The invention generally relates to polyunsaturated fatty acid (PUFA) polyketide synthase (PKS) systems isolated from or derived from Schizochytrium sp., to homologues thereof, to isolated nucleic acid molecules and recombinant nucleic acid molecules encoding biologically active domains of such a PUFA PKS system, to genetically modified organisms comprising PUFA PKS systems, to methods of making and using such systems for the production of bioactive molecules of interest, and to novel methods for identifying new bacterial and non-bacterial microorganisms having such a PUFA PKS system.