Chloroplast omega-3 fatty acid desaturase introduces the third double bond in the biosynthesis of 16:3 and 18:3 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. A number of isoforms are produced. According to EST sequences. By low temperature. 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.