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1. Probing the Bioavailability of Dissolved Iron to Marine Eukaryotic Phytoplankton Using In Situ Single Cell Iron Quotas

   https://doi.org/10.1029/2021GB006979  

   Shaked, Yeala ; Twining, Benjamin S. ; Tagliabue, Alessandro ; Maldonado, Maria T. ( August 2021 , Global Biogeochemical Cycles)

   We present a new approach for quantifying the bioavailability of dissolved iron (dFe) to oceanic phytoplankton. Bioavailability is defined using an uptake rate constant (k_in‐app) computed by combining data on: (a) Fe content of individual in situ phytoplankton cells; (b) concurrently determined seawater dFe concentrations; and (c) growth rates estimated from the PISCES model. We examined 930 phytoplankton cells, collected between 2002 and 2016 from 45 surface stations during 11 research cruises. This approach is only valid for cells that have upregulated their high‐affinity Fe uptake system, so data were screened, yielding 560 single cellk_in‐appvalues from 31 low‐Fe stations. We normalizedk_in‐appto cell surface area (S.A.) to account for cell‐size differences.

   The resulting bioavailability proxy (k_in‐app/S.A.) varies among cells, but all values are within bioavailability limits predicted from defined Fe complexes. In situ dFe bioavailability is higher than model Fe‐siderophore complexes and often approaches that of highly available inorganic Fe′. Station averagedk_in‐app/S.A. are also variable but show no systematic changes across location, temperature, dFe, and phytoplankton taxa. Given the relative consistency ofk_in‐app/S.A. among stations (ca. five‐fold variation), we computed a grand‐averaged dFe availability, which upon normalization to cell carbon (C) yieldsk_in‐app/C of 42,200 ± 11,000 L mol C⁻¹ d⁻¹. We utilizek_in‐app/C to calculate dFe uptake rates and residence times in low Fe oceanic regions. Finally, we demonstrate the applicability ofk_in‐app/C for constraining Fe uptake rates in earth system models, such as those predicting climate mediated changes in net primary production in the Fe‐limited Equatorial Pacific.

    

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2. Seasonal metabolism and carbon export potential of a key coastal habitat: The perennial canopy‐forming macroalga Fucus vesiculosus

   https://doi.org/10.1002/lno.11026  

   Attard, K. M. ; Rodil, I. F. ; Berg, P. ; Norkko, J. ; Norkko, A. ; Glud, R. N. ( October 2018 , Limnology and Oceanography)

   The important role of macroalgal canopies in the oceanic carbon (C) cycle is increasingly being recognized, but direct assessments of community productivity remain scarce. We conducted a seasonal study on a sublittoral Baltic Sea canopy of the brown algaFucus vesiculosus, a prominent species in temperate and Arctic waters. We investigated community production on hourly, daily, and seasonal timescales. Aquatic eddy covariance (AEC) oxygen flux measurements integrated ~ 40 m²of the seabed surface area and documented considerable oxygen production by the canopy year‐round. High net oxygen production rates of up to 35 ± 9 mmol m⁻²h⁻¹were measured under peak irradiance of ~ 1200 μmol photosynthetically active radiation (PAR) m⁻²s⁻¹in summer. However, high rates > 15 mmol m⁻²h⁻¹were also measured in late winter (March) under low light intensities < 250 μmol PAR m⁻²s⁻¹and water temperatures of ~ 1°C. In some cases, hourly AEC fluxes documented an apparent release of oxygen by the canopy under dark conditions, which may be due to gas storage dynamics within internal air spaces ofF. vesiculosus.Daily net ecosystem metabolism (NEM) was positive (net autotrophic) in all but one of the five measurement campaigns (December). A simple regression model predicted a net autotrophic canopy for two‐thirds of the year, and annual canopyNEMamounted to 25 mol O₂m⁻²yr⁻¹, approximately six‐fold higher than net phytoplankton production. Canopy C export was ~ 0.3 kg C m⁻²yr⁻¹, comparable to canopy standing biomass in summer. Macroalgal canopies thus represent regions of intensified C assimilation and export in coastal waters.

