Tryptophanol enhances nitrogen assimilation in marine diatoms

Diatoms exhibit high competitive capacity in nitrogen assimilation, but the underlying mechanisms remain unclear. Here, we identify a non-ribosomal peptide synthase-like gene (PtNRPS1) with an atypical domain structure (A-T-R₁-R₂) in the marine diatom Phaeodactylum tricornutum, crucial for short-term nitrogen assimilation. In vitro enzyme assays show PtNRPS1 catalyzes conversion of L-tryptophan to tryptophanol, a tryptophan-derived indole compound that promotes diatom growth at concentrations far lower than indole-3-acetic acid. Transcriptomic, metabolomic analyses, and stable-isotope analyses indicate tryptophanol enhances short-term nitrogen assimilation. CRISPR-Cas9 knockout of PtNRPS1 abolishes tryptophanol biosynthesis and reduces nitrogen-assimilation enzymes activities, which are restored by exogenous tryptophanol. PtNRPS1 overexpression results in delayed but sustained enzyme elevation. Global distribution of PtNRPS1 homologues in stramenopiles positively correlates with nitrogen-assimilation gene abundance. Our findings suggest tryptophanol, synthesized by a diatom NRPS, accelerates nitrogen assimilation, providing a competitive edge in oceanic nitrogen acquisition.