Xanthophyll cycle dynamics in the model diatom Phaeodactylum tricornutum : molecular characterisation and combined effects of temperature and light
Year: 2025
Type: dissertation
Abstract: Photosynthesis is the natural process through which plants and algae convert water and carbon dioxide into sugars and molecular oxygen. While sunlight is essential to power this process, an excess of photons can cause the formation of highly reactive radicals with the potential of serious physiological harm. Thus, photosynthetic organisms have evolved a wide variety of photoprotection mechanisms to avoid light-induced damage. One of these is the xanthophyll cycle, a reversible interconversion of carotenoid pigments which contributes to photoprotection by regulating Non-Photochemical Quenching (NPQ), a form of excess energy dissipation as heat. Diatoms, a major group of marine and freshwater microalgae, thrive in turbulent waters, complex environments in which light is subject to frequent and extreme changes, and where this stress may be exacerbated by other environmental drivers (such as temperature). In response, these photosynthetic heterokonts display a highly efficient NPQ, which has been often linked to the ecological success of this taxon, albeit this hypothesis still lacks substantial experimental proof. This fast and rapidly reversible NPQ relies on the diadinoxanthin cycle, a characteristic form of xanthophyll cycling. Interestingly, diatoms also contain minor amounts of pigments of the violaxanthin cycle (the ubiquitous xanthophyll cycle found in land plants and most algae), thought to serve only as precursors in carotenoid biosynthesis. Both cycles are catalysed by the same enzymes: violaxanthin de-epoxidase (VDE) and zeaxanthin epoxidase (ZEP). However, the corresponding genes have been subject to subsequent duplication events in diatoms, yielding several paralogs with specialised functions. In the first part of this work, I focused on the characterisation of selected VDE and ZEP isoforms (VDE, ZEP2 and ZEP3) in the model diatom Phaeodactylum tricornutum. Through the generation of the respective knockout lines and a series of physiological experiments, we identified VDE and ZEP3 as the main enzymes regulating photoprotective xanthophyll cycling in this diatom, while we attributed a primary role in de novo carotenoid biosynthesis to ZEP2. As ZEP2 knockouts significantly accumulate pigments of the violaxanthin cycle during high light, we investigated the role of these xanthophylls in photoprotection and obtained strong experimental evidence that both the diadinoxanthin and the violaxanthin cycle can contribute to NPQ in this diatom, with several implications for the evolution and regulation of this relevant photoprotection mechanism. Subsequently, I focused on the ecological relevance of xanthophyll-mediated NPQ, under the pressure of an additional parameter: temperature. Through combined temperature-light stress experiments, we analysed the simultaneous effect of these parameters on the molecular dynamics of NPQ and identified changes in the activity of xanthophyll cycle enzymes as the main driver. We conclude that diatoms thrive under new environmental pressures by efficiently tuning their photosynthetic and photoprotective apparatus, confirming that their tight NPQ regulation might have evolved in response to dynamic environments and might thus be linked with their ecological success.
Source: KOPS (University of Konstanz)
Author: Giossi, Chiara E.
Language: English
Subfield: Molecular Biology
Domain: Life Sciences
Open Access status: green
OpenAlex ID: W7149221470