Iron colloids dominate sedimentary supply to the ocean interior.

Dissolution of marine sediment is a key source of dissolved iron (Fe) that regulates the ocean carbon cycle. Currently, our prevailing understanding, encapsulated in ocean models, focuses on low-oxygen reductive supply mechanisms and neglects the emerging evidence from iron isotopes in seawater and sediment porewaters for additional nonreductive dissolution processes. Here, we combine measurements of Fe colloids and dissolved δ56Fe in shallow porewaters spanning the full depth of the South Atlantic Ocean to demonstrate that it is lithogenic colloid production that fuels sedimentary iron supply away from low-oxygen systems. Iron colloids are ubiquitous in these oxic ocean sediment porewaters and account for the lithogenic isotope signature of dissolved Fe (δ56Fe = +0.07 ± 0.07‰) within and between ocean basins. Isotope model experiments demonstrate that only lithogenic weathering in both oxic and nitrogenous zones, rather than precipitation or ligand complexation of reduced Fe species, can account for the production of these porewater Fe colloids. The broader covariance between colloidal Fe and organic carbon (OC) abundance suggests that sorption of OC may control the nanoscale stability of Fe minerals by inhibiting the loss of Fe(oxyhydr)oxides to more crystalline minerals in the sediment. Oxic ocean sediments can therefore generate a large exchangeable reservoir of organo-mineral Fe colloids at the sediment water interface (a "rusty source") that dominates the benthic supply of dissolved Fe to the ocean interior, alongside reductive supply pathways from shallower continental margins.

Anthropogenic lead pervasive in Canadian Arctic seawater.

Anthropogenic Pb is widespread in the environment including remote places. However, its presence in Canadian Arctic seawater is thought to be negligible based on low dissolved Pb (dPb) concentrations and proxy data. Here, we measured dPb isotopes in Arctic seawater with very low dPb concentrations (average ∼5 pmol ⋅ kg-1) and show that anthropogenic Pb is pervasive and often dominant in the western Arctic Ocean. Pb isotopes further reveal that historic aerosol Pb from Europe and Russia (Eurasia) deposited to the Arctic during the 20th century, and subsequently remobilized, is a significant source of dPb, particularly in water layers with relatively higher dPb concentrations (up to 16 pmol ⋅ kg-1). The 20th century Eurasian Pb is present predominantly in the upper 1,000 m near the shelf but is also detected in older deep water (2,000 to 2,500 m). These findings highlight the importance of the remobilization of anthropogenic Pb associated with previously deposited aerosols, especially those that were emitted during the peak of Pb emissions in the 20th century. This remobilization might be further enhanced because of accelerated melting of permafrost and ice along with increased coastal erosion in the Arctic. Additionally, the detection of 20th century Eurasian Pb in deep water helps constrain ventilation ages. Overall, this study shows that Pb isotopes in Arctic seawater are useful as a gauge of changing particulate and contaminant sources, such as those resulting from increased remobilization (e.g., coastal erosion) and potentially also those associated with increased human activities (e.g., mining and shipping).

Anthropogenic Asian aerosols provide Fe to the North Pacific Ocean.

Fossil-fuel emissions may impact phytoplankton primary productivity and carbon cycling by supplying bioavailable Fe to remote areas of the ocean via atmospheric aerosols. However, this pathway has not been confirmed by field observations of anthropogenic Fe in seawater. Here we present high-resolution trace-metal concentrations across the North Pacific Ocean (158°W from 25°to 42°N). A dissolved Fe maximum was observed around 35°N, coincident with high dissolved Pb and Pb isotope ratios matching Asian industrial sources and confirming recent aerosol deposition. Iron-stable isotopes reveal in situ evidence of anthropogenic Fe in seawater, with low δ56Fe (-0.23‰ > δ56Fe > -0.65‰) observed in the region that is most influenced by aerosol deposition. An isotope mass balance suggests that anthropogenic Fe contributes 21-59% of dissolved Fe measured between 35° and 40°N. Thus, anthropogenic aerosol Fe is likely to be an important Fe source to the North Pacific Ocean.

Quantification of dissolved iron sources to the North Atlantic Ocean.

