Global oceanic diazotroph database version 2 and elevated estimate of global oceanic N₂ fixation

Abstract

Abstract. Marine diazotrophs convert dinitrogen (N2) gas into bioavailable nitrogen (N), supporting life in the global ocean. In 2012, the first version of the global oceanic diazotroph database (version 1) was published. Here, we present an updated version of the database (version 2), significantly increasing the number of in situ diazotrophic measurements from 13 565 to 55 286. Data points for N2 fixation rates, diazotrophic cell abundance, and nifH gene copy abundance have increased by 184 %, 86 %, and 809 %, respectively. Version 2 includes two new data sheets for the nifH gene copy abundance of non-cyanobacterial diazotrophs and cell-specific N2 fixation rates. The measurements of N2 fixation rates approximately follow a log-normal distribution in both version 1 and version 2. However, version 2 considerably extends both the left and right tails of the distribution. Consequently, when estimating global oceanic N2 fixation rates using the geometric means of different ocean basins, version 1 and version 2 yield similar rates (43–57 versus 45–63 Tg N yr−1; ranges based on one geometric standard error). In contrast, when using arithmetic means, 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 are estimated 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 the Indian Ocean to be 35±14 Tg N yr−1, which could not be estimated using version 1 due to limited data availability. Furthermore, a comparison of N2 fixation rates obtained through different measurement methods at the same months, locations, and depths reveals that the conventional 15N2 bubble method yields lower rates in 69 % cases compared to the new 15N2 dissolution method. This updated version of the database can facilitate future studies in marine ecology and biogeochemistry. The database is stored at the Figshare repository (https://doi.org/10.6084/m9.figshare.21677687; Shao et al., 2022).

Continuous Zonal Gradients Characterize Epipelagic Plankton Assemblages and Hydrography in the Subtropical North Atlantic

“…Data Availability Statement: Hydrographic and nutrient data for this cruise can be found in the database of CLIVAR and CCHDO [27] at https://cchdo.ucsd.edu/cruise/29AH20110128, accessed on 1 March 2024. Trichodesmium abundance data was incorporated in Version 2 of the global oceanic diazotroph database [11,33] at https://figshare.com/ndownloader/articles/21677687/versions/3, accessed on 1 March 2024.…”

“…Particularly in the North Atlantic, the variability in the properties of subtropical waters has profound implications for climate change, showing decadal warming [2,3], slowdown of the Atlantic Meridional Overturning Circulation (AMOC [4]), and increasing oligotrophy [5][6][7], among other changes recently reported. Plankton communities in subtropical waters can be highly sensitive to climate changes because they are characterized by low abundance and biomass, dominance of small organisms [8,9], low primary production rates [5] but significant N fixation by cyanobacteria [10,11], predominance of microbial food webs [9], and food chains longer than those found in productive waters [12].…”

Distribution of the N2‐fixing cyanobacterium Candidatus Atelocyanobacterium thalassa in the Mexican Pacific upwelling system under two contrasting El Niño Southern Oscillation conditions

“…UCYN‐A is of widespread importance in the ocean as it has been recorded in diverse marine environments (e.g., Krupke et al, 2014 ; Li et al, 2021 ; Shao et al, 2023 ; Shiozaki et al, 2017 ; Tang et al, 2020 ; Turk‐Kubo et al, 2017 ), including regions not typically assumed to be important for biological N fixation, such as N‐enriched waters, especially polar seas (Harding et al, 2018 ; Shiozaki et al, 2018 ), coastal waters (Cabello et al, 2020 ; Mulholland et al, 2012 ) and upwelling regions (Agawin et al, 2014 ; Moreira‐Coello et al, 2019 ; Selden et al, 2022 ; Turk‐Kubo et al, 2021 ). However, UCYN‐A has not been detected in oxygen‐deficient zones (ODZs) under suboxic conditions (<20 μM), which may represent a favourable niche for N 2 fixation, since in these environments diazotrophs do not need to expend additional energy for oxygen removal from the surrounding water to protect nitrogenase (Fay, 1992 ; Robson & Postgate, 1980 ).…”

Possible transport pathway of diazotrophic Trichodesmium by Agulhas Leakage from the Indian into the Atlantic Ocean

“…It is assumed that biological fixation of atmospheric nitrogen (N 2 ) by diazotrophic cyanobacteria balances this loss in nutrient-poor regions, fuelling up to half of the new production 5 and therefore playing an important role in carbon uptake in the marine environment. Shao et al 6 compiled a database of estimated global oceanic N 2 fixation rates.…”