Periodic Reporting for period 4 - PhotoPHYTOMICS (Photosynthesis in PHYTOplankton Mixtures to Investigate Community Structuration.)
Reporting period: 2021-07-01 to 2022-12-31
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, …).
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, …).
In WP1, we focused on the allelopathic interactions between "red tide" dinoflagellates (Amphidinium carterae, Alexandrium minutum and Ostreopsis ovata) and co-occuring diatoms involving the release of allelochemicals targeting photosynthesis. Using the « photosynthesis in mixture » method, we especially focused on deciphering the allelopathy between the toxic dinoflagellate Amphidinium carterae and diatoms, including the nature of the secondary metabolite, its target, the inhibition mechanism. We performed a screening of the species sensitive to the secondary metabolite secreted by this dinoflagellate and started screening for other allelopathic interactions than the ones involving diatoms and dinoflagellates. One PhD thesis and four papers have been published, another one will be submitted at the end of 2023. This WP gave rise to 7 talks in national and international conferences and to collaboration with the startup Immunrise Biocontrol.
WP2, aiming at studying competition for nutrients, was modified during the course of the project. New ECS-based methods were developed to assess the PSII/PSI stoichiometry, the Cyclic Electron Flow and the regulation of the plastid ATPase, all rapidly identified as main signatures of nitrogen deficiency . We could achieve a proof of methods of the use of ECS deconvolution to study competition for nitrogen or phosphorous. Investigating the interplay between allelopathy and nutrient deficiencies revealed the important role of phosphorous for the production of the secondary metabolite by A. carterae. Three papers and two PhD thesis have been published and 5 manuscripts are in preparation. This WP gave rise to 8 talks in national and international conferences.
In WP3, we investigated the photosynthetic response of diatoms to light stress. We could establish for the first time the light-dependencies of the two enzymes involved in the regulation of photoprotection in diatoms, which revealed an additional level of light response that we also observed in another photosynthetic clade sister to diatoms. We developed a new method to probe lumenal pH in vivo and investigated the role of ion channels in the regulation of the lumenal pH and qE. We used a series of LHCX1 mutants in Phaeodactylum tricornutum to investigate the importance of qE in the overall regulation of the photosynthetic process, including photo-inhibition, cyclic electron flow and photosynthesis-modulated gene expression. Studies on other species were also performed, including the polar species Fragilariopsis cylindrus and the facultative phototroph Cyclotella cryptica. In this WP3, 4 reviews about photophysiology in diatoms and their green counterparts and 4 experimental papers have been published; 5 more are in preparation. This WP gave rise to 7 talks in national or international conferences.
WP4 is a transversal task which corresponds to the testing in the field of results and hypothesis coming from the laboratory. A library of ECS spectra has been reinforced and we could characterize ECS signals in cyanobacteria, extending the spectrum of application of the ECS deconvolution method. Thanks to several missions in Roscoff (France), in Bergen (Norway) and in the canadian Arctic (Dark Edge mission, Canada) we could test models developed in the laboratory for WP1 and WP3. We also managed to extract the light dependencies of photosynthesis of diatoms and dinoflagellates from natural assemblages in the field. One paper is published in this WP and a method paper about the ECS deconvolution method is in preparation which includes the library of ECS spectra, the proof of method of the ECS deconvolution and the screening of allelopathic interactions in the Roscoff Culture Collection. As forecasted at the beginning of the project, the transition from laboratory studies to in situ studies will take time but we developed a new high-sensitivity instrument allowing to work at low microalgal densities, paving the path to the use of our methodology in situ.
WP2, aiming at studying competition for nutrients, was modified during the course of the project. New ECS-based methods were developed to assess the PSII/PSI stoichiometry, the Cyclic Electron Flow and the regulation of the plastid ATPase, all rapidly identified as main signatures of nitrogen deficiency . We could achieve a proof of methods of the use of ECS deconvolution to study competition for nitrogen or phosphorous. Investigating the interplay between allelopathy and nutrient deficiencies revealed the important role of phosphorous for the production of the secondary metabolite by A. carterae. Three papers and two PhD thesis have been published and 5 manuscripts are in preparation. This WP gave rise to 8 talks in national and international conferences.
