Chloroplast omega-3 fatty acid desaturase introduces the third double bond in the biosynthesis of 18:3, and probably also 16:3 fatty acids, important constituents of plant membranes. It is thought to use ferredoxin as an electron donor and to act on fatty acids esterified to galactolipids, sulfolipids and phosphatidylglycerol. a (7Z,10Z)-hexadecadienoyl-containing glycerolipid + 2 H(+) + O2 + 2 reduced [2Fe-2S]-[ferredoxin] = a (7Z,10Z,13Z)-hexadecatrienoyl-containing glycerolipid + 2 H2O + 2 oxidized [2Fe-2S]-[ferredoxin] a (9Z,12Z)-octadecadienoyl-containing glycerolipid + 2 H(+) + O2 + 2 reduced [2Fe-2S]-[ferredoxin] = (9Z,12Z,15Z)-octadecatrienoyl-containing glycerolipid + 2 H2O + 2 oxidized [2Fe-2S]-[ferredoxin] Lipid metabolism; polyunsaturated fatty acid biosynthesis. Highly expressed in leaves and cotyledons, while no or little expression detected in mature seeds, roots and stems. The histidine box domains may contain the active site and/or be involved in metal ion binding. Belongs to the fatty acid desaturase type 1 family.