Description: Growth of the prominent nitrogen-fixing cyanobacterium Trichodesmium is often limited by phosphorus availability in the ocean. How nitrogen fixation by phosphorus-limited Trichodesmium may respond to ocean acidification remains poorly understood. Here, we use phosphate-limited chemostat experiments to show that acidification enhanced phosphorus demands and decreased phosphorus-specific nitrogen fixation rates in Trichodesmium. The increased phosphorus requirements were attributed primarily to elevated cellular polyphosphate contents, likely for maintaining cytosolic pH homeostasis in response to acidification. Alongside the accumulation of polyphosphate, decreased NADP(H):NAD(H) ratios and impaired chlorophyll synthesis and energy production were observed under acidified conditions. Consequently, the negative effects of acidification were amplified compared to those demonstrated previously under phosphorus sufficiency. Estimating the potential implications of this finding, using outputs from the Community Earth System Model, predicts that acidification and dissolved inorganic and organic phosphorus stress could synergistically cause an appreciable decrease in global Trichodesmium nitrogen fixation by 2100.

Description: The research cruise M187 with the RV METEOR sailed January 25 th to March 4 th 2023 from Walvis Bay to Walvis Bay (Namibia), with a focus on investigating the biogeochemical gradients that exist between the Benguela Upwelling zone and the South Atlantic Subtropical Gyre. In order to achieve this, the two specific foci of the research cruise were to (i) track upwelling filaments as they advect offshore and interact with the subtropical gyre, and (ii) perform a high-resolution transect from upwelling sites to the subtropical gyre. On the research cruise, two filaments were successfully mapped from cold water upwelling sites near or over the Namibian shelf through to warmer waters offshore. This was followed by a transect of twelve stations outwards into the subtropical gyre, reaching a maximum westward position of 5 °W. Sampling stations were conducted to a maximum depth of 1000 m and involved an array of deployments to investigate the biogeochemistry of the water column. Further nutrient addition bioassay experiments were conducted throughout the research cruise to assess the nutrients (co-)limiting to phytoplankton growth. Collectively our research will shed light on key mechanisms establishing the major oceanic biogeochemical gradients between upwelling and subtropical gyre regions, so that they can be included in models used to predict the impacts of climate change.

Description: Nitrogen fixation is critical for the biological productivity of the ocean, but clear mechanistic controls on this process remain elusive. Here, we investigate the abundance, activity, and drivers of nitrogen-fixing diazotrophs across the tropical western North Pacific. We find a basin-scale coherence of diazotroph abundances and N 2 fixation rates with the supply ratio of iron:nitrogen to the upper ocean. Across a threshold of increasing supply ratios, the abundance of nifH genes and N 2 fixation rates increased, phosphate concentrations decreased, and bioassay experiments demonstrated evidence for N 2 fixation switching from iron to phosphate limitation. In the northern South China Sea, supply ratios were hypothesized to fall around this critical threshold and bioassay experiments suggested colimitation by both iron and phosphate. Our results provide evidence for iron:nitrogen supply ratios being the most important factor in regulating the distribution of N 2 fixation across the tropical ocean.

Description: Fast repetition rate fluorometry (FRRf) allows for rapid non-destructive assessment of phytoplankton photophysiology in situ yet has rarely been applied to Trichodesmium. This gap reflects long-standing concerns that Trichodesmium (and other cyanobacteria) contain pigments that are less effective at absorbing blue light which is often used as the sole excitation source in FRR fluorometers—potentially leading to underestimation of key fluorescence parameters. In this study, we use a multi-excitation FRR fluorometer (equipped with blue, green, and orange LEDs) to investigate photophysiological variability in Trichodesmium assemblages from two sites. Using a multi-LED measurement protocol (447+519+634 nm combined), we assessed maximum photochemical efficiency (Fv/Fm), functional absorption cross section of PSII (σPSII), and electron transport rates (ETRs) for Trichodesmium assemblages in both the Northwest Pacific (NWP) and North Indian Ocean in the vicinity of Sri Lanka (NIO-SL). Evaluating fluorometer performance, we showed that use of a multi-LED measuring protocol yields a significant increase of Fv/Fm for Trichodesmium compared to blue-only excitation. We found distinct photophysiological differences for Trichodesmium at both locations with higher average Fv/Fm as well as lower σPSII and non-photochemical quenching (NPQNSV) observed in the NWP compared to the NIO-SL (Kruskal–Wallis t-test df = 1, p 〈 0.05). Fluorescence light response curves (FLCs) further revealed differences in ETR response with a lower initial slope (αETR) and higher maximum electron turnover rate ((Formula presented.)) observed for Trichodesmium in the NWP compared to the NIO-SL, translating to a higher averaged light saturation EK (= (Formula presented.) /αETR) for cells at this location. Spatial variations in physiological parameters were both observed between and within regions, likely linked to nutrient supply and physiological stress. Finally, we applied an algorithm to estimate primary productivity of Trichodesmium using FRRf-derived fluorescence parameters, yielding an estimated carbon-fixation rate ranging from 7.8 to 21.1 mgC mg Chl-a–1 h–1 across this dataset. Overall, our findings demonstrate that capacity of multi-excitation FRRf to advance the application of Chl-a fluorescence techniques in phytoplankton assemblages dominated by cyanobacteria and reveals novel insight into environmental regulation of photoacclimation in natural Trichodesmium populations

