The present invention relates to a method for increasing the content of polyunsaturated long-chain fatty acids in an organism by introducing into said organism nucleic acids coding for polypeptides or proteins exhibiting a phospholipase, ketoacyl-CoA reductase and/or dehydratase activity. Advantageously, said enzymes originate from Ostreococcus or Thraustochytrium. The present invention further relates to the nucleic acid sequences, nucleic acid constructs, vectors and organisms containing the nucleic acid sequences according to the present invention, vectors containing the nucleic acid sequences and/or the nucleic acid constructs as well as to transgenic organisms containing the above mentioned nucleic acid sequences, nucleic acid constructs and/or vectors. Advantageously, said previously mentioned nucleic acid sequences, nucleic acid constructs, vectors can optionally be expressed in the organism together with further nucleic acid sequences coding for polypeptides or proteins of the biosynthesis of the fatty acid or lipid metabolism. Particularly advantageous herein are those nucleic acid sequences of the fatty acid or lipid metabolism that code for a .DELTA.-9 elongase, .DELTA.-8 desaturase, .DELTA.-6 desaturase, a .DELTA.-5 desaturase, .DELTA.-4 desaturase, .DELTA.-12 desaturase, .DELTA.-5 elongase and/or .DELTA.-6 elongase activity. Advantageously, said desaturases and elongases originate from organisms like Thalassiosira, Euglena, Isochrysis, Physcomitrella, Thraustochytrium, Borago, Phytophthora, Crypthecodinium, Oncorhynchus, Primula, Xenopus, Ciona, Arabidopsis, Mortierella, Caenorhabditis, Phaeodactylum, Ceratodon or Ostreococcus. A further part of the present invention relates to oils, lipids and/or fatty acids produced according to the method of the present invention and the use thereof. Furthermore, the present invention relates to unsaturated fatty acids and triglycerides having an increased content of unsaturated fatty acids and the use thereof.