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).

Little is known about food insecurity and unmet subsistence needs and their association with antiretroviral therapy adherence among youth and young adults living with HIV (YLWH). Methods: We conducted a cross-sectional survey to examine this association in 101 YLWH (aged 18e29 years). Poisson regression models with robust standard errors were used to estimate adjusted risk ratios (RRs) and 95% confidence intervals (CIs).Results: Approximately 51.7% of participants experienced at least one unmet subsistence need (difficulty finding enough to eat [36.6%], clothing [22.8%], place to sleep [21.8%], place to wash [17.8%], and bathroom [15.8%]), and 64.2% reported being food insecure. For every additional unmet need, the risk of very good/excellent adherence was reduced by 15% (RR ¼ .85; 95% CI ¼ .72e.99; p value ¼ .04). The risk of very good/excellent adherence was lowered by 39% (RR ¼ .61; 95% CI ¼ .43e.87; p value ¼ .005) among food insecure youth, compared with those who were food secure. Conclusions: These data highlight the need for more research and structural interventions targeting food assistance strategies among YLWH.

Oxygen minimum zones (OMZs) have been suggested as a suitable niche for the oxygen-sensitive process of biological fixation of dinitrogen (N2) gas. However, most N2 fixation rates reported from such waters are low. This low N2 fixation activity has been proposed to result from the unusual community of N2 fixers, in which cyanobacteria were typically underrepresented. The Northern Benguela Upwelling System (North BUS) is part of one of the most productive marine ecosystems and hosts a well-developed OMZ. Although previous observations indicated low to absent N2 fixation rates, the community composition of diazotrophs needed to understand the North BUS has not been described. Here, we present a first detailed analysis of the diazotrophic diversity in the North BUS OMZ and the Angola tropical zone (ATZ), based on genetic data and isotope speciation. Consistent with a previous study, we detected a slight N deficit in the OMZ, but isotope data did not indicate any active or past N2 fixation. The diazotroph community in the North BUS was dominated by non-cyanobacterial microbes clustering with members of gamma-proteobacteria, as is typical for other OMZ regions. However, we found a strikingly high diversity of Cluster III diazotrophs not yet described in other OMZs. In contrast to previous observations, we could also identify cyanobacteria of the clades Trichodesmium sp., UCYN-A and Cyanothece sp., in surface waters connected to or above the OMZ, which were potentially active as shown by the presence of genes and transcripts of the key functional marker gene for N2 fixation, nifH. While the detection of diazotrophs and the absence of active N2 fixation (based on isotopic speciation) are consistent with other OMZ observations, the detected regional variation in the diversity and presence of cyanobacteria indicate that we still are far from understanding the role of diazotrophs in OMZs, which, however, is relevant for understanding the N cycle in OMZ waters, as well for predicting the future development of OMZ biogeochemistry in a changing ocean.

Biological dinitrogen (N2) fixation is the pathway making the large pool of atmospheric N2 available to marine life. Besides direct rate measurements, a common approach to explore the potential for N2 fixation in the ocean is a screening-based targeting the key functional marker gene nifH, coding for a subunit of the nitrogenase reductase. As novel sequencing techniques improved, our understanding of the diversity of marine N2 fixers grew exponentially. However, one aspect of N2 fixation in the ocean is often underexplored, which are the two alternative types of the key enzyme of N2 fixation, the nitrogenase. Altogether there are three isoenzymes, the most common Mo-Fe nitrogenase Nif, the Fe-Fe nitrogenase Anf, and the V-Fe nitrogenase Vnf, which differ regarding their genetic organization, as well as their metal co-enzymes. While Mo is only available in the presence of at least traces of oxygen (O2), V and Fe are available if O2 is absent. Therefore, low O2 and anoxic ocean environments could be an ideal place to explore the diversity of the different isotypes of the nitrogenases. Most phylogenetic studies, however, were only based on the functional marker gene nifH, encoding for a subunit of the Nif nitrogenase, and thus limited in representing the diversity of alternative nitrogenases. Here, we screened metagenomes and -transcriptomes from O2 minimum zones off Peru, from the Bay of Bengal, and the anoxic Saanich Inlet to explore the diversity of genes involved in N2 fixation. We identified genes related to all three nitrogenases, and a generally increased diversity as compared to our previous nifH based on studies from OMZ waters. While we could not confirm gene expression of alternative nitrogenases from our transcriptomic, we detected diazotrophs harboring the genetic potential for alternative nitrogenases. We suggest that alternative nitrogenases may not be used under conditions present in those waters, however, depending on trace metal availability they may become active under future ocean deoxygenation.

Microbial plankton is essential for ocean biogeochemistry. As part of the prokaryotic phototrophic microbial community, both oxygenic phototrophs (OP) and anoxygenic phototrophs (AP) are widely distributed in the ocean and may play a significant role in carbon flow and oxygen production. However, comparative studies of microbial OP and AP have received very little attention, even though their different roles might be important in various marine environments, especially in oxygen minimum zones (OMZ). We explored the spatial distribution of the microbial community in the Baltic Sea, including an OMZ region, with a particular focus on the distribution and activity of OP and AP. We used 16S rRNA amplicon sequencing in combination with a qPCR-based quantification of photosynthesis marker genes. We found that specific bacterial groups dominated surface and intermediate depths, the OMZ, and deep waters, respectively. Salinity, temperature, oxygen, and depth were significant factors explaining the microbial community composition and distribution. A high diversity of OP and AP was observed, including OP-Chlorophyta, Diatoms, Cyanobacteria and Cryptomonads, and AP-Proteobacteria and Chloroflexota. OP were more abundant at most stations compared to AP. OP showed high photosynthetic activity and more photosynthesis activity in higher temperature and upper waters, while AP photosynthesis cannot be detected in most stations. Both, cyanobacterial and eukaryotic OP preferred to live in higher temperature and upper waters, but Cyanobacteria also preferred to live in oxic water while the whole OP community showed preference to live in higher salinity area. However, AP did not show any significant hydrochemical preference but prefer to live with OP community. The Baltic Sea is exposed to multiple climate change related stressors, such as warming, decreasing salinity, and deoxygenation. This study contributes to understanding and interpretation of how microbial community, especially phototrophic groups, might shift in their distribution and activity in a changing ocean like the Baltic Sea.