Abstract
Although planktonic organisms have been the topic of scientific research for centuries, some organisms have fallen through the cracks. This is the case of Rhizaria, unicellular eukaryotes that are particularly delicate and often crushed by classical plankton nets. Yet, their substantial contribution to planktonic biomass was recently brought to light thanks to in situ imaging. Such an approach allows the study of these organisms in their undisturbed environment. Beyond their substantial biomass, their trophic ecology is poorly described (some taxa are mixotrophic and host photosynthetic symbionts, others do not) and knowledge regarding their reproductive cycle is even scarcer. Leveraging high frequency in situ imaging, we investigated 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. We brought to light differences in vertical distribution between subgroups, likely revealing different life strategies and contrasted abilities for buoyancy control. We also detected a previously undocumented preferential orientation of some organisms in each taxon. Finally, we infer from some of our observations presumptive steps of the obscure life cycle of Collodaria, which seems to involve fine buoyancy control to reach new environments and enable de novo symbiont acquisition. Altogether, these unprecedented results highlight that complex ecological behaviour can be achieved by “simple” unicellular organisms.