Aubrey L. Zerkle

Partitioning of Bacterial Communities between Seawater and Healthy, Black Band Diseased, and Dead Coral Surfaces

ABSTRACT Distinct partitioning has been observed in the composition and diversity of bacterial communities inhabiting the surface and overlying seawater of three coral species infected with black band disease (BBD) on the southern Caribbean island of Curaçao, Netherlands Antilles. PCR amplification and sequencing of bacterial 16S rRNA genes (rDNA) with universally conserved primers have identified over 524 unique bacterial sequences affiliated with 12 bacterial divisions. The molecular sequences exhibited less than 5% similarity in bacterial community composition between seawater and the healthy, black band diseased, and dead coral surfaces. The BBD bacterial mat rapidly migrates across and kills the coral tissue. Clone libraries constructed from the BBD mat were comprised of eight bacterial divisions and 13% unknowns. Several sequences representing bacteria previously found in other marine and terrestrial organisms (including humans) were isolated from the infected coral surfaces, including Clostridium spp., Arcobacter spp., Campylobacter spp., Cytophaga fermentans, Cytophaga columnaris, and Trichodesmium tenue.

High isotope fractionations during sulfate reduction in a low-sulfate euxinic ocean analog

A detailed record of the early-Earth sulfur (S) cycle is chronicled by the S isotope values of sulfide and sulfate preserved in the rock record. Interpretation of this record rests on our understanding of sulfur cycling in modern systems, experiments, and the resulting isotopic signatures. Very large fractionations in δ34S of ≥70‰ are commonly measured between sulfide and sulfate in modern systems and in ancient sediments. Theoretical calculations suggest that sulfate-reducing prokaryotes are capable of producing such large fractionations during the reduction of sulfate to sulfide, although they have only been demonstrated to generate fractionations up to 48‰. Here we report the first direct determination of 60‰–70‰ fractionations by natural populations of sulfate reducers. These high fractionations occur under the relatively low-sulfate conditions (1.1–2 mM) of meromictic Lago di Cadagno in Switzerland. The major and minor isotopic composition of sulfide and sulfate in the lake water is consistent with sulfide produced by sulfate reduction, with little evidence for modification by further oxidative sulfur cycling. These observations help us to constrain the evolution of seawater sulfate concentrations.

Sulfate was a trace constituent of Archean seawater

Dissecting ancient microbial sulfur cycling Before the rise of oxygen, life on Earth depended on the marine sulfur cycle. The fractionation of different sulfur isotopes provides clues to which biogeochemical cycles were active long ago (see the Perspective by Ueno). Zhelezinskaia et al. found negative isotope anomalies in Archean rocks from Brazil and posit that metabolic fluxes from sulfate-reducing microorganisms influenced the global sulfur cycle, including sulfur in the atmosphere. In contrast, Paris et al. found positive isotope anomalies in Archean sediments from South Africa, implying that the marine sulfate pool was more disconnected from atmospheric sulfur. As an analog for the Archean ocean, Crowe et al. measured sulfur isotope signatures in modern Lake Matano, Indonesia, and suggest that low seawater sulfate concentrations restricted early microbial activity. Science, this issue p. 703, p. 742, p. 739; see also p. 735 Low levels of sulfate allowed for the preservation of mass-independent isotope signatures in the Archean. [Also see Perspective by Ueno] In the low-oxygen Archean world (>2400 million years ago), seawater sulfate concentrations were much lower than today, yet open questions frustrate the translation of modern measurements of sulfur isotope fractionations into estimates of Archean seawater sulfate concentrations. In the water column of Lake Matano, Indonesia, a low-sulfate analog for the Archean ocean, we find large (>20 per mil) sulfur isotope fractionations between sulfate and sulfide, but the underlying sediment sulfides preserve a muted range of δ34S values. Using models informed by sulfur cycling in Lake Matano, we infer Archean seawater sulfate concentrations of less than 2.5 micromolar. At these low concentrations, marine sulfate residence times were likely 103 to 104 years, and sulfate scarcity would have shaped early global biogeochemical cycles, possibly restricting biological productivity in Archean oceans.

Pathways for Neoarchean pyrite formation constrained by mass-independent sulfur isotopes

It is generally thought that the sulfate reduction metabolism is ancient and would have been established well before the Neoarchean. It is puzzling, therefore, that the sulfur isotope record of the Neoarchean is characterized by a signal of atmospheric mass-independent chemistry rather than a strong overprint by sulfate reducers. Here, we present a study of the four sulfur isotopes obtained using secondary ion MS that seeks to reconcile a number of features seen in the Neoarchean sulfur isotope record. We suggest that Neoarchean ocean basins had two coexisting, significantly sized sulfur pools and that the pathways forming pyrite precursors played an important role in establishing how the isotopic characteristics of each of these pools was transferred to the sedimentary rock record. One of these pools is suggested to be a soluble (sulfate) pool, and the other pool (atmospherically derived elemental sulfur) is suggested to be largely insoluble and unreactive until it reacts with hydrogen sulfide. We suggest that the relative contributions of these pools to the formation of pyrite depend on both the accumulation of the insoluble pool and the rate of sulfide production in the pyrite-forming environments. We also suggest that the existence of a significant nonsulfate pool of reactive sulfur has masked isotopic evidence for the widespread activity of sulfate reducers in the rock record.

