Chemically mediated mechanisms underpinning the formation and maintenance of photosymbiosis of plankton Radiolaria and microalgae

• Applicant: Professor Dr. Georg Pohnert
• Subject Area: Organismic Interactions, Chemical Ecology and Microbiomes of Plant Systems ; Biological and Biomimetic Chemistry ; Microbial Ecology and Applied Microbiology
• Term: since 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 566341480

Marine phytoplankton comprises all microscopic photosynthesizing algae and is responsible for around 50% of oxygen production on the Earth. These microalgae exist in the ocean as free-living forms or in association with heterotrophic plankton community members. In such associations algae receive inorganic nutrients from the partner and have the benefit of being protected from predators. These associations, known as photosymbiosis, are well-documented in corals but remain poorly understood in planktonic systems, where they are frequently observed between heterotrophic protists Radiolaria and various classes of microalgae. Radiolaria harboring microalgae are ubiquitous, particularly in oligotrophic waters, and can constitute up to 1/3rd of the total zooplankton community. As key primary producers in nutrient-poor environments, their role in marine ecosystems is significant. The presence of the photosymbionts is obligate for Radiolaria, however, the mechanisms driving Radiolaria-microalgae photosymbiosis remain largely unexplored. This is primarily due to challenges in culturing Radiolaria and their fragility during sampling. We propose a project in which the underlying principles of chemically mediated processes in the photosymbiosis are addressed. Therefore, we will investigate i) the chemotactic association of algae and Radiolaria that follows up our initial observation of accumulation of algal cells from free-living cultures around the Radiolaria; ii) the regulation of algal growth and performance through the Radiolarian host. We will explore whether growth and productivity of the algae is supported by the host and/or, if cell division is inhibited after association to avoid overgrowth during symbiosis. In addition, iii) the substrate shuttling between algae and host will be addressed. According to the most accepted scenario, photosymbionts deliver organic nutrients to the host, but the host could also benefit from essential vitamins or defensive secondary metabolites provided by symbionts. Our project aims to unravel the complex interdependency in the photosymbiosis by using stable isotope labeling, mass spectrometry, and ecological experiments. We can base this project on extensive preliminary studies where we established two Radiolaria / algal model systems and showed how the holobiont metabolism is re-wired in comparison to that of the free-living form of algae. We will develop elaborate labeling experiments in combination with metabolomic studies and bioassays to obtain a comprehensive picture of the role of chemically mediated interactions in the photosymbiosis. These findings will contribute to the understanding of symbiotic strategies in marine ecosystems and to the prediction of factors that could disrupt the plankton photosymbioses and thereby disturb the ecosystem functioning in oligotrophic oceans.

• DFG Programme: Research Grants