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Abstract. Marine diazotrophs convert dinitrogen (N2) gas intobioavailable nitrogen (N), supporting life in the global ocean. In 2012, thefirst version of the global oceanic diazotroph database (version 1) waspublished. Here, we present an updated version of the database (version 2),significantly increasing the number of in situ diazotrophic measurements from13 565 to 55 286. Data points for N2 fixation rates, diazotrophic cellabundance, and nifH gene copy abundance have increased by 184 %, 86 %, and809 %, respectively. Version 2 includes two new data sheets for the nifH genecopy abundance of non-cyanobacterial diazotrophs and cell-specific N2fixation rates. The measurements of N2 fixation rates approximatelyfollow a log-normal distribution in both version 1 and version 2. However,version 2 considerably extends both the left and right tails of thedistribution. Consequently, when estimating global oceanic N2 fixationrates using the geometric means of different ocean basins, version 1 andversion 2 yield similar rates (43–57 versus 45–63 Tg N yr−1; rangesbased on one geometric standard error). In contrast, when using arithmeticmeans, version 2 suggests a significantly higher rate of 223±30 Tg N yr−1 (mean ± standard error; same hereafter) compared to version 1(74±7 Tg N yr−1). Specifically, substantial rate increases areestimated for the South Pacific Ocean (88±23 versus 20±2 Tg N yr−1), primarily driven by measurements in the southwestern subtropics,and for the North Atlantic Ocean (40±9 versus 10±2 Tg N yr−1). Moreover, version 2 estimates the N2 fixation rate in theIndian Ocean to be 35±14 Tg N yr−1, which could not be estimatedusing version 1 due to limited data availability. Furthermore, a comparisonof N2 fixation rates obtained through different measurement methods atthe same months, locations, and depths reveals that the conventional15N2 bubble method yields lower rates in 69 % cases compared tothe new 15N2 dissolution method. This updated version of thedatabase can facilitate future studies in marine ecology andbiogeochemistry. The database is stored at the Figshare repository(https://doi.org/10.6084/m9.figshare.21677687; Shao etal., 2022).

The denitrification efficiencies of two continuous flow mixed-culture denitrification reactors with Fe0 alone and carbon fiber reinforced Fe0 were compared by treating the synthetic wastewater containing NO3--N. The results showed that the average removal efficiencies of total nitrogen(TN) and NO3--N were as high as 89.04% and 97.13% for the experimental group R1 with carbon fiber and Fe0 injection at COD/NO3--N of 2.9-3.1 and hydraulic retention time(HRT) of 24 h, which were significantly higher than the control group R0 with Fe0 injection alone. The extracellular polymeric substances(EPS) and electron transfer activity(ETSA) variation patterns indicated that the input of carbon fiber further promoted the synthesis of EPS and enhanced the utilization of electrons by microorganisms. Scanning electron microscopy-energy dispersive spectrometry(SEM-EDS) and X-ray diffractometer(XRD) analyses revealed that there was obvious microbial corrosion on the surface of Fe0 in R1 reactor, and FeO(OH) and bioorganic-Fe complexes were the main corrosion products. Microbiological analysis showed that the increase of organic carbon source and the injection of carbon fiber effectively increased the abundance of iron autotrophic denitrifying bacteria genus and promoted the enrichment of denitrifying functional genes.

Biological N2 fixation has been crucial for sustaining early life on Earth. Very negative δ15N values detected in Archean sediments, which are not observed in present‐day environments, have been attributed to the low efficiency of proto‐nitrogenases. Alternatively, variations in early atmospheric CO2 may also play a role. Here we examine the effects of CO2 concentrations on the biomass δ15N signatures of the diazotrophs Trichodesmium erythraeum and Crocosphaera watsonii, which utilize Mo‐Fe nitrogenase (the most common form of the enzyme). Our results show that these organisms produce biomass with δ15N values up to ∼3‰ lower under both decreased and elevated CO2 concentrations compared to modern levels (∼380 μatm). These deviations from modern CO2 levels reduce nitrogenase enzyme efficiency, leading to increased organismal isotopic fractionation during N2 fixation. This study offers an alternative explanation for the observed fluctuations in geological δ15N records and provides new insights into the past nitrogen cycle on Earth. Plain Language Summary: The isotope effect of biological N2 fixation is crucial for understanding the nitrogen cycle, but its regulation under different atmospheric CO2 levels as appeared in Earth's history is not well understood. Our research shows that CO2 levels significantly influence the nitrogen isotope composition in the biomass of the diazotrophic cyanobacteria Trichodesmium and Crocosphaera by affecting the nitrogenase efficiency and thus the growth rate. This study sheds light on the geological changes in the δ15N records and provides new insights into the historical nitrogen cycle on Earth. Key Points: The effects of CO2 on the biomass δ15N signatures of the diazotrophs Trichodesmium erythraeum and Crocosphaera watsonii are examinedBoth species produce biomass with δ15N values lower under both decreased and elevated CO2 concentrations compared to modern CO2 levelsCO2‐controlled nitrogenase efficiency significantly influences organismal isotopic fractionation during N2 fixation [ABSTRACT FROM AUTHOR]

