Early photosynthetic eukaryotes inhabited low-salinity habitats

Abstract

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 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, a basal cyanobacterial lineage, ∼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 ∼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.

On the origin of oxygenic photosynthesis and Cyanobacteria

“…3) (S anchez-Baracaldo, 2015). Age estimates of marine green algae (S anchez- Baracaldo et al, 2017) at the end of the Precambrian era and before the origin of animals are consistent with eukaryote biomarker data (Brocks et al, 2017). Gloeomargarita) of the Archaeplastida, a monophyletic group that includes the glaucophytes, red algae, the green algae and land plants, emerged c. 1.9 Ba (S anchez- Baracaldo et al, 2017).…”

“…Other basal lineages, such as Gloeomargarita (Ponce-Toledo et al, 2017), have revealed the closest known relatives of the chloroplast and helped to infer the habitat of early photosynthetic eukaryotes (S anchez- Baracaldo et al, 2017). Biological evidence supports the view that early forms of oxygenic photosynthesis were present throughout the Archean eon.…”

“…Synechococcus, and Cyanothece), filamentous freshwater (e.g. Age estimates for PSI (Cardona, 2018), PSII , crown group Cyanobacteria (Schirrmeister et al, 2015), major clades and taxa (S anchez-Baracaldo, 2015; S anchez- Baracaldo et al, 2017). Hydrocoleum) (S anchez- S anchez-Baracaldo, 2015).…”

Geological and Geochemical Constraints on the Origin and Evolution of Life

“…(2) Algal eukaryotes captured cyanobacteria and, hence, photosynthesis. This innovation may have occurred soon after cyanobacteria evolved and in freshwater (Ponce-Toledo et al, 2017;Sánchez-Baracaldo et al, 2017). (3) Our own species captured photosynthesis by agriculture *10,000 years ago and has become very abundant over a short geological time (e.g., Colledge et al, 2004).…”

Biosilicification Drives a Decline of Dissolved Si in the Oceans through Geologic Time

“…The major lineage of planktonic marine Synechococcus evolved approximately 650-600 Ma, based on molecular clock analyses (Sanchez-Baracaldo, 2015;Sanchez-Baracaldoa et al, 2017), with alternative clock analyses based on large genomic datasets suggesting an origin ∼1.5 Ga (Dvorák et al, 2014). Today this cyanobacterial group accounts for 25% of oceanic net primary production (Flombaum et al, 2013).…”