Ali Drew, a tourist from Wales, asks :-

Why are the ponds at the Grassmere Salt Works pink?

Robert Kooistra, a chemist at the Grassmere Salt Works, responded.

The ponds where we crystallise salt from sea-water are pink because of the presence of two types of microorganisms. The first are single-cell micro-algae that survive in the highly concentrated ponds. The algae produce glycerine in their cells, which stops them from drying out in the concentrated salt water. This ability to survive where others cannot enables them to bloom.

The algae produce beta-carotene (as well as other pigments), which is a deep red pigment. Beta-carotene is found widely in nature and at low concentrations is yellow, as can be seen in butter. It also the orange colour in carrots, where it is more concentrated. In the algae beta-carotene is highly concentrated so has a red-orange hue.

The second microorganisms are bacteria. Smaller than the algae these single celled creatures use the same technique as the algae to avoid dehydration. The bacteria produce a pink pigment called a xanthophyll. This class of pigments is related to the carotenoids.

In cooler temperatures and lower sunlight the bacteria tend to dominate, however as the temperature rises and the light intensifies the algae will grow to large numbers. This effect gives the varying shades of pink to red-orange that can be seen in the Lake Grassmere ponds.

Lake Grassmere is in a temperate climate for a saltworks, and other works around the world have much higher temperature and light intensity. These works are more dominated by the algae, and some have harvested the algae to produce beta-carotene.