Light microscope footage of a Haematococcus pluvialis freshwater alga. This is a single-celled green alga, which photosynthesises using chlorophyll in good conditions. When stressed it produces the orange pigment astaxanthin, as seen 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.