Light microscope footage of the freshwater algae Haematococcus pluvialis. This single-celled algae is known for producing the orange pigment astaxanthin when the cells are stressed, as here. Astaxanthin is an antioxidant and is thought to act as a sunscreen for the cells. The chemical is extracted and widely used in animal feeds, human dietary supplements and some cosmetics. H. pluvialis cells average around 20 micrometres in diameter, and are the richest natural source of astaxanthin. Despite this, the chemical can still be produced more cheaply from petroleum products.