Production of 15N‐depleted biomass during cyanobacterial N2‐fixation at high Fe concentrations

[1] In this study we examine the effects of varying Fe, Mo, and P concentrations on δ 15 N fractionation during N 2 fixation in the cyanobacterium Anabaena variabilis. We show that when grown in Fe-enriched media ([Fe] > 50 nM), this organism produces biomass up to 3%o lower in δ 15 N than when grown in Fe-limited media ([Fe] < 50 nM). A compilation of our data with previous measurements of δ 15 N in N 2 -fixing cyanobacteria reveals a general trend toward the production of more 15 N-depleted biomass at higher Fe concentrations. We discuss our results in the context of negative δ 15 N values preserved in Archean and some Phanerozoic sediments, generally attributed to the production of marine organic matter with low δ 15 N by N 2 fixation (and potentially NH4 regeneration) during periods of fluctuating nutrient dynamics. We suggest that enhanced Fe availability during periods of widespread ocean anoxia can further stimulate the production of 15 N-depleted biomass by N 2 -fixing organisms, contributing to the isotopic record.

Molybdenum isotope fractionation by cyanobacterial assimilation during nitrate utilization and N2fixation

We measured the δ98Mo of cells and media from molybdenum (Mo) assimilation experiments with the freshwater cyanobacterium Anabaena variabilis, grown with nitrate as a nitrogen (N) source or fixing atmospheric N2. This organism uses a Mo-based nitrate reductase during nitrate utilization and a Mo-based dinitrogenase during N2 fixation under culture conditions here. We also demonstrate that it has a high-affinity Mo uptake system (ModABC) similar to other cyanobacteria, including marine N2-fixing strains. Anabaena variabilis preferentially assimilated light isotopes of Mo in all experiments, resulting in fractionations of −0.2‰ to −1.0‰ ± 0.2‰ between cells and media (εcells–media), extending the range of biological Mo fractionations previously reported. The fractionations were internally consistent within experiments, but varied with the N source utilized and for different growth phases sampled. During growth on nitrate, A. variabilis consistently produced fractionations of −0.3 ± 0.1‰ (mean ± standard deviation between experiments). When fixing N2, A. variabilis produced fractionations of −0.9 ± 0.1‰ during exponential growth, and −0.5 ± 0.1‰ during stationary phase. This pattern is inconsistent with a simple kinetic isotope effect associated with Mo transport, because Mo is likely transported through the ModABC uptake system under all conditions studied. We present a reaction network model for Mo isotope fractionation that demonstrates how Mo transport and storage, coordination changes during enzymatic incorporation, and the distribution of Mo inside the cell could all contribute to the total biological fractionations. Additionally, we discuss the potential importance of biologically incorporated Mo to organic matter-bound Mo in marine sediments.

The Role of Calcining and Basal Fluidization in the Long Runout of Carbonate Slides: An Example from the Heart Mountain Slide Block, Wyoming and Montana, U.S.A.

In order to understand the movement of large rock masses or allochthons on low-angle surfaces, we have studied the 3400-km2 Heart Mountain slide block of northwestern Wyoming and southwestern Montana. The Heart Mountain slide block was initiated on a 2° gradient, with its toe thrust a minimum of 45 km across an early Eocene landscape. The slide block moved on a basal layer that ranges in thickness from a few tens of centimeters to several meters. This basal layer commonly has a concrete-like appearance of rounded, mixed-lithology grains in a fine-grained carbonate matrix, and in some locations it has features similar to sedimentary deposits, including both normal and inverse grading, flow banding, turbidite-like structures, and clastic dikes containing pieces of carbonized wood. Nowhere did we observe crosscutting relationships in the basal layer or overlying clastic dikes, as would be expected from incremental or noncatastrophic emplacement. Results from cathodoluminescence and &dgr;18O, &dgr;13C, and 87Sr/86Sr isotopic compositions from the basal layer support a single movement event followed by hydrothermal and meteoric fluids percolating through a permeable basal layer. These observations suggest that a catastrophic movement on the detachment resulted in frictional heating at the base of the slide. When the generated heat was at least 800°C, calcining of carbonates occurred, yielding calcium and magnesium oxide powders and carbon dioxide gas. The calcium oxide powder became mechanically fluidized by the pressurized carbon dioxide gas, leading to a reduced coefficient of friction at the base of the slide, which in turn permitted the long runout on such a low-angle surface. This mechanism might be applied to explain a wide range of catastrophic sliding events where carbonate rocks are involved.

