Sea ice is white...right? Well, mostly it is. Take this piece of sea ice for example. It's not dirty, it's covered in algae that loves living in freezing conditions.
This particular algae goes my several names - 'snow algae' (makes sense), 'pink algae' (also makes sense), 'watermelon algae' (I have NO idea if it tastes like watermelon... I don't suggest you find out - rumour has it that it helps you… go to the bathroom… a lot…), and the somewhat macabre 'blood snow' (it's definitely not blood!).
Oh and in case you were thinking this is a pink algae, it's not. It's green. Really. The pink comes from a pigment called astaxanthin - a carotenoid with an orangy-red colour. Yup - that's what makes carrots orange!
Now the pink colour isn't there just to amuse us. It has a very important role. Carotenoids are good at absorbing UV - really good. Essentially it acts like a natural sunscreen for the algae. Without it, the algae would suffer tremendously from the sun's rays. It also turns out that the pigment is really important for melting snow.
When the suns' rays hit a light colour, its wavelengths are reflected away, but when the rays hit a dark colour, the wavelengths are absorbed. This means that light objects stay cooler than dark colours. So whilst the snow (or ice) remains cool, the darker-coloured algae get warm. The result - the snow around the algae melts much quicker!