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"Addison, J."
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North Pacific deglacial hypoxic events linked to abrupt ocean warming
The processes responsible for driving the expansion of the ocean's oxygen minimum zones remain uncertain; here sediment core data from the Gulf of Alaska suggest that reduced oxygen solubility was a result of ocean warming initiating the expansion of the North Pacific oxygen minimum zone, leading to increased marine productivity and carbon export and, in turn, further reductions in dissolved oxygen levels.
A thermal trigger for ocean hypoxia
The processes responsible for driving the expansion of the ocean's oxygen minimum zones — also known as hypoxic or 'dead' zones — remain uncertain. This paper describes two warming events during the last deglacial transition that coincide with a shift to hypoxia, based on sediment core data from the Gulf of Alaska. The data suggest that reduced oxygen solubility was a result of ocean warming initiating the expansion of the North Pacific oxygen minimum zone, leading to increased marine productivity and carbon export and, in turn, further reductions in dissolved oxygen levels.
Marine sediments from the North Pacific document two episodes of expansion and strengthening of the subsurface oxygen minimum zone (OMZ) accompanied by seafloor hypoxia during the last deglacial transition
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. We present a new high-resolution alkenone palaeotemperature reconstruction from the Gulf of Alaska that reveals two abrupt warming events of 4–5 degrees Celsius at the onset of the Bølling and Holocene intervals that coincide with sudden shifts to hypoxia at intermediate depths. The presence of diatomaceous laminations and hypoxia-tolerant benthic foraminiferal species, peaks in redox-sensitive trace metals
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, collectively suggest association with high export production. A decrease in
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O values of benthic foraminifera accompanying the most severe deoxygenation event indicates subsurface warming of up to about 2 degrees Celsius. We infer that abrupt warming triggered expansion of the North Pacific OMZ through reduced oxygen solubility and increased marine productivity via physiological effects; following initiation of hypoxia, remobilization of iron from hypoxic sediments could have provided a positive feedback on ocean deoxygenation through increased nutrient utilization and carbon export. Such a biogeochemical amplification process implies high sensitivity of OMZ expansion to warming.
Journal Article
Hierarchical spidroin micellar nanoparticles as the fundamental precursors of spider silks
Many natural silks produced by spiders and insects are unique materials in their exceptional toughness and tensile strength, while being lightweight and biodegradable–properties that are currently unparalleled in synthetic materials. Myriad approaches have been attempted to prepare artificial silks from recombinant spider silk spidroins but have each failed to achieve the advantageous properties of the natural material. This is because of an incomplete understanding of the in vivo spidroin-to-fiber spinning process and, particularly, because of a lack of knowledge of the true morphological nature of spidroin nanostructures in the precursor dope solution and the mechanisms by which these nanostructures transform into micrometer-scale silk fibers. Herein we determine the physical form of the natural spidroin precursor nanostructures stored within spider glands that seed the formation of their silks and reveal the fundamental structural transformations that occur during the initial stages of extrusion en route to fiber formation. Using a combination of solution phase diffusion NMR and cryogenic transmission electron microscopy (cryo-TEM), we reveal direct evidence that the concentrated spidroin proteins are stored in the silk glands of black widow spiders as complex, hierarchical nanoassemblies (∼300 nm diameter) that are composed of micellar subdomains, substructures that themselves are engaged in the initial nanoscale transformations that occur in response to shear. We find that the established micelle theory of silk fiber precursor storage is incomplete and that the first steps toward liquid crystalline organization during silk spinning involve the fibrillization of nanoscale hierarchical micelle subdomains.
Journal Article
Discovery, Biosynthesis and Stress-Related Accumulation of Dolabradiene-Derived Defenses in Maize
Terpenoids are a major component of maize (Zea mays) chemical defenses that mediate responses to herbivores, pathogens, and other environmental challenges. Here, we describe the biosynthesis and elicited production of a class of maize diterpenoids, named dolabralexins. Dolabralexin biosynthesis involves the sequential activity of two diterpene synthases, ENT-COPALYL DIPHOSPHATE SYNTHASE (ZmAN2) and KAURENE SYNTHASE-LIKE4 (ZmKSL4). Together, ZmAN2 and ZmKSL4 form the diterpene hydrocarbon dolabradiene. In addition, we biochemically characterized a cytochrome P450 monooxygenase, ZmCYP71Z16, which catalyzes the oxygenation of dolabradiene to yield the epoxides 15,16-epoxydolabrene (epoxydolabrene) and 3𝛽-hydroxy-15,16-epoxydolabrene (epoxydolabranol). The absence of dolabradiene and epoxydolabranol in Zman2 mutants under elicited conditions confirmed the in vivo biosynthetic requirement of ZmAN2. Combined mass spectrometry and NMR experiments demonstrated that much of the epoxydolabranol is further converted into 3𝛽,15,16-trihydroxydolabrene (trihydroxydolabrene). Metabolite profiling of field-grown maize root tissues indicated that dolabralexin biosynthesis is widespread across common maize cultivars, with trihydroxydolabrene as the predominant diterpenoid. Oxidative stress induced dolabralexin accumulation and transcript expression of ZmAN2 and ZmKSL4 in root tissues, and metabolite and transcript accumulation were up-regulated in response to elicitation with the fungal pathogens Fusarium verticillioides and Fusarium graminearum. Consistently, epoxydolabranol significantly inhibited the growth of both pathogens in vitro at 10 𝜇g mL⁻¹, while trihydroxydolabrene-mediated inhibition was specific to F. verticillioides. These findings suggest that dolabralexins have defense-related roles in maize stress interactions and expand the known chemical space of diterpenoid defenses as genetic targets for understanding and ultimately improving maize resilience.