In WP3, we investigated the photosynthetic response of diatoms to light stress. We could establish for the first time the light-dependencies of the two enzymes involved in the regulation of photoprotection in diatoms, which revealed an additional level of light response that we also observed in another photosynthetic clade sister to diatoms. We developed a new method to probe lumenal pH in vivo and investigated the role of ion channels in the regulation of the lumenal pH and qE. We used a series of LHCX1 mutants in Phaeodactylum tricornutum to investigate the importance of qE in the overall regulation of the photosynthetic process, including photo-inhibition, cyclic electron flow and photosynthesis-modulated gene expression. Studies on other species were also performed, including the polar species Fragilariopsis cylindrus and the facultative phototroph Cyclotella cryptica. In this WP3, 4 reviews about photophysiology in diatoms and their green counterparts and 4 experimental papers have been published; 5 more are in preparation. This WP gave rise to 7 talks in national or international conferences.
WP4 is a transversal task which corresponds to the testing in the field of results and hypothesis coming from the laboratory. A library of ECS spectra has been reinforced and we could characterize ECS signals in cyanobacteria, extending the spectrum of application of the ECS deconvolution method. Thanks to several missions in Roscoff (France), in Bergen (Norway) and in the canadian Arctic (Dark Edge mission, Canada) we could test models developed in the laboratory for WP1 and WP3. We also managed to extract the light dependencies of photosynthesis of diatoms and dinoflagellates from natural assemblages in the field. One paper is published in this WP and a method paper about the ECS deconvolution method is in preparation which includes the library of ECS spectra, the proof of method of the ECS deconvolution and the screening of allelopathic interactions in the Roscoff Culture Collection. As forecasted at the beginning of the project, the transition from laboratory studies to in situ studies will take time but we developed a new high-sensitivity instrument allowing to work at low microalgal densities, paving the path to the use of our methodology in situ.
The development of the ECS-based method to deconvolute photosynthetic contributions from a natural or artificial mixture is clearly beyond the state of the art. We could demonstrate the gain of time that our new method provides compared to classical approaches based on growth inhibition in allelopathy studies. We could characterize in detail several allelopathic interactions that could potentially play an important role on coastal ecosystems. The potential of this method attracted the interest of the start-up Immunrise Biocontrol which aims at replacing conventional pesticides by new biopesticides coming from phytoplanktonic species. We could show that some of the secondary metabolites we identified were good candidates as biopesticides for the vine downy mildiew.
Beyond the initial plan of the project, we could also show that cyanobacteria also possess ECS signals that we characterized and applied for basic characterization of photosynthesis. A postdoc in the project got a COFUND fellowship to continue the work started in WP2 till spring 2024. Results obtained during a mesocosm in Bergen revealed that measurements of photosynthesis in natural assemblages can reveal viral infection of the coccolithophore Emiliania huxleyi, another biotic interaction which was initially not formally explicited in the project. Last, we developed a high-sensitivity spectrometer for field studies which we successfully used in the lab to perform transient absorption spectroscopy on 100 fold more diluted samples than in commercial instruments, a very promising step towards the use of the method developed in the ERC PhotoPHYTOMICS in the field.
Beyond the initial plan of the project, we could also show that cyanobacteria also possess ECS signals that we characterized and applied for basic characterization of photosynthesis. A postdoc in the project got a COFUND fellowship to continue the work started in WP2 till spring 2024. Results obtained during a mesocosm in Bergen revealed that measurements of photosynthesis in natural assemblages can reveal viral infection of the coccolithophore Emiliania huxleyi, another biotic interaction which was initially not formally explicited in the project. Last, we developed a high-sensitivity spectrometer for field studies which we successfully used in the lab to perform transient absorption spectroscopy on 100 fold more diluted samples than in commercial instruments, a very promising step towards the use of the method developed in the ERC PhotoPHYTOMICS in the field.