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The role of O 2 in the evolution of early animals, as represented by some members of the Ediacara biota, has been heavily debated because current geochemical evidence paints a conflicting picture regarding global marine O 2 levels during key intervals of the rise and fall of the Ediacara biota. Fossil evidence indicates that the diversification the Ediacara biota occurred during or shortly after the Ediacaran Shuram negative C-isotope Excursion (SE), which is often interpreted to reflect ocean oxygenation. However, there is conflicting evidence regarding ocean oxygen levels during the SE and the middle Ediacaran Period. To help resolve this debate, we examined U isotope variations (δ 238 U) in three carbonate sections from South China, Siberia, and USA that record the SE. The δ 238 U data from all three sections are in excellent agreement and reveal the largest positive shift in δ 238 U ever reported in the geologic record (from ~ -0.74‰ to ~ -0.26‰). Quantitative modeling of these data suggests that the global ocean switched from a largely anoxic state (26%-100% of the seafloor overlain by anoxic waters) to near-modern levels of ocean oxygenation during the SE. This episode of ocean oxygenation is broadly coincident with the rise of the Ediacara biota. Following this initial radiation, the Ediacara biota persisted until the terminal Ediacaran period, when recently published U isotope data indicate a return to more widespread ocean anoxia. Taken together, it appears that global marine redox changes drove the rise and fall of the Ediacara biota., (© 2019 John Wiley & Sons Ltd.)

The ocean-atmosphere system is typically envisioned to have gone through a unidirectional oxygenation with significant oxygen increases in the earliest (ca. 635 Ma), middle (ca. 580 Ma), or late (ca. 560 Ma) Ediacaran Period. However, temporally discontinuous geochemical data and the patchy metazoan fossil record have been inadequate to chart the details of Ediacaran ocean oxygenation, raising fundamental debates about the timing of ocean oxygenation, its purported unidirectional rise, and its causal relationship, if any, with the evolution of early animal life. To better understand the Ediacaran ocean redox evolution, we have conducted a multi-proxy paleoredox study of a relatively continuous, deep-water section in South China that was paleogeographically connected with the open ocean. Iron speciation and pyrite morphology indicate locally euxinic (anoxic and sulfidic) environments throughout the Ediacaran in this section. In the same rocks, redox sensitive element enrichments and sulfur isotope data provide evidence for multiple oceanic oxygenation events (OOEs) in a predominantly anoxic global Ediacaran-early Cambrian ocean. This dynamic redox landscape contrasts with a recent view of a redox-static Ediacaran ocean without significant change in oxygen content. The duration of the Ediacaran OOEs may be comparable to those of the oceanic anoxic events (OAEs) in otherwise well-oxygenated Phanerozoic oceans. Anoxic events caused mass extinctions followed by fast recovery in biologically diversified Phanerozoic oceans. In contrast, oxygenation events in otherwise ecologically monotonous anoxic Ediacaran-early Cambrian oceans may have stimulated biotic innovations followed by prolonged evolutionary stasis., (© 2016 John Wiley & Sons Ltd.)

All life on Earth seems to be made of the same chemical elements in relatively conserved proportions (stoichiometry). Whether this stoichiometry is conserved in settings that differ radically in physicochemical conditions (extreme environments) from those commonly encountered elsewhere on the planet provides insight into possible stoichiometries for putative life beyond Earth. Here, we report measurements of elemental stoichiometry for extremophile microbes from hot springs of Yellowstone National Park (YNP). Phototrophic and chemotrophic microbes were collected in locations spanning large ranges of temperature (24 °C to boiling), pH (1.6-9.6), redox (0.1-7.2 mg L(-1) dissolved oxygen), and nutrient concentrations (0.01-0.25 mg L(-1) NO2-, 0.7-12.9 mg L(-1) NO3-, 0.01-42 mg L(-1) NH4 (+), 0.003-1.1 mg L(-1) P mostly as phosphate). Despite these extreme conditions, the microbial cells sampled had a major and trace element stoichiometry within the ranges commonly encountered for microbes living in the more moderate environments of lakes and surface oceans. The cells did have somewhat high C:P and N:P ratios that are consistent with phosphorus (P) limitation. Furthermore, chemotrophs and phototrophs had similar compositions with the exception of Mo content, which was enriched in cells derived from chemotrophic sites. Thus, despite the extraordinary physicochemical and biological diversity of YNP environments, life in these settings, in a stoichiometric sense, remains much the same as we know it elsewhere., (© 2015 John Wiley & Sons Ltd.)