The impact of environmental factors on nitrogen (N) and oxygen (O) isotope effects during algal nitrate assimilation causes uncertainty in the field application of sedimentary N isotope records and nitrate isotopes to understand the marine nitrogen cycle. Ocean acidification is predicted to change nitrogen cycling including nitrate assimilation, but how N and O isotope effects during algal nitrate assimilation vary in response to changes in seawater pH and partial pressure CO2 (pCO2) remains unknown. We measured N and O isotope effects during nitrate assimilation and physiological states of the marine diatom Thalassiosira weissflogii and Synechococcus under different pH (8.1 or 7.8) and pCO2 (400 or 800 μatm) conditions. Low pH and/or high pCO2 equally decreased N and O isotope effects during nitrate assimilation by diatoms possibly due to reducing cellular nitrate efflux/uptake ratio and decreased isotope effects for nitrate uptake, whereas they did not affect those by Synechococcus with low intracellular nitrate concentration and limited nitrate efflux. Our results provide compelling experimental evidence showing different changes in N and O isotope effects during nitrate assimilation by marine eukaryotic and prokaryotic phytoplankton at low pH and/or high pCO2. These findings suggest new insight into environmental controls on variability in the isotope effect during algal nitrate assimilation, and have implications for improving a predictive understanding of N and O isotope tools in acidified oceans. [ABSTRACT FROM AUTHOR]

Biological nitrogen (N2) fixation is the main input of fixed nitrogen to ecosystems on Earth. Nitrogen isotope fractionation during this process is a key parameter for understanding the nitrogen cycle, however, relatively little is known about its regulatory mechanisms. Here we examine the effects of varying CO2 concentrations on biomass δ15N signatures of the cyanobacterial diazotrophs Trichodesmium erythraeum and Crocosphaera watsonii. We show that these organisms produce biomass up to ~3 ‰ lower in δ15N under either decreased (~180 µatm) or elevated (~1400 µatm) CO2 concentrations in comparison to modern levels (~380 µatm). Our results pointed towards changes in nitrogenase enzyme efficiency in response to CO2 perturbations impacting isotopic fractionation during N2 fixation and thus the biomass δ15N. This study contributes to an improved interpretation of the observed fluctuations in the δ15N records, and thus the past nitrogen cycle on Earth. [ABSTRACT FROM AUTHOR]

The productivity of diatoms in the Southern Ocean plays a key role in the transfer of carbon (C) from the atmosphere to the ocean's interior, which impacts climate. However, diatom growth in the Southern Ocean is limited by several environmental factors including iron (Fe) and light. Ongoing increases in ocean CO2 concentrations may increase diatom carbon fixation, but it is uncertain how this will interact with extant Fe and light limitation. Here we grew the Southern Ocean diatom Fragilariopsis cylindrus under a matrix of growth sufficient and limiting Fe and light levels and current and elevated CO2 concentrations, and found that a decrease in Fe concentration at high light, or in light intensity at high Fe, caused a similar 28–35% decrease in growth rate. Combined low Fe and low light caused a much larger (71–75%) decrease in growth rate than occurred with low Fe or low light alone, indicating Fe and light co‐limitation. At a given concentration of bioavailable dissolved inorganic Fe (Fe′), increasing pCO2 from 400 to 750 μatm had no significant effect on growth or C‐fixation rates under any Fe and light conditions. These results suggest that unlike previous measurements in Fe‐ and light‐limited temperate diatoms, increased CO2 should have little effect on C‐fixation rates in Southern Ocean diatoms. The different physiological responses of cold‐water and temperate diatoms to the changing environment warrant further investigation for understanding and predicting changes in the efficiency of the biological carbon pump and the associated potential feedback to the climate change. [ABSTRACT FROM AUTHOR]

To inhibit the destructive evolution of the HfB2‐SiC coating during oxidation, La2O3 was added to modify the oxygen‐blocking capability of the HfB2‐SiC coating. The effect of La2O3 content on the oxygen barrier capacity of HfB2‐SiC was investigated. The addition of 5 vol.% La2O3 lowered the oxidation activity and strengthened the inert oxygen barrier capacity of the HfB2‐SiC coating, which made the oxygen permeability and maximum weight change rate of the coating decrease by 30.32% and 73.97%, respectively. Due to the solid solution reaction of the dispersed La2O3, HfO2, and SiO2 nanoparticles, a stable Hf‐B‐La‐Si‐O multiphase glass was formed on the surface of the coating. Besides, the high‐melting‐point particles, such as HfO2, La2Hf2O7, and HfSiO4, dispersed in the glass layer as hard particles and formed a mosaic structure based on the Hf‐B‐La‐Si‐O compound, which increased the cumulative protection efficiency of the HfB2‐SiC coating to 98.5% and reduced the inert factor value by 84.57%. However, the excessive consumption of SiO2 glass by La2O3 results in dendritization of the generated glass when the La2O3 content is above 10 vol.%, which reduced the self‐healing capability of the generated glass and instead weakened the oxygen barrier properties of the HfB2‐SiC‐La2O3 coatings. [ABSTRACT FROM AUTHOR]