Bossa, Rosanna (2025) Trophic transitions from dark to light in the unicellular red algae Galdieria phlegrea. [Tesi di dottorato]

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Abstract

Galdieria phlegrea (strain 734) is a unicellular and polyextremophilic red microalgae, endowed with high metabolic plasticity, which allows it to alternate trophic modalities (autotrophic, heterotrophic, and mixotrophic) and to thrive in extreme environmental conditions. The present study investigates the physiological and functional mechanisms of photosynthetic reactivation during the trophic transition from heterotrophy to autotrophy, to define temporal dynamics, reorganization of photosystems, and strategies of use of intracellular reserves. The results show that heterotrophic cells accumulate energy reserves in the form of lipids and carbohydrates, keeping the functional core of the photosystems unchanged. Cell density increases, while cell size decreases in the transition to autotrophy. Exposure to light initiates a coordinated process of reactivation of photosynthesis. In fact, following the transition in five days, it is observed that the maximum quantum yield of Photosystem II (Fv/Fm), together with the effective quantum yield of Photosystem II (ΦPSII), increases within five days, indicating the restoration of electronic transfer and oxygen production. The results are confirmed by spectroscopic, 77K fluorescence, and confocal microscopy analyses, which show that Photosystem I (PSI) maintains structural and functional integrity even in the absence of light, acting as a center for reorganization, while the antennal complexes and Photosystem II (PSII) recompose rapidly. In conclusion, we can argue that photosynthesis recovery takes place in three distinct phases: (i) latency; (ii) activation, and (iii) stabilization, optimizing the use of stored energy resources, which reflect an optimization of the deployment of intracellular energy reserves. These results highlight the exceptional resilience and versatility of G. phlegrea and suggest its potential as a model organism for trophic adaptation studies and for biotechnological applications.

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