Mounting evidence indicates the presence of a near complete biological nitrogen cycle in redox-stratified oceans during the late Archean to early Proterozoic (c. 2.5-2.0 Ga). It has been suggested that the iron (Fe)- or vanadium (V)-dependent nitrogenase rather than molybdenum (Mo)-dependent form was responsible for dinitrogen fixation during this time because oceans were depleted in Mo and rich in Fe. We evaluated this hypothesis by examining the phylogenetic relationships of proteins that are required for the biosynthesis of the active site cofactor of Mo-nitrogenase in relation to structural proteins required for Fe-, V- and Mo-nitrogenase. The results are highly suggestive that among extant nitrogen-fixing organisms for which genomic information exists, Mo-nitrogenase is unlikely to have been associated with the Last Universal Common Ancestor. Rather, the origin of Mo-nitrogenase can be traced to an ancestor of the anaerobic and hydrogenotrophic methanogens with acquisition in the bacterial domain via lateral gene transfer involving an anaerobic member of the Firmicutes. A comparison of substitution rates estimated for proteins required for the biosynthesis of the nitrogenase active site cofactor and for a set of paralogous proteins required for the biosynthesis of bacteriochlorophyll suggests that Nif emerged from a nitrogenase-like ancestor approximately 1.5-2.2 Ga. An origin and ensuing proliferation of Mo-nitrogenase under anoxic conditions would likely have occurred in an environment where anaerobic methanogens and Firmicutes coexisted and where Mo was at least episodically available, such as in a redox-stratified Proterozoic ocean basin., (© 2011 Blackwell Publishing Ltd.)

Marine primary producers adapted over eons to the changing chemistry of the oceans. Because a number of metalloenzymes are necessary for N assimilation, changes in the availability of transition metals posed a particular challenge to the supply of this critical nutrient that regulates marine biomass and productivity. Integrating recently developed geochemical, biochemical, and genetic evidence, we infer that the use of metals in N assimilation - particularly Fe and Mo - can be understood in terms of the history of metal availability through time. Anoxic, Fe-rich Archean oceans were conducive to the evolution of Fe-using enzymes that assimilate abiogenic NH(4)(+) and NO(2)(-). The N demands of an expanding biosphere were satisfied by the evolution of biological N(2) fixation, possibly utilizing only Fe. Trace O(2) in late Archean environments, and the eventual 'Great Oxidation Event' c. 2.3 Ga, mobilized metals such as Mo, enabling the evolution of Mo (or V)-based N(2) fixation and the Mo-dependent enzymes for NO(3)(-) assimilation and denitrification by prokaryotes. However, the subsequent onset of deep-sea euxinia, an increasingly-accepted idea, may have kept ocean Mo inventories low and depressed Fe, limiting the rate of N(2) fixation and the supply of fixed N. Eukaryotic ecosystems may have been particularly disadvantaged by N scarcity and the high Mo requirement of eukaryotic NO(3)(-) assimilation. Thorough ocean oxygenation in the Neoproterozoic led to Mo-rich oceans, possibly contributing to the proliferation of eukaryotes and thus the Cambrian explosion of metazoan life. These ideas can be tested by more intensive study of the metal requirements in N assimilation and the biological strategies for metal uptake, regulation, and storage.