Thriving Across Seawater Depths: How Blue Light Shapes a Large PSI Supercomplex and Specific Photosynthetic Traits in the seagrass Posidonia oceanica.

Authors

Quentin Charras Ferroussier, Charlie Mathiot, Dmitry A Semchonok, Eduard Elias, Ahmad Farhan Bhatti, Régine Lebrun, Dorian Guillemain, Marina I Siponen, Roberta Croce, Colette Jungas

Published in

Plant communications. Pages 101574. Oct 23, 2025. Epub Oct 23, 2025.

Abstract

Seagrasses are marine flowering plants that perform oxygenic photosynthesis both under high, white sunlight and under low, blue-green light, conditions fundamentally different from those experienced by land plants. Yet, how seagrasse's photosynthetic machinery adapts to such underwater light gradient remains poorly understood. Here, we investigate the Mediterranean seagrass Posidonia oceanica, an ecosystem engineer thriving from the surface down to 40 m depth, to uncover how it maintains efficient photosynthesis across this gradient. Combining spectroscopy, pigments and blue-native PAGE analysis, we show that P. oceanica maintains a high but stable PSI/PSII ratio and constant antenna size at all depths, together with a high abundance of LHCII. Electron microscopy observation showed that the adjustment of photosynthetic efficiency along the depth gradients is achieved primarily through structural remodeling of thylakoid architecture rather than through major changes in photosystem composition. We also identify a previously undescribed large PSI supercomplex (L-PSI-LHCII) that binds an additional Lhca1-Lhca4 dimer and a phosphorylated LHCII trimer. This complex, expressed at all tested depth, is enriched in chlorophyll b, lacks the far-red absorbing chlorophylls (red-forms) typical of land plants, and exhibits distinct energy-transfer dynamics optimized for blue-light harvesting. The presence of similar PSI supercomplexes in other marine seagrasses such as Zostera marina indicates a conserved strategy among deep-growing species. Together, these results reveal how seagrasses combine structural adaptation at the level of PSI and thylakoids architecture reorganisation to sustain efficient photosynthesis and long-term carbon fixation under blue-dominated marine light.

PMID:
41137398
Bibliographic data and abstract were imported from PubMed on 25 Oct 2025.

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