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1. Advances in Understanding the Marine Nitrogen Cycle in the GEOTRACES Era

   https://doi.org/10.5670/oceanog.2024.406  

   Casciotti, Karen; Marshall, Tanya; Fawcett, Sarah; Knapp, Angela (May 2024, Oceanography)

   Conway, T.; Fitzsimmons, J.; Middag, R; Noble, T.; Planquette, H. (Ed.)

   Because nitrogen availability limits primary production over much of the global ocean, understanding the controls on the marine nitrogen inventory and supply to the surface ocean is essential for understanding biological productivity and exchange of greenhouse gases with the atmosphere. Quantifying the ocean’s inputs, outputs, and internal cycling of nitrogen requires a variety of tools and approaches, including measurements of the nitrogen isotope ratio in organic and inorganic nitrogen species. The marine nitrogen cycle, which shapes nitrogen availability and speciation in the ocean, is linked to the elemental cycles of carbon, phosphorus, and trace elements. For example, the majority of nitrogen cycle oxidation and reduction reactions are mediated by enzymes that require trace metals for catalysis. Recent observations made through global-scale programs such as GEOTRACES have greatly expanded our knowledge of the marine nitrogen cycle. Though much work remains to be done, here we outline key advances in understanding the marine nitrogen cycle that have been achieved through these analyses, such as the distributions and rates of dinitrogen fixation, terrestrial nitrogen inputs, and nitrogen loss processes. 

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2. Fundamentally different global marine nitrogen cycling in response to severe ocean deoxygenation

   https://doi.org/10.1073/pnas.1905553116  

   Naafs, B. David; Monteiro, Fanny M.; Pearson, Ann; Higgins, Meytal B.; Pancost, Richard D.; Ridgwell, Andy (December 2019, Proceedings of the National Academy of Sciences)

   null (Ed.)

   The present-day marine nitrogen (N) cycle is strongly regulated by biology. Deficiencies in the availability of fixed and readily bioavailable nitrogen relative to phosphate (P) in the surface ocean are largely corrected by the activity of diazotrophs. This feedback system, termed the “nitrostat,” is thought to have provided close regulation of fixed-N speciation and inventory relative to P since the Proterozoic. In contrast, during intervals of intense deoxygenation such as Cretaceous ocean anoxic event (OAE) 2, a few regional sedimentary δ 15 N records hint at the existence of a different mode of marine N cycling in which ammonium plays a major role in regulating export production. However, the global-scale dynamics during this time remain unknown. Here, using an Earth System model and taking the example of OAE 2, we provide insights into the global marine nitrogen cycle under severe ocean deoxygenation. Specifically, we find that the ocean can exhibit fundamental transitions in the species of nitrogen dominating the fixed-N inventory––from nitrate (NO 3 − ) to ammonium (NH 4 + )––and that as this transition occurs, the inventory can partially collapse relative to P due to progressive spatial decoupling between the loci of NH 4 + oxidation, NO 3 − reduction, and nitrogen fixation. This finding is relatively independent of the specific state of ocean circulation and is consistent with nitrogen isotope and redox proxy data. The substantive reduction in the ocean fixed-N inventory at an intermediate state of deoxygenation may represent a biogeochemical vulnerability with potential implications for past and future (warmer) oceans. 

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3. First Record of Fungal Diversity in the Tropical and Warm-Temperate Middle Miocene Climate Optimum Forests of Eurasia

   https://doi.org/10.3389/ffgc.2021.768405  

   Romero, Ingrid C.; Nuñez Otaño, Noelia B.; Gibson, Martha E.; Spears, Tyler M.; Fairchild, C. Jolene; Tarlton, Laikin; Jones, Savannah; Belkin, Harvey E.; Warny, Sophie; Pound, Matthew J.; et al (December 2021, Frontiers in Forests and Global Change)

   The middle Miocene Climate Optimum (MMCO) was the warmest interval of the last 23 million years and is one of the best analogs for proposed future climate change scenarios. Fungi play a key role in the terrestrial carbon cycle as dominant decomposers of plant debris, and through their interactions with plants and other organisms as symbionts, parasites, and endobionts. Thus, their study in the fossil record, especially during the MMCO, is essential to better understand biodiversity changes and terrestrial carbon cycle dynamics in past analogous environments, as well as to model future ecological and climatic scenarios. The fossil record also offers a unique long-term, large-scale dataset to evaluate fungal assemblage dynamics across long temporal and spatial scales, providing a better understanding of how ecological factors influenced assemblage development through time. In this study, we assessed the fungal diversity and community composition recorded in two geological sections from the middle Miocene from the coal mines of Thailand and Slovakia. We used presence-absence data to quantify the fungal diversity of each locality. Spores and other fungal remains were identified to modern taxa whenever possible; laboratory codes and fossil names were used when this correlation was not possible. This study represents the first of its kind for Thailand, and it expands existing work from Slovakia. Our results indicate a total of 281 morphotaxa. This work will allow us to use modern ecological data to make inferences about ecosystem characteristics and community dynamics for the studied regions. It opens new horizons for the study of past fungal diversity based on modern fungal ecological analyses. It also sheds light on how global variations in fungal species richness and community composition were affected by different climatic conditions and under rapid increases of temperature in the past to make inferences for the near climatic future. 

