Catalysis of the reaction: a (9Z)-octadecenoyl-containing glycerolipid + 2 H+ + O2 + 2 reduced - = a (9Z,12Z)-octadecadienoyl-containing glycerolipid + 2 H2O + 2 oxidized -.
1-18:1-2-16:0-phosphatidylglycerol omega-6 desaturase activity