Influence of Nitrate and Light on Fucoxanthin Content and Key Gene Expression in the Marine Diatom Thalassiosira rotula

Fucoxanthin is the predominant carotenoid in diatoms, playing a central role in light harvesting and photoprotection, and is increasingly valued for its potential in pharmaceutical, nutraceutical, and cosmetic applications. In this study, we investigated the influence of high nitrate supplementation, low-light exposure, and combined treatment, on fucoxanthin content and on the expression of key genes involved in its biosynthetic pathway in the marine diatom <i>Thalassiosira rotula</i>. Fucoxanthin content was quantified using HPLC-based and spectrophotometric methods. Control culture at the exponential growth phase showed a fucoxanthin content of 4.7 mg g^-1 DW, reaching 5.2 mg g^-1 DW under low-light conditions at the late exponential phase. Gene expression analysis revealed condition-dependent modulation of major biosynthetic genes (<i>PSY</i>, <i>PDS</i>, <i>ZCIS</i>, <i>CRTISO</i>, <i>ZEP</i>, <i>VDL</i>, <i>DDE</i>). Early biosynthetic genes, <i>PSY</i> and <i>PDS</i>, were upregulated under low light, whereas <i>ZCIS</i> and <i>CRTISO</i> responded to high nitrate availability. <i>ZEP</i> exhibited treatment-specific induction and <i>VDL</i> isoforms showed differential regulation, highlighting distinct xanthophyll cycle gene expression patterns across treatments. These results demonstrate that both light and nitrate availability modulate fucoxanthin content and biosynthetic gene expression in <i>T. rotula</i>, providing insights into the regulatory mechanisms underlying carotenoid metabolism in diatoms and proposing <i>T. rotula</i> as a potential candidate for fucoxanthin production.