The hypothesis about the emergence of photosynthesis so far was that a single-celled eukaryote engulfed a cyanobacterium and integrated it into its cellular metabolism. This hypothesis is now strongly questioned by the recent placement of two relatively new phyla, Rhodelphidia and Picozoa, as closely related sister groups to Rhodophyta (red algae). The known Rhodelphidia species have a reduced remnant of a plastid, which seems not to be photosynthetic. Within Picozoa no traces of a remnant plastid have been found, neither in their genomes nor in cellular structures. However, only one Picozoan species has been microscopically studied to date despite the diversity of OTUs identified in environmental sequencing studies. Furthermore, there seem to be more potentially non-photosynthetic organisms, which group within the Archaeplastida that have not been further described. Those species could help to get an understanding of the dynamics in the evolution of photosynthesis in Archaeplastida. In this project, we want to address these questions by characterizing the Picozoan diversity using microscopy and single-cell genome and transcriptome sequencing.