Diatoms are unicellular eukaryotic algae that produce intricately structured cell walls made of nanopatterned amorphous silica (SiO2). Silica formation in diatoms appears to depend on long-chain polyamines (LCPAs), which contain a propylamine backbone with a variable degree of N-methylation and chain length. Genome analysis of Thalassiosira pseudonana recently revealed four gene fusions encoding the polyamine biosynthetic domains S-Adenosylmethionine decarboxylase (AdoMetDC) and aminopropyltransferase. One of these gene fusions, ThaAPT4 further contains an N-methyltransferase (SET domain). This enzymatic toolkit could be sufficient to synthesize LCPAs with all characteristic features found in T. pseudonana LCPAs. Using a CRISPR/Cas9 mediated knockout, I show that ThaAPT4 is indeed an LCPA synthase, capable of iterative elongation of the aminopropyl backbone of LCPAs and internal N-methylation. However, the ThaAPT4 knockout did not affect the terminal N-methylations found in T. pseudonana LCPAs. I have shown by Scanning electron microscopy and silicon content determination that the perturbations in LCPA biosynthesis did not affect the cell wall morphology.