Onset of the aerobic nitrogen cycle during the Great Oxidation Event

The rise of oxygen on the early Earth (about 2.4 billion years ago) caused a reorganization of marine nutrient cycles, including that of nitrogen, which is important for controlling global primary productivity. However, current geochemical records lack the temporal resolution to address the nature and timing of the biogeochemical response to oxygenation directly. Here we couple records of ocean redox chemistry with nitrogen isotope (15N/14N) values from approximately 2.31-billion-year-old shales of the Rooihoogte and Timeball Hill formations in South Africa, deposited during the early stages of the first rise in atmospheric oxygen on the Earth (the Great Oxidation Event). Our data fill a gap of about 400 million years in the temporal 15N/14N record and provide evidence for the emergence of a pervasive aerobic marine nitrogen cycle. The interpretation of our nitrogen isotope data in the context of iron speciation and carbon isotope data suggests biogeochemical cycling across a dynamic redox boundary, with primary productivity fuelled by chemoautotrophic production and a nitrogen cycle dominated by nitrogen loss processes using newly available marine oxidants. This chemostratigraphic trend constrains the onset of widespread nitrate availability associated with ocean oxygenation. The rise of marine nitrate could have allowed for the rapid diversification and proliferation of nitrate-using cyanobacteria and, potentially, eukaryotic phytoplankton.

Biological Regulation of Atmospheric Chemistry En Route to Planetary Oxygenation

Significance It has been proposed that enhanced methane fluxes to Earth’s early atmosphere could have altered atmospheric chemistry, initiating a hydrocarbon-rich haze reminiscent of Saturn’s moon Titan. The occurrence, cause, and significance of haze development, however, remain unknown. Here, we test and refine the “haze hypothesis” by combining an ultra-high-resolution sulfur- and carbon-isotope dataset with photochemical simulations to reveal the structure and timing of haze development. These data suggest that haze persisted for ∼1 million years, requiring a sustained biological driver. We propose that enhanced atmospheric CH4, implied by the presence of haze, could have had a significant impact on the escape of hydrogen from the atmosphere, effectively contributing to the terminal oxidation of Earth’s surficial environments ∼2.4 billion years ago. Emerging evidence suggests that atmospheric oxygen may have varied before rising irreversibly ∼2.4 billion years ago, during the Great Oxidation Event (GOE). Significantly, however, pre-GOE atmospheric aberrations toward more reducing conditions—featuring a methane-derived organic-haze—have recently been suggested, yet their occurrence, causes, and significance remain underexplored. To examine the role of haze formation in Earth’s history, we targeted an episode of inferred haze development. Our redox-controlled (Fe-speciation) carbon- and sulfur-isotope record reveals sustained systematic stratigraphic covariance, precluding nonatmospheric explanations. Photochemical models corroborate this inference, showing Δ36S/Δ33S ratios are sensitive to the presence of haze. Exploiting existing age constraints, we estimate that organic haze developed rapidly, stabilizing within ∼0.3 ± 0.1 million years (Myr), and persisted for upward of ∼1.4 ± 0.4 Myr. Given these temporal constraints, and the elevated atmospheric CO2 concentrations in the Archean, the sustained methane fluxes necessary for haze formation can only be reconciled with a biological source. Correlative δ13COrg and total organic carbon measurements support the interpretation that atmospheric haze was a transient response of the biosphere to increased nutrient availability, with methane fluxes controlled by the relative availability of organic carbon and sulfate. Elevated atmospheric methane concentrations during haze episodes would have expedited planetary hydrogen loss, with a single episode of haze development providing up to 2.6–18 × 1018 moles of O2 equivalents to the Earth system. Our findings suggest the Neoarchean likely represented a unique state of the Earth system where haze development played a pivotal role in planetary oxidation, hastening the contingent biological innovations that followed.

The geobiological nitrogen cycle : from microbes to the mantle

Abstract Nitrogen forms an integral part of the main building blocks of life, including DNA, RNA, and proteins. N2 is the dominant gas in Earths atmosphere, and nitrogen is stored in all of Earths geological reservoirs, including the crust, the mantle, and the core. As such, nitrogen geochemistry is fundamental to the evolution of planet Earth and the life it supports. Despite the importance of nitrogen in the Earth system, large gaps remain in our knowledge of how the surface and deep nitrogen cycles have evolved over geologic time. Here, we discuss the current understanding (or lack thereof) for how the unique interaction of biological innovation, geodynamics, and mantle petrology has acted to regulate Earths nitrogen cycle over geologic timescales. In particular, we explore how temporal variations in the external (biosphere and atmosphere) and internal (crust and mantle) nitrogen cycles could have regulated atmospheric pN2. We consider three potential scenarios for the evolution of the geobiological nitrogen cycle over Earths history: two in which atmospheric pN2 has changed unidirectionally (increased or decreased) over geologic time and one in which pN2 could have taken a dramatic deflection following the Great Oxidation Event. It is impossible to discriminate between these scenarios with the currently available models and datasets. However, we are optimistic that this problem can be solved, following a sustained, open‐minded, and multidisciplinary effort between surface and deep Earth communities.