    

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3. Light, ammonium, pH , and phytoplankton competition as environmental factors controlling nitrification

   https://doi.org/10.1002/lno.12359  

   Proctor, Claudette ; Coupel, Pierre ; Casciotti, Karen ; Tremblay, Jean‐Eric ; Zakem, Emily ; Arrigo, Kevin R. ; Mills, Matthew M. ( May 2023 , Limnology and Oceanography)

   The biogeochemical cycling of nitrogen (N) plays a critical role in supporting marine ecosystems and controlling primary production. Nitrification, the oxidation of ammonia (NH₃) by microorganisms, is an important process in the marine N cycle, supplying nitrate (), the primary source of N that fuels new phytoplankton growth, and the primary substrate for the microbial process of denitrification. Understanding nitrification in the Chukchi Sea, the shallow sea overlying the continental shelf north of Alaska and the Bering Strait, is particularly important as phytoplankton growth there has been shown to be limited by N. However, the controls on nitrification in the water column and potential effects of climate change remain unknown. This study seeks to characterize the controls on nitrification in the Chukchi Sea. We found light to be a strong control on nitrification rates. Nitrification was undetectable at light levels above 23 μmol photons m⁻² s⁻¹. Subsequently, sea ice concentration was related to nitrification, with rates being higher at stations with high ice cover where light transmission to the water column was reduced. High ammonium () concentrations also enhanced nitrification, suggesting that nitrifying organisms were substrate‐limited, likely due to competition for from phytoplankton. Unlike previous experimental studies, we found that nitrification rates were higher under low pH conditions. As the effects of ocean acidification and warming disproportionately impact the Arctic, nitrification rates will undoubtedly be affected. Our results will help guide future studies on potential implications of climate change on the biogeochemistry of N in the Chukchi Sea.

    

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4. Standing Crop, Turnover, and Production Dynamics of Macrocystis pyrifera and Understory Species Hedophyllum nigripes and Neoagarum fimbriatum in High Latitude Giant Kelp Forests

   https://doi.org/10.1111/jpy.13291  

   Bell, Lauren E. ; Kroeker, Kristy J. ( November 2022 , Journal of Phycology)

   Production rates reported for canopy‐forming kelps have highlighted the potential contributions of these foundational macroalgal species to carbon cycling and sequestration on a globally relevant scale. Yet, the production dynamics of many kelp species remain poorly resolved. For example, productivity estimates for the widely distributed giant kelpMacrocystis pyriferaare based on a few studies from the center of this species' range. To address this geospatial bias, we surveyed giant kelp beds in their high latitude fringe habitat in southeast Alaska to quantify foliar standing crop, growth and loss rates, and productivity ofM. pyriferaand co‐occurring understory kelpsHedophyllum nigripesandNeoagarum fimbriatum. We found that giant kelp beds at the poleward edge of their range produce ~150 g C · m⁻²· year⁻¹from a standing biomass that turns over an estimated 2.1 times per year, substantially lower rates than have been observed at lower latitudes. Although the productivity of high latitudeM. pyriferadwarfs production by associated understory kelps in both winter and summer seasons, phenological differences in growth and relative carbon and nitrogen content among the three kelp species suggests their complementary value as nutritional resources to consumers. This work represents the highest latitude consideration ofM. pyriferaforest production to date, providing a valuable quantification of kelp carbon cycling in this highly seasonal environment.

    

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5. Comparing DNA, RNA and protein levels for measuring microbial dynamics in soil microcosms amended with nitrogen fertilizer

   https://doi.org/10.1038/s41598-019-53679-0  

   Orellana, Luis H. ; Hatt, Janet K. ; Iyer, Ramsunder ; Chourey, Karuna ; Hettich, Robert L. ; Spain, Jim C. ; Yang, Wendy H. ; Chee-Sanford, Joanne C. ; Sanford, Robert A. ; Löffler, Frank E. ; et al ( November 2019 , Scientific Reports)

   To what extent multi-omic techniques could reflectin situmicrobial process rates remains unclear, especially for highly diverse habitats like soils. Here, we performed microcosm incubations using sandy soil from an agricultural site in Midwest USA. Microcosms amended with isotopically labeled ammonium and urea to simulate a fertilization event showed nitrification (up to 4.1 ± 0.87 µg N-NO₃⁻g⁻¹dry soil d⁻¹) and accumulation of N₂O after 192 hours of incubation. Nitrification activity (NH₄⁺ → NH₂OH → NO → NO₂^- → NO₃⁻) was accompanied by a 6-fold increase in relative expression of the 16S rRNA gene (RNA/DNA) between 10 and 192 hours of incubation for ammonia-oxidizing bacteriaNitrosomonasandNitrosospira, unlike archaea and comammox bacteria, which showed stable gene expression. A strong relationship between nitrification activity and betaproteobacterial ammonia monooxygenase and nitrite oxidoreductase transcript abundances revealed that mRNA quantitatively reflected measured activity and was generally more sensitive than DNA under these conditions. Although peptides related to housekeeping proteins from nitrite-oxidizing microorganisms were detected, their abundance was not significantly correlated with activity, revealing that meta-proteomics provided only a qualitative assessment of activity. Altogether, these findings underscore the strengths and limitations of multi-omic approaches for assessing diverse microbial communities in soils and provide new insights into nitrification.

    

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