Dissolved iron is an essential micronutrient for marine phytoplankton, and its availability controls patterns of primary productivity and carbon cycling throughout the oceans. The relative importance of different sources of iron to the oceans is not well known, however, and flux estimates from atmospheric dust, hydrothermal vents and oceanic sediments vary by orders of magnitude. Here we present a high-resolution transect of dissolved stable iron isotope ratios (δ(56)Fe) and iron concentrations ([Fe]) along a section of the North Atlantic Ocean. The different iron sources can be identified by their unique δ(56)Fe signatures, which persist throughout the water column. This allows us to calculate the relative contribution from dust, hydrothermal venting and reductive and non-reductive sedimentary release to the dissolved phase. We find that Saharan dust aerosol is the dominant source of dissolved iron along the section, contributing 71-87 per cent of dissolved iron. Additional sources of iron are non-reductive release from oxygenated sediments on the North American margin (10-19 per cent), reductive sedimentary dissolution on the African margin (1-4 per cent) and hydrothermal venting at the Mid-Atlantic Ridge (2-6 per cent). Our data also indicate that hydrothermal vents in the North Atlantic are a source of isotopically light iron, which travels thousands of kilometres from vent sites, potentially influencing surface productivity. Changes in the relative importance of the different iron sources through time may affect interactions between the carbon cycle and climate.

A new anion exchange purification method for Cu stable isotopes in blood samples.

The isotopic composition of iron, zinc, copper, and cadmium (δ56Fe, δ66Zn, δ65Cu, and δ114Cd) are novel and promising tools to study the metabolism and homeostasis of trace metals in the human body. Serum δ65Cu has been proposed as a potential tool for diagnosis of cancer in liquid biopsy, and other metals may have similar utility. However, accurate analysis of trace metal isotopes is challenging because of the difficulties in purifying the metals from biological samples. Here we developed a simple and rapid method for sequential purification of Cu, Fe, Zn, and Cd from a single blood plasma sample. By using a combination of 11 M acetic acid and 4 M HCl in the first steps of column chemistry on AG-MP1 resin, we dramatically improve the separation of Cu from matrix elements compared to previous methods which use concentrated HCl alone. Our new method achieves full recovery of Cu, Fe, Zn, and Cd to prevent column-induced isotope fractionation effects, and effectively separates analytes from the matrix in order to reduce polyatomic interferences during isotope analysis. Our methods were verified by the analysis of isotope standards, a whole blood reference material, and a preliminary sample set including five plasma samples from healthy individuals and five plasma samples from cancer patients. This new method simplifies preparation of blood samples for metal isotope analysis, accelerating multi-isotope approaches to medical studies and contributing to our understanding of the cycling of Fe, Zn, Cu, and Cd in the human body. Graphical abstract ᅟ.

Divergent responses of Atlantic coastal and oceanic Synechococcus to iron limitation.

Marine Synechococcus are some of the most diverse and ubiquitous phytoplankton, and iron (Fe) is an essential micronutrient that limits productivity in many parts of the ocean. To investigate how coastal and oceanic Atlantic Synechococcus strains acclimate to Fe availability, we compared the growth, photophysiology, and quantitative proteomics of two Synechococcus strains from different Fe regimes. Synechococcus strain WH8102, from a region in the southern Sargasso Sea that receives substantial dust deposition, showed impaired growth and photophysiology as Fe declined, yet used few acclimation responses. Coastal WH8020, from the dynamic, seasonally variable New England shelf, displayed a multitiered, hierarchical cascade of acclimation responses with different Fe thresholds. The multitiered response included changes in Fe acquisition, storage, and photosynthetic proteins, substitution of flavodoxin for ferredoxin, and modified photophysiology, all while maintaining remarkably stable growth rates over a range of Fe concentrations. Modulation of two distinct ferric uptake regulator (Fur) proteins that coincided with the multitiered proteome response was found, implying the coastal strain has different regulatory threshold responses to low Fe availability. Low nitrogen (N) and phosphorus (P) availability in the open ocean may favor the loss of Fe response genes when Fe availability is consistent over time, whereas these genes are retained in dynamic environments where Fe availability fluctuates and N and P are more abundant.

Undocumented water column sink for cadmium in open ocean oxygen-deficient zones.