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4. Acclimation of Phytoplankton Fe:C Ratios Dampens the Biogeochemical Response to Varying Atmospheric Deposition of Soluble Iron

   https://doi.org/10.1029/2022GB007491  

   Wiseman, N. A.; Moore, J. K.; Twining, B. S.; Hamilton, D. S.; Mahowald, N. M. (April 2023, Global Biogeochemical Cycles)

   Abstract Dissolved iron (dFe) plays an important role in regulating marine productivity. In high nutrient, low chlorophyll regions (>33% of the global ocean), iron is the primary growth limiting nutrient, and elsewhere iron can regulate nitrogen fixation by diazotrophs. The link between iron availability and carbon export is strongly dependent on the phytoplankton iron quotas or cellular Fe:C ratios. This ratio varies by more than an order of magnitude in the open ocean and is positively correlated with ambient dFe concentrations in field observations. Representing Fe:C ratios within models is necessary to investigate how ocean carbon cycling will interact with perturbations to iron cycling in a changing climate. The Community Earth System Model ocean component was modified to simulate dynamic, group‐specific, phytoplankton Fe:C that varies as a function of ambient iron concentration. The simulated Fe:C ratios improve the representation of the spatial trends in the observed Fe:C ratios. The acclimation of phytoplankton Fe:C ratios dampens the biogeochemical response to varying atmospheric deposition of soluble iron, compared to a fixed Fe:C ratio. However, varying atmospheric soluble iron supply has first order impacts on global carbon and nitrogen fluxes and on nutrient limitation spatial patterns. Our results suggest that pyrogenic Fe is a significant dFe source that rivals mineral dust inputs in some regions. Changes in dust flux and iron combustion sources (anthropogenic and wildfires) will modify atmospheric Fe inputs in the future. Accounting for dynamic phytoplankton iron quotas is critical for understanding ocean biogeochemistry and projecting its response to variations in atmospheric deposition. 

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5. Nutrient-Colimited Trichodesmium as a Nitrogen Source or Sink in a Future Ocean

   https://doi.org/10.1128/AEM.02137-17  

   Walworth, Nathan G.; Fu, Fei-Xue; Lee, Michael D.; Cai, Xiaoni; Saito, Mak A.; Webb, Eric A.; Hutchins, David A.; Liu, Shuang-Jiang (February 2018, Applied and Environmental Microbiology)

   ABSTRACT Nitrogen-fixing (N 2 ) cyanobacteria provide bioavailable nitrogen to vast ocean regions but are in turn limited by iron (Fe) and/or phosphorus (P), which may force them to employ alternative nitrogen acquisition strategies. The adaptive responses of nitrogen fixers to global-change drivers under nutrient-limited conditions could profoundly alter the current ocean nitrogen and carbon cycles. Here, we show that the globally important N 2 fixer Trichodesmium fundamentally shifts nitrogen metabolism toward organic-nitrogen scavenging following long-term high-CO 2 adaptation under iron and/or phosphorus (co)limitation. Global shifts in transcripts and proteins under high-CO 2 /Fe-limited and/or P-limited conditions include decreases in the N 2 -fixing nitrogenase enzyme, coupled with major increases in enzymes that oxidize trimethylamine (TMA). TMA is an abundant, biogeochemically important organic nitrogen compound that supports rapid Trichodesmium growth while inhibiting N 2 fixation. In a future high-CO 2 ocean, this whole-cell energetic reallocation toward organic nitrogen scavenging and away from N 2 fixation may reduce new-nitrogen inputs by Trichodesmium while simultaneously depleting the scarce fixed-nitrogen supplies of nitrogen-limited open-ocean ecosystems. IMPORTANCE Trichodesmium is among the most biogeochemically significant microorganisms in the ocean, since it supplies up to 50% of the new nitrogen supporting open-ocean food webs. We used Trichodesmium cultures adapted to high-CO 2 conditions for 7 years, followed by additional exposure to iron and/or phosphorus (co)limitation. We show that “future ocean” conditions of high CO 2 and concurrent nutrient limitation(s) fundamentally shift nitrogen metabolism away from nitrogen fixation and instead toward upregulation of organic nitrogen-scavenging pathways. We show that the responses of Trichodesmium to projected future ocean conditions include decreases in the nitrogen-fixing nitrogenase enzymes coupled with major increases in enzymes that oxidize the abundant organic nitrogen source trimethylamine (TMA). Such a shift toward organic nitrogen uptake and away from nitrogen fixation may substantially reduce new-nitrogen inputs by Trichodesmium to the rest of the microbial community in the future high-CO 2 ocean, with potential global implications for ocean carbon and nitrogen cycling. 

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