tend to lose their colour in the winters when the temperatures drop, and tend to resemble the blue-green colour characteristic of chlorophyll, while the colours can be seen clearly in summers. Truly,the Grand Prismatic Lake is a natural and scientific marvel and a brilliant example of the power of nature!