Cadmium (Cd) is a micronutrient and a tracer of biological productivity and circulation in the ocean. The correlation between dissolved Cd and the major algal nutrients in seawater has led to the use of Cd preserved in microfossils to constrain past ocean nutrient distributions. However, linking Cd to marine biological processes requires constraints on marine sources and sinks of Cd. Here, we show a decoupling between Cd and major nutrients within oxygen-deficient zones (ODZs) in both the Northeast Pacific and North Atlantic Oceans, which we attribute to Cd sulfide (CdS) precipitation in euxinic microenvironments around sinking biological particles. We find that dissolved Cd correlates well with dissolved phosphate in oxygenated waters, but is depleted compared with phosphate in ODZs. Additionally, suspended particles from the North Atlantic show high Cd content and light Cd stable isotope ratios within the ODZ, indicative of CdS precipitation. Globally, we calculate that CdS precipitation in ODZs is an important, and to our knowledge a previously undocumented marine sink of Cd. Our results suggest that water column oxygen depletion has a substantial impact on Cd biogeochemical cycling, impacting the global relationship between Cd and major nutrients and suggesting that Cd may be a previously unidentified tracer for water column oxygen deficiency on geological timescales. Similar depletions of copper and zinc in the Northeast Pacific indicate that sulfide precipitation in ODZs may also have an influence on the global distribution of other trace metals.

An inexpensive, accurate, and precise wet-mount method for enumerating aquatic viruses.

Viruses affect biogeochemical cycling, microbial mortality, gene flow, and metabolic functions in diverse environments through infection and lysis of microorganisms. Fundamental to quantitatively investigating these roles is the determination of viral abundance in both field and laboratory samples. One current, widely used method to accomplish this with aquatic samples is the "filter mount" method, in which samples are filtered onto costly 0.02-µm-pore-size ceramic filters for enumeration of viruses by epifluorescence microscopy. Here we describe a cost-effective (ca. 500-fold-lower materials cost) alternative virus enumeration method in which fluorescently stained samples are wet mounted directly onto slides, after optional chemical flocculation of viruses in samples with viral concentrations of <5×10(7) viruses ml(-1). The concentration of viruses in the sample is then determined from the ratio of viruses to a known concentration of added microsphere beads via epifluorescence microscopy. Virus concentrations obtained by using this wet-mount method, with and without chemical flocculation, were significantly correlated with, and had precision equivalent to, those obtained by the filter mount method across concentrations ranging from 2.17×10(6) to 1.37×10(8) viruses ml(-1) when tested by using cultivated viral isolates and natural samples from marine and freshwater environments. In summary, the wet-mount method is significantly less expensive than the filter mount method and is appropriate for rapid, precise, and accurate enumeration of aquatic viruses over a wide range of viral concentrations (≥1×10(6) viruses ml(-1)) encountered in field and laboratory samples.

Carboxylesterase 1-mediated drug-drug interactions between clopidogrel and simvastatin.

Patients with coronary artery disease often receive concurrent treatment with clopidogrel and a hydroxymethylglutaryl (HMG)-CoA reductase inhibitor medication. Accordingly, potential drug-drug interactions associated with the concomitant use of these agents present an area of concern. Both CYP enzymes and carboxylesterase 1 (CES1) are involved in the metabolism of clopidogrel, while CES1 is believed to be the enzyme responsible for the activation of simvastatin. Some in vitro studies have suggested that simvastatin could attenuate clopidogrel activation via inhibiting CYP3A activity. However, these findings have not found support in several recently published clinical investigations. The present study addresses these inconsistencies by exploring the potential role of CES1 in the metabolism of clopidogrel and simvastatin. Our in vitro human liver s9 fraction incubation study demonstrated that simvastatin significantly enhanced the formation of the intermediate metabolite 2-oxo-clopidogrel, and inhibited the CES1-mediated hydrolysis of clopidogrel, 2-oxo-clopidogrel, and the active metabolite. However, the production of the active metabolite remained unchanged. Conversely, clopidogrel was not found to influence the CES1 mediated hydrolysis (activation) of simvastatin. Moreover, we provided evidence that CES1 is not an efficient enzyme for catalyzing simvastatin activation. In summary, the inhibitory effect of simvastatin on the hydrolysis of clopidogrel and its principal metabolites may have offset the influence of simvastatin-mediated inhibition of CYP3A, and permitted the unaltered formation of the clopidogrel active metabolite. These data help explain the conflicting accounts in previous reports regarding clopidogrel and simvastatin interactions by taking into consideration CES1; they suggest that the interactions are unlikely to significantly influence clinical outcomes.

A rapid procedure for isotopic purification of copper and nickel from seawater using an automated chromatography system.