Description: Nutrients limiting phytoplankton growth in the ocean are a critical control on ocean productivity and can underpin predicted responses to climate change. The extensive western subtropical North Pacific is assumed to be under strong nitrogen limitation, but this is not well supported by experimental evidence. Here, we report the results of 14 factorial nitrogen–phosphorus–iron addition experiments through the Philippine Sea, which demonstrate a gradient from nitrogen limitation in the north to nitrogen–iron co-limitation in the south. While nitrogen limited sites responded weakly to nutrient supply, co-limited sites bloomed with up to ~60-fold increases in chlorophyll a biomass that was dominated by initially undetectable diatoms. The transition in limiting nutrients and phytoplankton growth capacity was driven by a gradient in deep water nutrient supply, which was undetectable in surface concentration fields. We hypothesize that this large-scale phytoplankton response gradient is both climate sensitive and potentially important for regulating the distribution of predatory fish.

Description: Marine diazotrophs convert dinitrogen (N-2) 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 N-2 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 N-2 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 N-2 fixation rates using the geometric means of different ocean basins, version 1 and version 2 yield similar rates (43-57 versus 45-63 TgNyr (-1); ranges based on one geometric standard error). In contrast, when using arithmetic means, version 2 suggests a significantly higher rate of 223 +/- 30 TgNyr (-1) (mean +/- standard error; same hereafter) compared to version 1 (74 +/- 7 TgNyr (-1)). Specifically, substantial rate increases are estimated for the South Pacific Ocean (88 +/- 23 versus 20 +/- 2 TgNyr 1), primarily driven by measurements in the southwestern subtropics, and for the North Atlantic Ocean (40 +/- 9 versus 10 +/- 2 TgNyr (-1)). Moreover, version 2 estimates the N-2 fixation rate in the Indian Ocean to be 35 +/- 14 TgNyr (-1), which could not be estimated using version 1 due to limited data availability. Furthermore, a comparison of N-2 fixation rates obtained through different measurement methods at the same months, locations, and depths reveals that the conventional N-15(2) bubble method yields lower rates in 69% cases compared to the new N-15(2) 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).

Description: Atmospheric aerosol deposition into the low latitude oligotrophic ocean is an important source of new nutrients for primary production. However, the resultant phytoplankton responses to aerosol deposition events, both in magnitude and changes in community composition, are poorly constrained. Here, we investigated this with 19 d of field and satellite observations for a site in the subtropical North Atlantic. During the observation period, surface dissolved aluminum concentrations alongside satellite-derived aerosol and precipitation data demonstrated the occurrence of both a dry deposition event associated with a dust storm and a wet deposition event associated with strong rainfall. The dry deposition event did not lead to any observable phytoplankton response, whereas the wet deposition event led to an approximate doubling of chlorophyll a, with Prochlorococcus becoming more dominant at the expense of Synechococcus. Bioassay experiments showed that phytoplankton were nitrogen limited, suggesting that the wet deposition event likely provided substantial aerosol-derived nitrogen, thereby alleviating the prevalent nutrient limitation and leading to the rapid observed phytoplankton response. These findings highlight the important role of wet deposition in driving rapid responses in both ocean productivity and phytoplankton community composition.

Description: Subtropical gyres cover 26%-29% of the world's surface ocean and are conventionally regarded as ocean deserts due to their permanent stratification, depleted surface nutrients, and low biological productivity. Despite tremendous advances over the past three decades, particularly through the Hawaii Ocean Time-series and the Bermuda Atlantic Time-series Study, which have revolutionized our understanding of the biogeochemistry in oligotrophic marine ecosystems, the gyres remain understudied. We review current understanding of upper ocean biogeochemistry in the North Pacific Subtropical Gyre, considering other subtropical gyres for comparison. We focus our synthesis on spatial variability, which shows larger than expected dynamic ranges of properties such as nutrient concentrations, rates of N-2 fixation, and biological production. This review provides new insights into how nutrient sources drive community structure and export in upper subtropical gyres. We examine the euphotic zone (EZ) in subtropical gyres as a two-layered vertically structured system: a nutrient-depleted layer above the top of the nutricline in the well-lit upper ocean and a nutrient-replete layer below in the dimly lit waters. These layers vary in nutrient supply and stoichiometries and physical forcing, promoting differences in community structure and food webs, with direct impacts on the magnitude and composition of export production. We evaluate long-term variations in key biogeochemical parameters in both of these EZ layers. Finally, we identify major knowledge gaps and research challenges in these vast and unique systems that offer opportunities for future studies. Key Points Subtropical gyres display larger spatiotemporal dynamics in biogeochemical properties than previously considered An improved two-layer framework is proposed for the study of nutrient-driven and biologically mediated carbon export in the euphotic zone Future research will benefit from high-resolution samplings, improved sensitivity of nutrient analyses, and advanced modeling capabilities