On the following subject: Ecophotobiology of the seagrass Posidonia oceanica: Exploring the ecological and evolutive dimension of photosynthetic adaptation mechanisms in-situ.

Composition of the jury

Mr Youri TIMSIT Mediterranean Institute of Oceanography (MIO) Thesis supervisor.
Ms Colette JUNGAS, BIAM, CEA Thesis co-supervisor.
Ms Anja KRIEGER-LISZKAY, Institute for Integrative Biology of the Cell, Université Paris-Saclay, Rapporteur.
Mr Giovanni FINAZZI, Interdisciplinary Research Institute of Grenoble, IRIG-LPCV, CNRS Rapporteur.
Mr Benjamin BAILLEUL, Institut de Biologie Physico-Chimique (IBPC), CNRS Examiner.
Ms Marina SIPONEN BIAM, CNRS Examiner.
Ms Heike MOLENAAR Université Côte d'Azur Examiner
Mr Rainer HIENERWADEL, BIAM, Aix Marseille University, Chairman of the jury

Abstract

In a context of global environmental changes, predicting the resilience and trajectories of natural plant population changes is of primordial importance. Vegetated ecosystems are at the basis of very rich and diversified networks. They are amongst the most important sources of the molecular oxygen we breathe and are considered as major carbon sinks contributing to buffering of anthropic greenhouse gas production. In the need of their conservation, the comprehension of the physiology of wild plant species is therefore necessary to predict the way they will respond to the actual anthropic crisis. In the framework of this PhD project, we focused on the photobiology of seagrass meadows formed by Posidonia oceanica, a species endemic from the medietarrenean sea. This plant flourishes from the surface up to 45m depth and is fascinating for many reasons. Not only does it engineer a rich ecosystem that provides goods and services for millions of people but it also flourishes under contrasted light irradiances, depending on the growing depth, a feature unique to the marine environment. Indeed, P. oceanica has to adapt to optimise its photosynthesis under irradiance similar to that of the land environment in shallow water but also under an irradiance that can be 50 times less intense than at the surface and on top of that, only composed of blue light in deep water.
The initial aim of this project was therefore to unveil some of the key photosynthetic adaptation mechanisms expressed by P. oceanica to cope with the unique light environment in the sea. However, underwater, not only light influences the biology of plants but also many biotic and abiotic factors such as temperature, salinity, CO2 limitation or competition. This is important since we came up against many biochemical locks that prevented the correct analysis of the photosynthetic apparatus through conventional biochemical approaches. Very early, we consequently investigated the plant cell wall composition and the impact of polyphenols on the process of photosystems isolation. Therefore, in this project we present our results on the photosynthesis of P. oceanica but we also improved knowledge on the plant cell wall, carbon allocation and polyphenol localization and composition in this plant.

The analysis of the plant cell wall and photoassimilates highlighted that one of the key strategies of P. oceanica to insure its success in deep water was the ability to store a large part of the fixed carbon in the below ground tissues as compared to shallow plants. Such studies raise questions about the different strategies of carbon allocation that respond to constraints specific to the shallow and deep meadows.

While we were trying to extract the photosynthetic membranes from P. oceanica, we noticed that the final preparation turned brown and that thylakoids appeared trapped in a mucilaginous mass. If we first blamed the cell wall components, we rapidly suspected that polyphenols prevented the extraction of thylakoids and photosynthetic complexes. We managed to alleviate these issues by using Poly Ethylene Glycol (PEG) and vitamin C (ascorbic acid), which inhibit the harmful effect of polyphenols. We extended our protocol to other wild plants such as Q. pubescens (oak) and V. vinifera (Wine), known to be refractory to the conventional extraction procedure.

The technical lock being opened, we then focused on the adaptive mechanisms of photosynthesis regulation implemented by P. oceanica to cope with the constraints of the marine light environment. In that aim, we selected meadows exposed to high irradiance (2 m depth), meadows that grew under an irradiance similar to the one used for lab grown model but only composed of cyan-blue light (15 m depth) and finally meadows that grow under low and blue-cyan light irradiance (26 m depth). By integrating electron microscopic observation of chloroplasts and biochemical approaches coupled with functional analyses, we highlight that despite drastic change in the thylakoids organization, the PSs stoichiometry, LHCII content and the respective antenna size remain fewly changed through depth. However, P. oceanica is distinguished from land plants by a higher PSI/PSII stoichiometry and higher LHCII content. Furthermore, analysis of photosystem supercomplexes by Blue-native gels revealed the presence of a blue-shifted Large-PSI-LHCII complex in addition to the classical PSI (PSI-LHCI), which is also blue shifted. The Large-PSI-LHCII complex was characterized by an extra Lhca1-Lhca4 dimer and accumulated at all tested depths. We expanded our analysis to other seagrasses that differed in their bathymetric distribution. All together, our results highlight that shallow and deep growing seagrasses differ in terms of photosynthetic strategies ensuring their ecological success in specific ecological niches.