BACKGROUND: Trace metals such as iron, nickel, copper, zinc, and cadmium (Fe, Ni, Cu, Zn, and Cd) are essential micronutrients (and sometimes toxins) for phytoplankton, and the analysis of trace-metal stable isotopes in seawater is a valuable tool for exploring the biogeochemical cycling of these elements in the ocean. However, the complex and often time-consuming chromatography process required to purify these elements from seawater has limited the number of trace-metal isotope samples which can be easily processed in biogeochemical studies. To facilitate the trace-metal stable isotope analysis, here, we describe a new rapid procedure that utilizes automated chromatography for extracting and purifying Ni and Cu from seawater for isotope analysis using a prepFAST-MC™ system (Elemental Scientific Inc.). RESULTS: We have tested the matrix removal effectiveness, recoveries, and procedural blanks of the new purification procedure with satisfactory results. A nearly complete recovery of Ni and a quantitative recovery of Cu are achieved. The total procedural blanks are 0.33 ± 0.24 ng for Ni and 0.42 ± 0.18 ng for Cu, which is negligible for natural seawater samples. The new procedure cleanly separates Ni and Cu from key seawater matrix elements that may cause interferences during mass spectrometry analysis. When the new procedure was used to purify seawater samples for Ni and Cu stable isotope analysis by multi-collector ICP-MS, we achieved an overall uncertainty of 0.07 ‰ for δ60Ni and 0.09 ‰ for δ65Cu (2 SD). The new purification procedure was also tested using natural seawater samples from the South Pacific, for comparison of δ60Ni and δ65Cu achieved in the same samples purified by traditional hand columns. Both methods produced similar results, and the results from both methods are consistent with analyses of δ60Ni and δ65Cu from other ocean locations as reported by other laboratories. SIGNIFICANCE: This study presents a new rapid procedure for seawater stable-metal isotope analysis by automating the chromatography step. We anticipate that the automated chromatography described here will facilitate the rapid and accurate analysis of seawater δ60Ni and δ65Cu in future studies, and may be adapted in the future to automate chromatographic purification of Fe, Zn, and Cd isotopes from seawater.

Proteomics analysis reveals differential acclimation of coastal and oceanic Synechococcus to climate warming and iron limitation.

In many oceanic regions, anthropogenic warming will coincide with iron (Fe) limitation. Interactive effects between warming and Fe limitation on phytoplankton physiology and biochemical function are likely, as temperature and Fe availability affect many of the same essential cellular pathways. However, we lack a clear understanding of how globally significant phytoplankton such as the picocyanobacteria Synechococcus will respond to these co-occurring stressors, and what underlying molecular mechanisms will drive this response. Moreover, ecotype-specific adaptations can lead to nuanced differences in responses between strains. In this study, Synechococcus isolates YX04-1 (oceanic) and XM-24 (coastal) from the South China Sea were acclimated to Fe limitation at two temperatures, and their physiological and proteomic responses were compared. Both strains exhibited reduced growth due to warming and Fe limitation. However, coastal XM-24 maintained relatively higher growth rates in response to warming under replete Fe, while its growth was notably more compromised under Fe limitation at both temperatures compared with YX04-1. In response to concurrent heat and Fe stress, oceanic YX04-1 was better able to adjust its photosynthetic proteins and minimize the generation of reactive oxygen species while reducing proteome Fe demand. Its intricate proteomic response likely enabled oceanic YX04-1 to mitigate some of the negative impact of warming on its growth during Fe limitation. Our study highlights how ecologically-shaped adaptations in Synechococcus strains even from proximate oceanic regions can lead to differing physiological and proteomic responses to these climate stressors.

Open science resources from the Tara Pacific expedition across coral reef and surface ocean ecosystems.

The Tara Pacific expedition (2016-2018) sampled coral ecosystems around 32 islands in the Pacific Ocean and the ocean surface waters at 249 locations, resulting in the collection of nearly 58 000 samples. The expedition was designed to systematically study warm-water coral reefs and included the collection of corals, fish, plankton, and seawater samples for advanced biogeochemical, molecular, and imaging analysis. Here we provide a complete description of the sampling methodology, and we explain how to explore and access the different datasets generated by the expedition. Environmental context data were obtained from taxonomic registries, gazetteers, almanacs, climatologies, operational biogeochemical models, and satellite observations. The quality of the different environmental measures has been validated not only by various quality control steps, but also through a global analysis allowing the comparison with known environmental large-scale structures. Such publicly released datasets open the perspective to address a wide range of scientific questions.

