ChloroMito · Chloroplast and Mitochondria interactions for microalgal acclimation

Photosynthesis emerged as an energy-harvesting process at least 3.5 billion years ago, first in anoxygenic bacteria and then in oxygen-producing organisms, which led to the evolution of complex life forms with oxygen-based metabolisms (e.g. humans). Oxygenic photosynthesis produces ATP and NADPH, and the correct balance between these energy-rich molecules allows assimilation of CO2 into organic matter. Although the mechanisms of ATP/NADPH synthesis are well understood, less is known about how CO2 assimilation was optimised. This process was essential to the successful phototrophic colonisation of land (by Plantae) and the oceans (by phytoplankton). Plants optimised CO2 assimilation using chloroplast-localised ATP-generating processes to control the ATP/NADPH ratio, but the strategies developed by phytoplankton are poorly understood. However, diatoms—ecologically successful ocean organisms—are known to control this ratio by exchanging energy between plastids and mitochondria. Is this mechanism a paradigm for optimisation of photosynthesis in the ocean? The ChloroMito project aims to first decipher the mechanism(s) behind plastid-mitochondria interactions. Thanks to a novel combination of whole-cell approaches, including (opto)genetics, cellular tomography and single-cell spectroscopy, we will identify the nature of the exchanges occurring in diatoms and assess their contribution to dynamic responses to environmental stimuli (light, temperature, nutrients). We will then assess conservation of this mechanism in ecologically relevant phytoplankton taxa, test its role in supporting different lifestyles (autotrophy, mixotrophy, photosymbiosis) encountered in the ocean, and track transitions between these different lifestyles as part of an unprecedented effort to visualise ocean dynamics. Overall, the ChloroMito project will alter our understanding of ocean photosynthesis, challenging textbook concepts which are often inferred from plant-based concepts