Grand Prismatic Spring: what makes it so colourful?

What lends this geothermal site in Yellowstone National Park its rainbow-like appearance?

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Grand Prismatic Spring

Yellowstone Park, Wyoming, became the world’s first national park when President Ulysses S Grant signed it into law in 1872. It’s not hard to see why the government wanted to preserve this area of great natural beauty, especially with features like this: the world’s third-largest hot spring.
The Grand Prismatic Spring is Yellowstone’s largest at 90 metres (295 feet) wide and 50 metres (164 feet) deep, and works like many of the park’s hydrothermal features. Water deep beneath the ground is heated by magma and rises to the surface unhindered by mineral deposits. As it bubbles to the top it cools and then sinks, only to be replaced by hotter water coming from the depths in a continuous cycle. The hot water also dissolves some of the silica in the rhyolite rocks in the ground, creating a solution that’s deposited as a whitish siliceous sinter onto the immediate land surrounding the spring.
So what makes all the pretty colours? That’s not due to chemicals, anyway. The iridescent pigments are caused by bands of microbes – cyanobacteria – that thrive in these warm to hot waters. Moving from the coolest edge of the spring along the temperature gradient, the calothrix cyanobacteria lives in temperatures of no less than 30 degrees Celsius (86 degrees Fahrenheit), can live out of the water too and produces the brown pigment that frames the spring. Phormidium, meanwhile, prefers a 45-60-degree-Celsius (113-140-degree-Fahrenheit) range and creates the orange pigment, while synechococcus enjoys temperatures of up to 72 degrees Celsius (162 degrees Fahrenheit) and is yellow-green. The deep blue colour seen in the centre is the natural colour of the water and is too hot for most bacteria, although it’s suspected that aquifex, a microbe that thrives in near-boiling water, lives off the hydrogen gas dissolved in the emerging Grand Prismatic Spring’s waters.

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