title: A neoproterozoic transition in the marine nitrogen cycle.
abstract: The Neoproterozoic (1000-542 million years ago, Mya) was characterized by profound global environmental and evolutionary changes, not least of which included a major rise in atmospheric oxygen concentrations [1, 2], extreme climatic fluctuations and global-scale glaciation [3], and the emergence of metazoan life in the oceans [4, 5]. We present here phylogenomic (135 proteins and two ribosomal RNAs, SSU and LSU) and relaxed molecular clock (SSU, LSU, and rpoC1) analyses that identify this interval as a key transition in the marine nitrogen cycle. Specifically, we identify the Cryogenian (850-635 Mya) as heralding the first appearance of both marine planktonic unicellular nitrogen-fixing cyanobacteria and non-nitrogen-fixing picocyanobacteria (Synechococcus and Prochlorococcus [6]). Our findings are consistent with the existence of open-ocean environmental conditions earlier in the Proterozoic adverse to nitrogen-fixers and their evolution-specifically, insufficient availability of molybdenum and vanadium, elements essential to the production of high-yielding nitrogenases. As these elements became more abundant during the Cryogenian [7, 8], both nitrogen-fixing cyanobacteria and planktonic picocyanobacteria diversified. The subsequent emergence of a strong biological pump in the ocean implied by our evolutionary reconstruction may help in explaining increased oxygenation of the Earth's surface at this time, as well as tendency for glaciation.
authors.affiliation: School of Biological Sciences, University of Bristol, Woodland Road, Bristol BS8 1UG, UK; School of Geographical Sciences, University of Bristol, Bristol BS8 1SS, UK. Electronic address: p.sanchez-baracaldo@bristol.ac.uk.
authors.forename: Patricia
authors.initials: P
authors.lastname: Sánchez-Baracaldo
authors.affiliation: School of Geographical Sciences, University of Bristol, Bristol BS8 1SS, UK.
authors.forename: Andy
authors.initials: A
authors.lastname: Ridgwell
authors.affiliation: Division of Plant Science, University of Dundee at the James Hutton Institute, Invergowrie, Dundee DD2 5DA, UK; School of Plant Biology, University of Western Australia, Crawley, WA 6009, Australia.
authors.forename: John A
authors.initials: JA
authors.lastname: Raven
mesh_terms: D059001:Aquatic Organisms / Q000378:metabolism; D005075:Biological Evolution; D000458:Cyanobacteria / Q000378:metabolism*; D008982:Molybdenum / Q000378:metabolism; D058458:Nitrogen Cycle; D009586:Nitrogen Fixation; D009792:Oceans and Seas; D010802:Phylogeny; D010933:Plankton / Q000378:metabolism; D048929:Prochlorococcus / Q000378:metabolism; D046940:Synechococcus / Q000378:metabolism
chemical_list: D008982:Molybdenum