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Metabolic interactions in oceanic photosymbioses

Periodic Reporting for period 1 - MINOTAUR (Metabolic interactions in oceanic photosymbioses)

Reporting period: 2016-01-01 to 2017-12-31

The ocean contains an immense reservoir of planktonic microorganisms that play fundamental roles in the food web and global biogeochemistry. Their contribution to planetary primary production of about 50% strongly influences the Earth's chemical and ecological stasis. While knowledge of their diversity has greatly improved in recent decades, biotic interactions in the plankton remain poorly understood. Symbiosis, whereby different biological species live together in close and long-term association, is a key ecological interaction for ecosystem functioning and diversification of life. In the open ocean, symbiotic partnerships of host organisms with photosynthetic microalgae (photosymbioses) are widespread and particularly prevalent at the surface of nutrient-depleted waters. Photosymbiosis is not only a key evolutionary process that led to the acquisition of photosynthesis in eukaryotes, but is also central in marine ecosystems. Photosymbiosis provides a competitive advantage in nutritionally demanding habitats like the open ocean, and contributes significantly to both predation and primary production. The partnership is typically considered mutually beneficial for the two partners: the algal symbiont provides photosynthetically-derived products to the host, which in turn maintains a sheltered and relatively nutrient-rich environment for the symbiont. This general postulate mainly relies on our understanding of coral symbioses, but in plankton we still lack fundamental information about the basic physiology of the partnership. More particularly, metabolic interactions between symbiotic partners, including nutrient assimilation, translocation and utilization, have barely been studied in plankton to date. Thus, the MINOTAUR project aimed to explore the metabolic basis of photosymbiosis in plankton to understand the functioning and metabolism, of the partnership, and to improve our understanding about its biogeochemical role in the open ocean, one of the largest ecosystems on Earth. This knowledge is important to better assess the impact of symbioses in marine ecosystems and help to predict their response to global warming.

The three main objectives of this project are:
1- Characterize the elemental and isotopic composition of symbiotic Radiolaria, focusing on carbon, nitrogen, phosphorous, sulfur and metals
2- Quantify the uptake and flux of carbon and nitrogen between the partners at the subcellular level
3- Compare the morphology and the metabolic features of the symbiont inside its host and outside (free-living) to shed light on its putative metabolic dependency, as well as on the host control over the symbiont metabolism.

The project unveiled the metabolic role of each partner in the symbiotic relationship, and more particularly the control of the host to enhance the photosynthetic activity of its intracellular microalgae. Key elements that play major roles in marine ecosystems have been mapped and quantified in the organelles of the host and the intracellular microalgae.
Symbiotic organisms have been collected in the Mediterranean Sea in surface waters with a plankton net and then subjected to a specific preparation to study them with state-of-the-art microscopy techniques. For instance, protocols have been developed to conserve the morphology and the chemical composition of cells. Cryo-fixation at high pressure was therefore the method used. At the final step, cells have been embedded in a plastic resin in order to generate very thin sections of the cells with an ultra-microtome. These sections containing the symbiotic association (host and microalgae) were then analyzed using different microscopy platforms, such as electron microscopy to have the ultrastructure of cells, and chemical imaging (NanoSIMS and ToFSIMS, available at the Helmholtz Centre for Environmental Research, ProVIS, Leipzig, Germany) to have the isotopic, elemental and molecular composition of cells at the nanoscale. Electron microscopy observations first unveiled a very complex ultrastructural organization of the host cells, and represented a basic work prior to chemical imaging analyses. Nutrient mapping with nanoSIMS unveiled the trophic microenvironment of the symbiotic microalgae within a host and free-living microalgae (i.e. in culture), focusing on nitrogen, phosphorous, and sulfur, as well as their stoichiometric ratios. Both electron microscopy and nanoSIMS imaging unveiled a significant change in microalgae before (i.e. free-living) and during interaction within a host with respect to the morphology and elemental composition, respectively. This change is probably caused by the host cell to boost the photosynthesis, so the energy produced by the microalgae and delivered by the host. Some sections have been also analyzed in parallel with X-ray fluorescence at the Synchrotron of Grenoble (ESRF, France). This experiment allowed us to map and quantify the trace metals in cells at high resolution and also unveiled major differences between free-living and symbiotic microalgae. Regarding the objective 2, preliminary results show the C fixation in symbiotic microalgae and translocation of this carbon in specific locations of the host cell. Three papers are in preparation and results have been disseminated during two nanoSIMS user meetings (Utrecht 2017 and Leipzig 2017) and at an international conference in Prague 15th International Congress of Protistology – Summer 2017).
Symbiotic interactions between unicellular eukaryotes have received very little scrutiny despite being highly pertinent for the study of basic eukaryotic processes and thus for biomedicine and biotechnology. The models of the MINOTAUR project between two single-celled eukaryotes are not only ecologically relevant, but also fundamental to better understand chloroplast acquisition in eukaryotes, which is heralded as one of the most important biological innovations in the history of life. The results we obtained improved our knowledge about the mechanisms by which a host can accommodate and exploit microalgal cells, having ecological and evolutionary perspectives. We also pinpointed the morphological and biochemical plasticity of microalgae from the free-living to the symbiotic stages. This brings new insights on photosynthetic plasticity in response to biotic constrains, and not only to abiotic constrains as it has traditionally been studying since several decades. More analyses are ongoing to complete the overall story for publication. The protocols and analyses conducted during this project have a great potential to inspire and have direct applications in the medical field, for instance in the case of parasitic infections, to help scientists understanding the functioning of intricate and complex interactions between two cells.
In this project, we specifically studied the elemental composition of previously overlooked symbiotic organisms, which provide fundamental information about the quality of the energy that is transferred up the food web and exported to the deep ocean. Therefore, results of the project will very likely draw the attention and foster collaborations with oceanographers and biogeochemists in the near future, and stress the need to reconsider the biogeochemical impact of photosymbiosis in the trace metal cycling, which drives marine productivity and the global climate.
NanoSIMS image showing the subcellular distribution of nitrogen in the symbiotic association