Hold the salt: Freshwater origin of primary plastids.

The evolution of oxygenic photosynthesis by cyanobacteria was arguably one of the most significant biological events in Earth’s history, shaping the atmosphere and subsequently leading to diverse ecosystems (1). The permanent endosymbiotic merger between a cyanobacterium and a unicellular heterotrophic eukaryote in deep evolutionary time set the stage for the stunning diversity of photosynthetic eukaryotes and ecosystems seen today, giving rise to the supergroup Archaeplastida, the red, glaucophyte, and green algae and their descendent land plants. Later transfers of photosynthesis to other lineages of heterotrophic eukaryotes through eukaryote–eukaryote mergers (secondary and tertiary endosymbiosis) led to many near-shore and open-ocean species, including kelps, diatoms, coccolithophors, and dinoflagellates (2). The cyanobacterial ancestry of primary plastids is no longer debated, but the precise donor of primary plastids, the timing and ecological context of the merger, and modifications since the event have received much attention (3–6). In PNAS, Sánchez-Baracaldo et al. (7) examine the evolution of primary photosynthesis and its habitat of origin using the most comprehensive dataset thus far from photosynthetic cyanobacteria and eukaryotes. Reconstructing the history of primary endosymbiosis is challenging due to its deep evolutionary time and long separation of descendent lineages. The oldest eukaryotic fossils (about 1.7 billion y ago) cannot be unequivocally assigned and most of the age constraints used for time-tree analyses are from Phanerozoic fossils. All three lineages of Archaeplastida possess primary plastids of cyanobacterial origin but, as seen from newly abundant plastid genomic data from diverse photosynthetic eukaryotes, each group has evolved distinct modifications of the inherited cyanobacterial genetic “toolkit” for plastid functions, with independent gene losses and transfers to the host nucleus of plastid targeted genes, thus solidifying the integration (3, 8, 9). Likewise, free-living cyanobacteria further diversified since their cousins participated in primary endosymbiosis. Alternative candidates for the sister group to Archaeplastida plastids range from among morphologically simple, early-diverging (6) to more derived and morphologically complex cyanobacteria (5), to the possibility that primary plastids of Archaeplastida have multiple origins (10, 11). Two exciting discoveries of novel and deeply diverging lineages of cyanobacteria and green algae, as well as growing availability of genomic data from diverse photosynthetic species, prompted a reinvestigation of these fundamental questions. The recent discovery of Gloeomargarita lithophora, a cyanobacterium found in microbiolites of alkaline lakes in Mexico, made a splash because this species is among the early-diverging cyanobacterial lineages and is implicated as the closest relative to Archaeplastida (6). Second, an early-diverging lineage Red Green Glauco. Gloeomargarita Archaeplastida