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1. Assessment of C, N, and Si Isotopes as Tracers of Past Ocean Nutrient and Carbon Cycling

   https://doi.org/10.1029/2020GB006775  

   Farmer, J. R. ; Hertzberg, J. E. ; Cardinal, D. ; Fietz, S. ; Hendry, K. ; Jaccard, S. L. ; Paytan, A. ; Rafter, P. A. ; Ren, H. ; Somes, C. J. ; et al ( July 2021 , Global Biogeochemical Cycles)

   Biological productivity in the ocean directly influences the partitioning of carbon between the atmosphere and ocean interior. Through this carbon cycle feedback, changing ocean productivity has long been hypothesized as a key pathway for modulating past atmospheric carbon dioxide levels and hence global climate. Because phytoplankton preferentially assimilate the light isotopes of carbon and the major nutrients nitrate and silicic acid, stable isotopes of carbon (C), nitrogen (N), and silicon (Si) in seawater and marine sediments can inform on ocean carbon and nutrient cycling, and by extension the relationship with biological productivity and global climate. Here, we compile water column C, N, and Si stable isotopes from GEOTRACES‐era data in four key ocean regions to review geochemical proxies of oceanic carbon and nutrient cycling based on the C, N, and Si isotopic composition of marine sediments. External sources and sinks as well as internal cycling (including assimilation, particulate matter export, and regeneration) are discussed as likely drivers of observed C, N, and Si isotope distributions in the ocean. The potential for C, N, and Si isotope measurements in sedimentary archives to record aspects of past ocean C and nutrient cycling is evaluated, along with key uncertainties and limitations associated with each proxy. Constraints on ocean C and nutrient cycling during late Quaternary glacial‐interglacial cycles and over the Cenozoic are examined. This review highlights opportunities for future research using multielement stable isotope proxy applications and emphasizes the importance of such applications to reconstructing past changes in the oceans and climate system.

    

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2. Surface Ocean Biogeochemistry Regulates the Impact of Anthropogenic Aerosol Fe Deposition on the Cycling of Iron and Iron Isotopes in the North Pacific

   https://doi.org/10.1029/2022GL098016  

   König, D. ; Conway, T. M. ; Hamilton, D. S. ; Tagliabue, A. ( July 2022 , Geophysical Research Letters)

   Distinctively‐light isotopic signatures associated with Fe released from anthropogenic activity have been used to trace basin‐scale impacts. However, this approach is complicated by the way Fe cycle processes modulate oceanic dissolved Fe (dFe) signatures (δ⁵⁶Fe_diss) post deposition. Here we include dust, wildfire, and anthropogenic aerosol Fe deposition in a global ocean biogeochemical model with active Fe isotope cycling, to quantify how anthropogenic Fe impacts surface ocean dFe and δ⁵⁶Fe_diss. Using the North Pacific as a natural laboratory, the response of dFe, δ⁵⁶Fe_diss, and primary productivity are spatially and seasonally variable and do not simply follow the footprint of atmospheric deposition. Instead, the effect of anthropogenic Fe is regulated by the biogeochemical regime, specifically the degree of Fe limitation and rates of primary production. Overall, we find that while δ⁵⁶Fe_dissdoes trace anthropogenic input, the response is muted by fractionation during phytoplankton uptake, but amplified by other isotopically‐light Fe sources.

    

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3. Ocean Dust Deposition Rates Constrained in a Data‐Assimilation Model of the Marine Aluminum Cycle

   https://doi.org/10.1029/2021GB007049  

   Xu, Hairong ; Weber, Thomas ( September 2021 , Global Biogeochemical Cycles)

   Aluminum (Al) is delivered to surface ocean waters by aeolian dust, making it a promising tracer to constrain dust deposition rates and the atmospheric supply of trace metal micronutrients. Over recent years, dissolved Al has been mapped along the GEOTRACES transects, providing unparalleled coverage of the world ocean. However, inferring atmospheric input rates from these observations is complicated by a suite of additional processes that influence the Al distribution, including reversible particle scavenging, biological uptake by diatoms, hydrothermal sources, sediment resuspension. Here we employ a data‐assimilation model of the oceanic Al cycle that explicitly accounts for these processes, allowing the atmospheric signal to be extracted. We conduct an ensemble of model optimizations that test different dust deposition distributions and consider spatial variations in Al solubility, thereby inferring the atmospheric Al supply that is most consistent with GEOTRACES observations. We find that 37.2 ± 11.0 Gmol/yr of soluble Al is added to the global ocean, dominated in the Atlantic Ocean, and that Al fractional solubility varies strongly as a function of atmospheric dust concentration. Our model also suggests that 6.1 ± 2.4 Gmol Al/yr is injected from hydrothermal vents, and that vertical Al redistribution through the water column is dominated by abiotic reversible scavenging rather than uptake by diatoms. Our results have important implications for the oceanic iron (Fe) budget: based on the soluble Fe:Al ratio of dust, we infer that aeolian Fe inputs lie between 3.82 and 9.25 Gmol/yr globally, and fall short of the biological Fe demand in most ocean regions.

    

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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)

   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. Defining the Realized Niche of the Two Major Clades of Trichodesmium: A Study on the West Florida Shelf

   https://doi.org/10.3389/fmars.2022.821655  

   Confesor, Kristina A. ; Selden, Corday R. ; Powell, Kimberly E. ; Donahue, Laura A. ; Mellett, Travis ; Caprara, Salvatore ; Knapp, Angela N. ; Buck, Kristen N. ; Chappell, P. Dreux ( June 2022 , Frontiers in Marine Science)

   The cyanobacterium  Trichodesmium  plays an essential role supporting ocean productivity by relieving nitrogen limitation via dinitrogen (N 2 ) fixation. The two common Trichodesmium clades,  T. erythraeum  and  T. thiebautii , are both observed in waters along the West Florida Shelf (WFS). We hypothesized that these taxa occupy distinct realized niches, where  T. thiebautii  is the more oceanic clade. Samples for DNA and water chemistry analyses were collected on three separate WFS expeditions (2015, 2018, and 2019) spanning multiple seasons; abundances of the single copy housekeeping gene  rnpB  from both clades were enumerated via quantitative PCR. We conducted a suite of statistical analyses to assess Trichodesmium  clade abundances in the context of the physicochemical data. We observed a consistent coastal vs. open ocean separation of the two clades:  T. erythraeum  was found in shallow waters where the concentrations of dissolved iron (dFe) and the groundwater tracer Ba were significantly higher, while  T. thiebautii  abundance was positively correlated with water column depth. The Loop Current intrusion in 2015 with entrained Missisippi River water brought higher dFe and elevated abundance of both clades offshore of the 50 m isobath, suggesting that both clades are subject to Fe limitation on the outer shelf. Whereas, previous work has observed that  T. thiebautii  is more abundant than  T. erythraeum  in open ocean surface waters, this is the first study to examine  Trichodesmium  niche differentiation in a coastal environment. Understanding the environmental niches of these two key taxa bears important implications for their contributions to global nitrogen and carbon cycling and their response to global climate change. 

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