1-acyl-2-(9Z,12Z-octadecadienoyl)-3-[alpha-D-galactosyl-(1->6)-beta-D-galactosyl]-sn-glycerol + 2 reduced [2Fe-2S]-[ferredoxin] + O2 + 2 H(+) = 1-acyl-2-(9Z,12Z,15Z-octadecatrienoyl)-3-[alpha-D-galactosyl-(1->6)-beta-D-galactosyl]-sn-glycerol + 2 oxidized [2Fe-2S]-[ferredoxin] + 2 H2O