Does activation of the anti proton, rather than concertedness, determine the stereochemistry of base-catalyzed 1,2-elimination reactions? Anti stereospecificity in E1cB eliminations of ß-3-trifluoromethylphenoxy esters, thioesters, and ketones.

As part of a comprehensive investigation on the stereochemical aspects of base-catalyzed 1,2-elimination reactions, we have studied a set of acyclic carbonyl substrates that react by an irreversible E1cB mechanism with largely anti stereospecificity. (2)H NMR data show that these reactions using KOH in EtOH/H(2)O under non-ion-pairing conditions produce a minimum of 85-89% anti elimination on stereospecifically labeled tert-butyl (2R*,3R*)- and (2R*,3S*)-3-(3-trifluoromethylphenoxy)-2,3-(2)H(2)-butanoate, S-tert-butyl (2R*,3R*)- and (2R*,3S*)-3-(3-trifluoromethylphenoxy)-2,3-(2)H(2)-butanethioate, and the related ketones, (4R*,5R*)- and (4R*,5S*)-5-(3-trifluoromethylphenoxy)-4,5-(2)H(2)-3-hexanone. With both diastereomers of each substrate available, the KIEs can be calculated and the innate stereoselectivities determined. The elimination reactions of the ß-3-trifluoromethylphenoxy substrates occur by E1cB mechanisms with diffusionally equilibrated enolate-anion intermediates. Thus, it is clear that anti elimination does not depend solely upon concerted E2 mechanisms. Negative hyperconjugation provides a satisfactory explanation for the anti stereospecificity exhibited by our carbonyl substrates, where the leaving group activates the anti proton, leading to the enolate intermediate. The activation of the anti proton by negative hyperconjugation may also play a role in the concerted pathways of E2 mechanisms. We have also measured the rates of the hydroxide-catalyzed elimination reactions of butanoate, thiobutanoate, and ketone substrates in EtOH/H(2)O, with ß-tosyloxy, acetoxy, and 3-trifluoromethylphenoxy nucleofuges.

Independent iron and light limitation in a low-light-adapted Prochlorococcus from the deep chlorophyll maximum.

Throughout the open ocean, a minimum in dissolved iron concentration (dFe) overlaps with the deep chlorophyll maximum (DCM), which marks the lower limit of the euphotic zone. Maximizing light capture in these dim waters is expected to require upregulation of Fe-bearing photosystems, further depleting dFe and possibly leading to co-limitation by both iron and light. However, this effect has not been quantified for important phytoplankton groups like Prochlorococcus, which contributes most of the productivity in the oligotrophic DCM. Here, we present culture experiments with Prochlorococcus strain MIT1214, a member of the Low Light 1 ecotype isolated from the DCM in the North Pacific subtropical gyre. Under a matrix of iron and irradiance matching those found at the DCM, the ratio of Fe to carbon in Prochlorococcus MIT1214 cells ranged from 10-40 × 10-6 mol Fe:mol C and increased with light intensity and growth rate. These results challenge theoretical models predicting highest Fe:C at lowest light intensity, and are best explained by a large photosynthetic Fe demand that is not downregulated at higher light. To sustain primary production in the DCM with the rigid Fe requirements of low-light-adapted Prochlorococcus, dFe must be recycled rapidly and at high efficiency.

Metabolic and biogeochemical consequences of viral infection in aquatic ecosystems.

Ecosystems are controlled by 'bottom-up' (resources) and 'top-down' (predation) forces. Viral infection is now recognized as a ubiquitous top-down control of microbial growth across ecosystems but, at the same time, cell death by viral predation influences, and is influenced by, resource availability. In this Review, we discuss recent advances in understanding the biogeochemical impact of viruses, focusing on how metabolic reprogramming of host cells during lytic viral infection alters the flow of energy and nutrients in aquatic ecosystems. Our synthesis revealed several emerging themes. First, viral infection transforms host metabolism, in part through virus-encoded metabolic genes; the functions performed by these genes appear to alleviate energetic and biosynthetic bottlenecks to viral production. Second, viral infection depends on the physiological state of the host cell and on environmental conditions, which are challenging to replicate in the laboratory. Last, metabolic reprogramming of infected cells and viral lysis alter nutrient cycling and carbon export in the oceans, although the net impacts remain uncertain. This Review highlights the need for understanding viral infection dynamics in realistic physiological and environmental contexts to better predict their biogeochemical consequences.

Tracing and constraining anthropogenic aerosol iron fluxes to the North Atlantic Ocean using iron isotopes.

