The present invention concerns a transformed diatom comprising a nucleic acid sequence operatively linked to a promoter, wherein said nucleic acid sequence encodes an amino acid sequence comprising (i) an heterologous signal peptide and (ii) a polypeptide, said heterologous signal peptide leading to the secretion of said polypeptide in the extracellular medium of said transformed diatom ; a method for producing a polypeptide which is secreted in the extracellular medium, said method comprising the steps of (i) culturing a transformed diatom, (ii) harvesting the extracellular medium of said culture and (iii) purifying the secreted polypeptide in said extracellular medium; and use of said transformed diatom for the secretion of a polypeptide in the extracellular medium.