Journal Article
Applying the social–ecological systems framework to the evaluation and design of payment for ecosystem service schemes in the Eurasian steppe
The application of payment for ecosystem services schemes to dryland areas has been limited, particularly for schemes that seek to improve the grassland upon which resource users in these landscapes depend. The high levels of climatic and resource variability, strength of informal institutions governing resource use and contested nature of resource decline are examples of defining characteristics that may challenge the application of more conventional payment for ecosystem services schemes in dryland contexts. We used a social–ecological systems framework to (i) help identify design criteria for effective and efficient payment for ecosystem services schemes in drylands under a pastoral land-use, and (ii) explore the applicability of the framework to dryland areas. Using eco-compensation schemes in the Chinese governed areas of the Eurasian steppe as a case study, we found that the framework adequately identified the need for non-equilibrium dynamics to be incorporated into scheme design. However, the framework was less able to explicitly enunciate the importance of micro-economics and cultural values for scheme viability. In contexts like the Eurasian steppe where some level of grazing may maximise the species richness of grasslands, acknowledging history of use in the resource unit subsystem component of the framework, not just the user subsystem component, would improve the framework. The explicit incorporation of contested issues into the framework is also needed, as dryland areas have a history of being misunderstood by non-dryland cultures, researchers and policymakers. We conclude that tailoring a general diagnostic tool towards the specific social–ecological attributes of the drylands under a pastoral land-use will improve the ability of payment for ecosystem services schemes to reach their conservation aims.
Journal Article
Bølling‐Allerød Productivity in the Subarctic Pacific Driven by Seasonal Upwelling
The Bølling‐Allerød deglacial event is marked by high diatom productivity and opal deposition throughout the subarctic Pacific. This opal could either constitute a strengthened biological pump and thus carbon sequestration, or a weakened biological pump and release of marine‐sequestered CO2 to the atmosphere. We quantify silicic acid supply at IODP Site U1340 in the Bering Sea using biogenic opal and δ30Si of Coscinodiscus, a diatom genus. These records, along with diatom environmental indicators, suggest the Bølling‐Allerød had high silicic acid availability related to a shift from stratification to seasonal upwelling dynamics. We thus propose the primary cause of the high productivity event was increased macronutrient supply from vertical exchange that injected old, nutrient‐rich, CO2‐rich waters into the surface. Enhanced CO2 release from the subarctic Pacific may help explain critical intervals of CO2 rise that occur at the onsets of the Bølling‐Allerød and PreBoreal. Plain Language Summary The subarctic Pacific experienced a period of remarkably high primary productivity from 14.7 to 12.9 thousand years ago. The growth and burial of diatoms, single‐celled algae with opal cell walls, deposited an opal‐rich layer in marine sediments across the entire region, including our site in the southern Bering Sea. To determine the cause of this productivity, we analyzed diatom opal and the diatom species present. The silicon isotopic composition of the opal suggests surface nutrients were more abundant during the high‐productivity event. This points to an increased connection between surface waters, where algae grow, and deep waters, which are rich in nutrients. The species present suggest deep mixing occurred each winter followed by a large spring bloom, indicating a different mode of oceanic circulation than the modern ocean. Summer production may have been limited by the rate of iron delivery, similar to the modern ocean. The combined effect of increased mixing and iron limitation is that, despite high productivity which sequestered some carbon, these events represent times when the subarctic Pacific was a net source of carbon to the atmosphere. The carbon released from the subarctic Pacific occurred during critical intervals of global change which ended the last glacial age. Key Points Diatom silicon isotopes suggest increased silicic acid availability during Bølling‐Allerød productivity in subarctic Pacific Diatom indicators suggest increased upwelling and iron limitation during high productivity events in the Subarctic Pacific Vertical mixing during the Bølling‐Allerød made the subarctic Pacific a source of atmospheric CO2 which contributed to global deglaciation
Journal Article
Can auxiliary indicators improve COVID-19 forecasting and hotspot prediction?
Short-term forecasts of traditional streams from public health reporting (such as cases, hospitalizations, and deaths) are a key input to public health decision-making during a pandemic. Since early 2020, our research group has worked with data partners to collect, curate, and make publicly available numerous real-time COVID-19 indicators, providing multiple views of pandemic activity in the United States. This paper studies the utility of five such indicators—derived from deidentified medical insurance claims, self-reported symptoms from online surveys, and COVID-related Google search activity—from a forecasting perspective. For each indicator, we ask whether its inclusion in an autoregressive (AR) model leads to improved predictive accuracy relative to the same model excluding it. Such an AR model, without external features, is already competitive with many top COVID-19 forecasting models in use today. Our analysis reveals that 1) inclusion of each of these five indicators improves on the overall predictive accuracy of the AR model; 2) predictive gains are in general most pronounced during times in which COVID cases are trending in “flat” or “down” directions; and 3) one indicator, based on Google searches, seems to be particularly helpful during “up” trends.
Journal Article