Atmospheric dust is an important source of the micronutrient Fe to the oceans. Although relatively insoluble mineral Fe is assumed to be the most important component of dust, a relatively small yet highly soluble anthropogenic component may also be significant. However, quantifying the importance of anthropogenic Fe to the global oceans requires a tracer which can be used to identify and constrain anthropogenic aerosols in situ. Here, we present Fe isotope (δ56Fe) data from North Atlantic aerosol samples from the GEOTRACES GA03 section. While soluble aerosol samples collected near the Sahara have near-crustal δ56Fe, soluble aerosols from near North America and Europe instead have remarkably fractionated δ56Fe values (as light as -1.6‰). Here, we use these observations to fingerprint anthropogenic combustion sources, and to refine aerosol deposition modeling. We show that soluble anthropogenic aerosol Fe flux to the global surface oceans is highly likely to be underestimated, even in the dusty North Atlantic.

Kilauea lava fuels phytoplankton bloom in the North Pacific Ocean.

From June to August 2018, the eruption of Kilauea volcano on the island of Hawai'i injected millions of cubic meters of molten lava into the nutrient-poor waters of the North Pacific Subtropical Gyre. The lava-impacted seawater was characterized by high concentrations of metals and nutrients that stimulated phytoplankton growth, resulting in an extensive plume of chlorophyll a that was detectable by satellite. Chemical and molecular evidence revealed that this biological response hinged on unexpectedly high concentrations of nitrate, despite the negligible quantities of nitrogen in basaltic lava. We hypothesize that the high nitrate was caused by buoyant plumes of nutrient-rich deep waters created by the substantial input of lava into the ocean. This large-scale ocean fertilization was therefore a unique perturbation event that revealed how marine ecosystems respond to exogenous inputs of nutrients.

Biological uptake and reversible scavenging of zinc in the global ocean.

Zinc (Zn) is a key micronutrient for marine phytoplankton, with a global distribution that is similar to silicic acid. The processes that govern this relationship, despite the very different biological cycling of Zn and silica, remain poorly understood. Here, we use diagnostic and mechanistic models to show that only a combination of Southern Ocean biological uptake and reversible scavenging of Zn onto sinking particles can explain the observations. The distinction between organic and adsorbed Zn can also reconcile the vertical distribution and mass balance of Zn isotopes, which previously appeared at odds. This holistic understanding explains the Zn deficiencies observed throughout the low-latitude ocean and implies a greater sensitivity of the marine Zn cycle to climate-driven changes in organic matter cycling than previously recognized.

Iron Chloride Flocculation of Bacteriophages from Seawater.

Viruses influence ecosystem dynamics by modulating microbial host population dynamics, evolutionary trajectories and metabolic outputs. While they are ecologically important across diverse ecosystems, viruses are challenging to study due to minimal biomass often obtained when sampling natural communities. Here we describe a technique using chemical flocculation, filtration and resuspension to recover bacteriophages from seawater and other natural waters. The method uses iron to precipitate viruses which are recovered by filtration onto large-pore size membranes and then resuspended using a buffer containing magnesium and a reductant (ascorbic acid or oxalic acid) at slightly acid pH (6-6.5). The recovery of bacteriophages using iron flocculation is efficient (>90%), inexpensive and reliable, resulting in preparations that are amenable to downstream analysis by next generation DNA sequencing, proteomics and, in some cases, can be used to study virus-host interactions.

Tracking the rise of eukaryotes to ecological dominance with zinc isotopes.

The biogeochemical cycling of zinc (Zn) is intimately coupled with organic carbon in the ocean. Based on an extensive new sedimentary Zn isotope record across Earth's history, we provide evidence for a fundamental shift in the marine Zn cycle ~800 million years ago. We discuss a wide range of potential drivers for this transition and propose that, within available constraints, a restructuring of marine ecosystems is the most parsimonious explanation for this shift. Using a global isotope mass balance approach, we show that a change in the organic Zn/C ratio is required to account for observed Zn isotope trends through time. Given the higher affinity of eukaryotes for Zn relative to prokaryotes, we suggest that a shift toward a more eukaryote-rich ecosystem could have provided a means of more efficiently sequestering organic-derived Zn. Despite the much earlier appearance of eukaryotes in the microfossil record (~1700 to 1600 million years ago), our data suggest a delayed rise to ecological prominence during the Neoproterozoic, consistent with the currently accepted organic biomarker records.