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1. 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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2. 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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3. Inorganic and organic carbon and nitrogen uptake strategies of picoplankton groups in the northwestern Atlantic Ocean

   https://doi.org/10.1002/lno.11909  

   Berthelot, Hugo ; Duhamel, Solange ; L'Helguen, Stéphane ; Maguer, Jean‐François ; Cassar, Nicolas ( August 2021 , Limnology and Oceanography)

   Picoplankton populations dominate the planktonic community in the surface oligotrophic ocean. Yet, their strategies in the acquisition and the partitioning of organic and inorganic sources of nitrogen (N) and carbon (C) are poorly described. Here, we measured at the single‐cell level the uptake of dissolved inorganic C (C‐fixation), C‐leucine, N‐leucine, nitrate (NO₃⁻), ammonium (NH₄⁺), and N‐urea in pigmented and nonpigmented picoplankton groups at six low‐N stations in the northwestern Atlantic Ocean. Our study highlights important differences in trophic strategies betweenProchlorococcus,Synechococcus, photosynthetic pico‐eukaryotes, and nonpigmented prokaryotes. Nonpigmented prokaryotes were characterized by high leucine uptake rates, nonsignificant C‐fixation and relatively low NH₄⁺, N‐urea, and NO₃⁻uptake rates. Nonpigmented prokaryotes contributed to 7% ± 3%, 2% ± 2%, and 9% ± 5% of the NH₄⁺, NO₃⁻, and N‐urea community uptake, respectively. In contrast, pigmented groups displayed relatively high C‐fixation rates, NH₄⁺and N‐urea uptake rates, but lower leucine uptake rates than nonpigmented prokaryotes.Synechococcusand photosynthetic pico‐eukaryotes NO₃⁻uptake rates were higher thanProchlorococcusones. Pico‐sized pigmented groups accounted for a significant fraction of the community C‐fixation (63% ± 27%), NH₄⁺uptake (47% ± 27%), NO₃⁻uptake (62% ± 49%), and N‐urea uptake (81% ± 35%). Interestingly,Prochlorococcusand photosynthetic pico‐eukaryotes showed a greater reliance on C‐ and N‐leucine thanSynechococcuson average, suggesting a greater reliance on organic C and N sources. Taken together, our single‐cell results decipher the wide diversity of C and N trophic strategies between and within marine picoplankton groups, but a clear partitioning between pigmented and nonpigmented groups still remains.

    

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4. Heterotrophic nitrogen fixation in response to nitrate loading and sediment organic matter in an emerging coastal deltaic floodplain within the Mississippi River Delta plain

   https://doi.org/10.1002/lno.11737  

   Li, Song ; Twilley, Robert R. ; Hou, Aixin ( April 2021 , Limnology and Oceanography)

   Increasing nitrate (NO₃⁻) loading in rivers due to agricultural fertilization alters benthic nitrogen (N) cycling and shifts coastal wetlands from being a net source to net sink of reactive N. Heterotrophic N₂fixation that converts N₂to reactive N is often assumed negligible in eutrophic ecosystems and excluded in coastal N budget evaluations. We investigated N₂fixation and denitrification in response to increasing NO₃⁻loading (0, 10, and 100 μM) and sediment organic matter (OM_sediment) concentrations in the emerging Wax Lake Delta. Continuous flow‐through incubations with³⁰N₂addition was applied to measure N₂fixation. The variation of N₂fixation rates from 0 to 437 μmol N m⁻²h⁻¹among different NO₃⁻and OM_sedimentconcentrations were comparable to the estimated denitrification rates of 141–377 μmol N m⁻²h⁻¹. Increasing overlying NO₃⁻concentrations reduced N₂fixation rates and facilitated denitrification rates at each OM_sedimentconcentration. However, 100 μM of overlying NO₃⁻did not thoroughly inhibit N₂fixation rates in sites with intermediate and higher OM_sedimentconcentrations (189 and 99 μmol N m⁻²h⁻¹, respectively). Both N₂fixation and denitrification increased with increasing OM_sedimentconcentrations, but the relative importance of these processes was impacted mostly by overlying NO₃⁻concentration as increasing NO₃⁻switched the dominance of N₂fixation to denitrification in benthic N cycling. This study highlights the importance of heterotrophic N₂fixation in coastal deltaic floodplains and emphasizes the necessity of including N₂fixation quantification in coastal N budget evaluation, not only in oligotrophic environment but also in eutrophic 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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