Early photosynthetic eukaryotes inhabited low-salinity habitats

The early evolutionary history of the chloroplast lineage remains an open question. It is widely accepted that the endosymbiosis that established the chloroplast lineage in eukaryotes can be traced back to a single event, in which a cyanobacterium was incorporated into a protistan host. It is still unclear, however, which Cyanobacteria are most closely related to the chloroplast, when the plastid lineage first evolved, and in what habitats this endosymbiotic event occurred. We present phylogenomic and molecular clock analyses, including data from cyanobacterial and chloroplast genomes using a Bayesian approach, with the aim of estimating the age for the primary endosymbiotic event, the ages of crown groups for photosynthetic eukaryotes, and the independent incorporation of a cyanobacterial endosymbiont by Paulinella. Our analyses include both broad taxon sampling (119 taxa) and 18 fossil calibrations across all Cyanobacteria and photosynthetic eukaryotes. Phylogenomic analyses support the hypothesis that the chloroplast lineage diverged from its closet relative Gloeomargarita, a basal cyanobacterial lineage, similar to 2.1 billion y ago (Bya). Our analyses suggest that the Archaeplastida, consisting of glaucophytes, red algae, green algae, and land plants, share a common ancestor that lived similar to 1.9 Bya. Whereas crown group Rhodophyta evolved in the Mesoproterozoic Era (1,600-1,000 Mya), crown groups Chlorophyta and Streptophyta began to radiate early in the Neoproterozoic (1,000-542 Mya). Stochastic mapping analyses indicate that the first endosymbiotic event occurred in low-salinity environments. Both red and green algae colonized marine environments early in their histories, with prasinophyte green phytoplankton diversifying 850-650 Mya.