Phytochelatins (PCs) are oligomers of glutathione, produced by the enzyme phytochelatin synthase (PCS). They are found in plants, fungi, nematodes and all groups of algae including cyanobacteria and are important for heavy metal detoxification. 
PCs have the general structure of (g-glutamyl-cysteinyl)n-glycine (n=2-11) and are therefore abbreviated as PC2 to PC11.

Chemical structure of phytochelatin with n=2-11

Phytochelatins act as chelators, binding metal ions through thiol coordination and by this way, limit damage to metabolic processes in reducing the cytotoxic free metal ion. Because phytochelatin synthase uses glutathione with a blocked thiol group in the synthesis of phytochelatin, the presence of heavy metal ions that bind to glutathione causes the enzyme to work faster. Therefore the amount of phytochelatin increases when the cell needs more phytochelatin to survive in an environment with high concentrations of metal ions. Its widespread presence in microorganisms suggests that PC-based metal detoxification might be an ancient type of defense mechanism established in micro-algae or micro-fungi.