Green alga cyst (Haematococcus pluvialis), SEM

Coloured scanning electron micrograph (SEM) of green alga (chlorophyte) cyst stage (Haematococcus pluvialis). This alga occurs in many different aquatic environments including fresh and brackish water. It is believed to be the world's richest known source of astaxanthin. Astaxanthin is a natural carotenoid pigment and a biological antioxidant. When environmental conditions become adverse (i.e. when nutrients start becoming scarce, the water environment starts to dry out and the algae are increasingly exposed to direct sunlight), they enter a resting phase (cyst phase) that allows them to survive for prolonged periods, until the environment becomes more favourable for growth. In the resting phase the alga accumulates high amounts of astaxanthin. In the vegetative phase Haematococcus is a unicellular, microscopic green alga that has two flagella. It contains a single, cup-shaped chloroplast, with several pyrenoids. Magnification: x220 when shortest