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.
