Balancing photoprotection and carotenoid biosynthesis: the role of specialised zeaxanthin epoxidases in the photosynthetic metabolism of diatoms.

• Applicant: Dr. Chiara Giossi
• Subject Area: Plant Physiology
• Term: since 2026
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 582859046

Diatoms are amongst the main primary producers on Earth, dominating aquatic communities in turbulent waters characterised by sudden and extreme light changes. To thrive under such complex conditions and avoid light induced damage, these microalgae perform different strategies. One of these involves the xanthophyll cycle, a carotenoids interconvertion catalysed by two antagonist enzymes: violaxanthin de-epoxidase (VDE) and zeaxanthin epoxidase (ZEP). This cycle regulates the photoprotective energy dissipation as heat, in the form of a rapid and higly efficient non-photochemical quenching (NPQ). Diatoms display several paralogs of VDE and ZEP, employed in photoprotion but also in a highly complex xanthophyll biosynthesis pathway. Because of this dual role in photoprotection and carotenoid metabolism, the enzymatic regulation of xanthophyll cycling must be constantly tuned to meet the physiological needs of the cell in response to the changing light environment. While VDE has traditionally been viewed as the primary regulator of xanthophyll cycling, more and more studies now point towards ZEPs as the critical determinant of this process across photosynthetic taxa, including both plants and algae. In diatoms this enzyme family may further underpin the successful physiological balance between metabolic and photoprotective needs. Recently, we proposed that two specific isoforms, ZEP2 and ZEP3, may orchestrate this process in the model diatom Phaeodactylum tricornutum. This project aims to elucidate the functional roles of ZEP2 and ZEP3 isoforms in balancing the needs of carotenoid biosynthesis and photoprotective xanthophyll cycling in P. tricornutum. The main hypothesis, based on our previous work, is that this diatom splits the functions of carotenoid biosynthesis and photoprotective xanthophyll cycling between ZEP2 and ZEP3 via the functional specialisation and/or physical separation of the two isoforms. To validate these models, the project will investigate the functional differentiation of ZEP2 and ZEP3 through the analysis of four key molecular properties: enzymatic activity, functional domains, protein stability and regulatory network. Furthermore, it will investigate the localisation of ZEP2 and ZEP3 within the chloroplast. The different ZEP isoforms of P. tricornutum share some similarities and a common evolutionary origin with the homologs of land plants and other algae, but harbour significant differences in their protein domains and regulation. Indeed, diatoms ZEPs may have undergone specific adjustments allowing rapid and efficient tuning of NPQ in response to their natural light environment. This work aims to finally elucidate the key features of ZEPs in diatoms, and how the specialisation of selected isoforms might have contributed to the ecological success of this relevant microalgal taxon in highly perturbed habitats.

• DFG Programme: Position