![]() ![]() ![]() From these lineages, N-fixing cyanobacteria are particularly important because they exert a control on primary productivity and the export of organic carbon to the deep ocean 14, by converting nitrogen gas (N 2) into ammonium (NH 4 +), which is later used to make amino acids and proteins. Within the cyanobacteria, only a few lineages colonized the open-ocean (i.e., Crocosphaera and relatives, cyanobacterium UCYN-A, Trichodesmium, as well as Prochlorococcus and Synechococcus) 14, 15, 16, 17. Marine phytoplankton today contribute to almost half of the Earth’s total primary production 13. So far, it seems that ocean geochemistry (e.g., euxinic conditions during the early- to mid-Proterozoic) 4, 7, 11 and nutrient availability 12 likely contributed to the apparent delay in diversification and widespread colonization of open ocean environments by planktonic cyanobacteria during the Neoproterozoic 9. It remains unclear, however, what evolutionary events led to the emergence of open-ocean planktonic forms within cyanobacteria and how these events relate to geochemical evidence during the Pre-Cambrian 4. Understanding the evolution of planktonic cyanobacteria is an essential question because their origin fundamentally transformed the nitrogen and carbon cycles towards the end of the Pre-Cambrian 9. Recent chromium (Cr) isotope data point to low levels of atmospheric oxygen in the Earth’s surface during the mid-Proterozoic 3, which is consistent with the late evolution of marine planktonic cyanobacteria during the Cryogenian 9 both types of evidence help explain the late emergence and diversification of metazoans 10. A second but much steeper increase in oxygen levels, known as the Neoproterozoic Oxygenation Event (NOE) 4, 6, 7, occurred at around 800 to 500 Mya 5, 8. Geochemical evidence has indicated that there was a first step-increase in the oxygenation of the Earth’s surface, which is known as the Great Oxidation Event (GOE), in the early Paleoproterozoic (2,500–1,600 Mya) 1, 2. While it is widely accepted that the presence of molecular oxygen in the early fossil record was the result of cyanobacteria activity, little is known about how cyanobacteria evolution (e.g., habitat preference) may have contributed to changes in biogeochemical cycles through Earth history. Multiple lines of geochemical evidence support the occurrence of intervals of profound global environmental change at the beginning and end of the Proterozoic (2,500–542 Mya) 3, 4, 5. Similar content being viewed by othersĬyanobacteria have fundamentally transformed the geochemistry 1, 2 of our planet. ![]() This study shows that marine planktonic cyanobacteria evolved from benthic marine and some diverged from freshwater ancestors during the Neoproterozoic (1,000–542 Mya). Decrease in cell diameter and loss of filamentous forms contributed to the evolution of unicellular planktonic lineages during the middle of the Mesoproterozoic (1,600–1,000 Mya) in freshwater environments. Crown groups of modern terrestrial and/or benthic coastal cyanobacteria appeared during the late Paleoproterozoic to early Mesoproterozoic. While filamentous cell types evolved early on at around 2,600–2,300 Mya and likely dominated microbial mats in benthic environments for most of the Proterozoic (2,500–542 Mya), marine planktonic cyanobacteria evolved towards the end of the Proterozoic and early Phanerozoic. I present here phylogenomic (135 proteins and two ribosomal RNAs), Bayesian relaxed molecular clock (18 proteins, SSU and LSU) and Bayesian stochastic character mapping analyses from 131 cyanobacteria genomes with the aim to unravel key evolutionary steps involved in the origin of marine planktonic cyanobacteria. ![]() Little is known, however, about the evolutionary events that led to the appearance of marine planktonic cyanobacteria. Marine planktonic cyanobacteria contributed to the widespread oxygenation of the oceans towards the end of the Pre-Cambrian and their evolutionary origin represents a key transition in the geochemical evolution of the Earth surface. ![]()
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