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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 2 - PhotoPHYTOMICS (Photosynthesis in PHYTOplankton Mixtures to Investigate Community Structuration.)

Teaser

Summary

Phytoplankton is not only at the basis of the marine food chain but also an important buffer of climate changes, given that marine photosynthesis is responsible of 50% of the CO2 fixation on Earth. Our understanding of the forces that shape the dynamics and structuration of phytoplankton communities is still limited by major methodological constraints in the marine environment. Prominent among them is the difficulty to simultaneously measure physiological responses from each micro-algal species within mixtures, which prevail in the Ocean. In this project,a physical phenomenon, the Electro-Chromic Shift (ECS) of the photosynthetic pigments, is used to extract the photosynthetic responses of each species within an assembly. Photosynthesis is a gateway to investigate both biotic interactions and abiotic stresses which are major determinants of the structuration of the phytoplankton community, because it is one of the prime targets of those external factors.
We use this method to analyze the effects of three major parameters that contribute to shaping the ecological patterns at sea: competition for nutrients; direct inhibition of the metabolism of competitors through the use of secondary metabolites (allelopathy), and the responses of phytoplankton species to the rapid and intense light changes they experience. These studies will be conducted both in laboratory conditions and in the field for validation. The overall goal is to provide a new tool allowing the study in situ and in real time of the cellular mechanisms giving rise to the ecological patterns observed in the Ocean. This is crucial for fundamental research but also to better understand and anticipate climate changes (eutrophication, changes of phytoplankton communities with the arrival of new toxic species, â€¦).

Work performed

\"At the basis of this ERC project is the development of a methodology allowing to measure photosynthetic activities of each microalga in a mixture. In the first 36 monthes of this project, we could demonstrate the validity of this method and its applicability to most conditions. This means showing that most photosynthetic clades possess an ECS signal and that their spectra are different enough to allow proper deconvolution : a library of ECS spectra for diatoms, dinoflagellates, haptophytes, chlorophytes, prasinophytes, cyanobacteria and others has been obtained. This also means that we can measure the features of photosynthesis responses to abiotic and biotic stress with ECS: we propose new ECS-based methodologies to probe the cyclic electron flow around PSI, the kinetic properties of the chloroplastic ATPase orthe photosynthetic architecture. With this in hands, we could achieve a proof of methods, on mixtures of microalgae without biotic interactions. We also provide several examples of application of the method for allelopathic interactions, competition for nutrients, and use in the field.

In WP1, we focused on the allelopathic interactions between \"\"red tide\"\" dinoflagellates and co-occuring diatoms. In all cases, the dinoflagellates release allelochemicals which target the diatom and inhibit photosynthesis. Using the Â« photosynthesis in mixture Â» method, and thanks to new collaboration with ecological biochemistry, we first decipghered the allelopathic interaction between the toxic dinoflagellate Amphidinium carterae and diatoms. We discovered the nature of the secondary metabolite secreted by the dinoflagellate, its target and the mechanism of photosynthesis inhibition. Using dinoflagellate/diatom co-cultures, we could show that the presence of the dinoflagellate immediately arrests photosynthesis and growth. Finally, a screening revealed that this dinoflagellate was toxic to more diatoms, some prasinophytes or natural diatom populations in the field. The two other projects are focusing on the inhibition of the photosynthesis of the diatom Chaetoceros muelleri and Licmophora by the dinoflagellates Alexandrium minutum and Ostreopsis ovata, respectively.

WP2 was supposed to start on the second half of the ERC project but we could already achieve a proof of methods: the use of ECS signals to study photosynthetic responses in experiments of competition for nitrogen.

Regarding WP3, we could establish for the first time the light-dependencies of the two enzymes involved in the regulation of photoprotection in diatoms : the diatoxanthin epoxidase and the diadinoxanthin deepoxidase. The light dependencies of those two enzymes are modified depending on the light conditions the diatoms experience, revealing an additional a level of light acclimation. The very peculiar regulation of the epoxidase was confirmed in the field. In collaboration with University of LiÃ¨ge, we observed a similar behavior in another photosynthetic clade, sister to diatoms.

WP4 is a transversal task which corresponds to the testing in the field of results and hypothesis coming from the laboratory. Thanks to several missions in Roscoff (France) and in Bergen (Norway), we could test our models for WP1 and WP3. At that occasion, new allelopathic interactions were identified in the field and strains were isolated in collaboration with the biological station of Roscoff (Fance), the Weizmann Institute (Israel) and the university of Bergen (Norway). Those allelopathic interactions are now being studied in the laboratory (WP1). We also managed to extract the light dependencies of photosynthesis of diatoms and dinoflagellates from natural assemblages in the field, paving the path to the use of our methodology in situ.\"

Final results

In WP1, through the example of the allelopathy between a dinoflagellate and diatoms, we could demonstrate the gain of time that our new method provides compared to classical approaches based on growth inhibition. The new method is fast for discovering new allelopathic interactions, deciphering the mechanism of inhibition and identifying the secondary metabolite responsible for it. In the remaining time of the project, we will extend this to other microalgal couples, starting with a large screen using the microalgae collection of the Biological Station in Roscoff, France, which will take place in September 2020.

In WP2, we aim at applying the same procedure used for allelopathy to competition for nutrients. The goal is to provide the community of researchers interested in nutrient competition in the marine environment with a robust method allowing to add physiological information to the growth phenotypes classically obtained in nutrient stressed co-cultures experiments.

In WP3, we aim at understanding the molecular determinants of the epoxidase and deepoxidase peculiar light regulation. Then, we will extend the work to another group of photosynthetic microalgae : dinoflagellates.

In WP4, we will continue studying allelopathic interactions in the field, trying to isolate new couples of strains showing allelopathic interactions targeting photosynthesis in connection with WP1. And we will work more on the light responses of diatoms and dinoflagellates in the field in connection with WP3. Also, unexpected results obtained during a mesocosm in Bergen prompt us to investigate the importance of photosynthesis in the mechanism of viral infection of the coccolithophore Emiliania huxleyi, another biotic interaction which was initially not formally explicited in the project.