diatoms, dinoflagellates and apicomplexan parasites) 5. These higher-order endosymbioses resulted in complex plastids surrounded by additional membranes, some even retaining the endosymbiont nucleus (the nucleomorph) and led to the diversification of many photosynthetic lineages of global ecological importance, especially those with red algal-derived plastids (e.g. the uptake of primary plastid-containing algae by eukaryotic hosts. Subsequently, to the primary endosymbiosis, plastids spread to other eukaryote groups from green and red algae by eukaryote-to-eukaryote endosymbioses, i.e. There are three main lineages with primary plastids: red algae, green algae (including land plants) and glaucophytes-altogether forming a large group known as Archaeplastida 3, 4. Plastids originated from primary endosymbiosis between a cyanobacterium and a heterotrophic eukaryotic host, leading to primary plastids in the first photosynthetic eukaryotes. Eukaryotes later acquired the capacity to photosynthesise with the establishment of plastids by endosymbiosis. Oxygenic photosynthesis (hereafter simply photosynthesis) evolved in cyanobacteria around 2.4 billion years ago (bya), leading to the Great Oxidation Event-a rise of oxygen that profoundly transformed the Earth’s atmosphere and shallow ocean 1, 2. chloroplasts) are organelles that allow eukaryotes to perform oxygenic photosynthesis. This period in the Meso- and Neoproterozoic Eras set the stage for the later expansion to dominance of red algal-derived primary production in the contemporary oceans, which profoundly altered the global geochemical and ecological conditions of the Earth. We find that the hypotheses of serial endosymbiosis are chronologically possible, as the stem lineages of all red plastid-containing groups overlap in time. Here, we establish a timeframe for the origin of red algal-derived plastids under scenarios of serial endosymbiosis, using Bayesian molecular clock analyses applied on a phylogenomic dataset with broad sampling of eukaryote diversity. New hypotheses have emerged to explain the acquisition of red algal-derived plastids by serial endosymbiosis, but the chronology of these putative independent plastid acquisitions remains untested. Despite the ecological importance of these groups and many others representing a huge diversity of forms and lifestyles, we still lack a comprehensive understanding of their evolution and how they obtained their plastids. In modern oceans, eukaryotic phytoplankton is dominated by lineages with red algal-derived plastids such as diatoms, dinoflagellates, and coccolithophores.
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