High throughput in situ imaging reveals complex ecological behaviour of giant marine mixotrophic protists

Different trophic modes coexist among planktonic organisms, with a continuum between strict heterotrophy and strict autotrophy. Indeed, mixotrophic organisms have the ability to combine photosynthetic and heterotrophic nutrition. Here, we focus on an iconic group of mixotrophic protists, Rhizaria. They are particularly delicate and are often crushed by standard plankton nets. Rhizaria are a very good model organism to study mixotrophy in plankton because they are easily observed in their environment by in situ imaging, allowing us to monitor their true ecological mixotrophic behaviour. While their substantial contribution to planktonic biomass has recently been revealed, their trophic ecology is poorly described — some taxa are mixotrophic and host photosynthetic symbionts, others are not — and knowledge of their reproductive cycle is even scarcer. Using high-frequency in situ imaging, we study the ecology of these mixotrophic organisms directly in their undisturbed environment. We examined the fine-scale distribution and orientation of ~230,000 organisms belonging to three groups of Rhizaria, including the mixotrophic taxa Acantharia and Collodaria, and the non-mixotrophic Phaeodaria. Overall, our results suggest that mixotrophic protists have better ability to control their position within the water column (depth, orientation) than non-mixotrophic ones. Our observations also point to several steps in the obscure life cycle of the mixotrophic Collodaria, during which de novo symbiont acquisition appears to involve active fine-scale buoyancy control. Taken together, these unprecedented results demonstrate that complex ecological behaviour can be achieved by "simple" single-celled organisms.