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1. How well do global ocean biogeochemistry models simulate dissolved iron distributions?

   https://doi.org/10.1002/2015gb005289  

   Tagliabue, Alessandro; Aumont, Olivier; DeAth, Ros; Dunne, John P; Dutkiewicz, Stephanie; Galbraith, Eric; Misumi, Kazuhiro; Moore, J Keith; Ridgwell, Andy; Sherman, Elliot; et al (February 2016, Global Biogeochemical Cycles)

   Abstract Numerical models of ocean biogeochemistry are relied upon to make projections about the impact of climate change on marine resources and test hypotheses regarding the drivers of past changes in climate and ecosystems. In large areas of the ocean, iron availability regulates the functioning of marine ecosystems and hence the ocean carbon cycle. Accordingly, our ability to quantify the drivers and impacts of fluctuations in ocean ecosystems and carbon cycling in space and time relies on first achieving an appropriate representation of the modern marine iron cycle in models. When the iron distributions from 13 global ocean biogeochemistry models are compared against the latest oceanic sections from the GEOTRACES program, we find that all models struggle to reproduce many aspects of the observed spatial patterns. Models that reflect the emerging evidence for multiple iron sources or subtleties of its internal cycling perform much better in capturing observed features than their simpler contemporaries, particularly in the ocean interior. We show that the substantial uncertainty in the input fluxes of iron results in a very wide range of residence times across models, which has implications for the response of ecosystems and global carbon cycling to perturbations. Given this large uncertainty, iron fertilization experiments based on any single current generation model should be interpreted with caution. Improvements to how such models represent iron scavenging and also biological cycling are needed to raise confidence in their projections of global biogeochemical change in the ocean. 

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2. Novel Insights into Ocean Trace Element Cycling from Biogeochemical Models

   https://doi.org/10.5670/oceanog.2024.418  

   Tagliabue, Alessandro; Weber, Thomas (January 2024, Oceanography)

   Ocean biogeochemical models have become critical tools for interpreting trace element and isotope (TEI) distributions observed during the GEOTRACES program and understanding their driving processes. Models stimulate new research questions that cannot be addressed with observations alone, for instance, concerning processes that occur over vast spatial scales and linkages between TEIs and other elemental cycles. A spectrum of modeling approaches has been applied to date, including (1) fully prognostic models that couple TEIs to broader biogeochemical frameworks, (2) simpler element-specific mechanistic models that allow for assimilation of observations, and (3) machine learning models that have no mechanistic underpinning but allow for skillful extrapolation of sparse data. Here, we evaluate the strengths and weaknesses of these approaches and review three sets of novel insights they have facilitated. First, models have advanced our understanding of global-scale micronutrient distributions, and their deviations from macronutrients, in terms of a “ventilation-regeneration-scavenging” balance. Second, models have yielded global-scale estimates of TEI inputs to and losses from the ocean, revealing, for instance, a rapid iron (Fe) cycle with an oceanic residence time on the order of decades. Third, models have identified novel links among various TEI cycling processes and the global ocean carbon cycle, such as tracing the supply of hydrothermally sourced Fe to iron-starved microbial communities in the Southern Ocean. We foresee additional important roles for modeling work in the next stages of trace element research, including synthesizing understanding from the GEOTRACES program in the form of TEI state estimates, and projecting the responses of TEI cycles to global climate change. 

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3. Shelf‐Basin Interactions and Water Mass Residence Times in the Western Arctic Ocean: Insights Provided by Radium Isotopes

   https://doi.org/10.1029/2019JC014988  

   Kipp, Lauren E.; Kadko, David C.; Pickart, Robert S.; Henderson, Paul B.; Moore, Willard S.; Charette, Matthew A. (May 2019, Journal of Geophysical Research: Oceans)

   Abstract Radium isotopes are produced through the decay of thorium in sediments and are soluble in seawater; thus, they are useful for tracing ocean boundary‐derived inputs to the ocean. Here we apply radium isotopes to study continental inputs and water residence times in the Arctic Ocean, where land‐ocean interactions are currently changing in response to rising air and sea temperatures. We present the distributions of radium isotopes measured on the 2015 U.S. GEOTRACES transect in the Western Arctic Ocean and combine this data set with historical radium observations in the Chukchi Sea and Canada Basin. The highest activities of radium‐228 were observed in the Transpolar Drift and the Chukchi shelfbreak jet, signaling that these currents are heavily influenced by interactions with shelf sediments. The ventilation of the halocline with respect to inputs from the Chukchi shelf occurs on time scales of ≤19–23 years. Intermediate water ventilation time scales for the Makarov and Canada Basins were determined to be ~20 and >30 years, respectively, while deep water residence times in these basins were on the order of centuries. The radium distributions and residence times described in this study serve as a baseline for future studies investigating the impacts of climate change on the Arctic Ocean. 

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4. Petrobactin, a siderophore produced by Alteromonas , mediates community iron acquisition in the global ocean

   https://doi.org/10.1038/s41396-021-01065-y  

   Manck, Lauren E.; Park, Jiwoon; Tully, Benjamin J.; Poire, Alfonso M.; Bundy, Randelle M.; Dupont, Christopher L.; Barbeau, Katherine A. (August 2021, The ISME Journal)

   Abstract It is now widely accepted that siderophores play a role in marine iron biogeochemical cycling. However, the mechanisms by which siderophores affect the availability of iron from specific sources and the resulting significance of these processes on iron biogeochemical cycling as a whole have remained largely untested. In this study, we develop a model system for testing the effects of siderophore production on iron bioavailability using the marine copiotroph Alteromonas macleodii ATCC 27126. Through the generation of the knockout cell line ΔasbB::kmr, which lacks siderophore biosynthetic capabilities, we demonstrate that the production of the siderophore petrobactin enables the acquisition of iron from mineral sources and weaker iron-ligand complexes. Notably, the utilization of lithogenic iron, such as that from atmospheric dust, indicates a significant role for siderophores in the incorporation of new iron into marine systems. We have also detected petrobactin, a photoreactive siderophore, directly from seawater in the mid-latitudes of the North Pacific and have identified the biosynthetic pathway for petrobactin in bacterial metagenome-assembled genomes widely distributed across the global ocean. Together, these results improve our mechanistic understanding of the role of siderophore production in iron biogeochemical cycling in the marine environment wherein iron speciation, bioavailability, and residence time can be directly influenced by microbial activities. 

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5. A Global Perspective on Iron and Plankton Through the Tara Oceans Lens

   https://doi.org/10.1029/2019GB006181  

   Marchetti, Adrian (March 2019, Global Biogeochemical Cycles)

   Abstract The Tara Oceans program has delivered major advances in our knowledge of ocean plankton diversity and complexity, shedding light on key interactions that explain their success on a planetary scale. In this issue, Caputi et al. (2019,https://doi.org/10.1029/2018GB006022) further contribute to this knowledge through combining comprehensive bio‐oceanographic genomic and transcriptomic Tara Oceans data sets with iron distributions derived from two global‐scale biogeochemical models. Their findings reveal the prevalence of iron as a limiting nutrient in pelagic ecosystems at both local and global scales, exerting a considerable force that drives plankton evolution and shapes community structure. Integration of omics data (i.e., genomics, transcriptomics, proteomics, and metabolomics) with oceanographic properties and biogeochemical models will transform our view of the ocean ecosystem and its role on a changing planet. 

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