Explore the vast range of titles available.

Organic Cu-binding ligands have a fundamental influence on Cu distributions in the global ocean and they complex >99% of the dissolved Cu in seawater. Cu-binding ligands however, represent a large diversity of compounds with distinct sources, sinks and chemical properties. This heterogeneity makes the organic Cu-binding ligand pool difficult to study at the global scale. In this review, we provide an overview of the diversity of compounds that compose the marine Cu-ligand pool, and their dominant sources and sinks. We also summarize the most common analytical methods to measure ligands in marine water column samples. Generally, ligands are classified according to their conditional binding strength to Cu. However, the lack of a common definition for Cu ligand categories has previously complicated data intercomparison. To address this, we provide a general classification for Cu-binding ligands according to their binding strength and discuss emerging patterns in organic Cu-binding ligand distributions in the ocean according to this classification. To date, there is no global biogeochemical model that explicitly represents Cu ligands. We provide estimates of organic Cu-binding ligand fluxes at key interfaces as first order estimates and a first step for future modeling efforts focused on Cu and Cu-binding ligands.

Primary production by phytoplankton represents a major pathway whereby atmospheric CO 2 is sequestered in the ocean, but this requires iron, which is in scarce supply. As over 99% of iron is complexed to organic ligands, which increase iron solubility and microbial availability, understanding the processes governing ligand dynamics is of fundamental importance. Ligands within humic-like substances have long been considered important for iron complexation, but their role has never been explained in an oceanographically consistent manner. Here we show iron co-varying with electroactive humic substances at multiple open ocean sites, with the ratio of iron to humics increasing with depth. Our results agree with humic ligands composing a large fraction of the iron-binding ligand pool throughout the water column. We demonstrate how maximum dissolved iron concentrations could be limited by the concentration and binding capacity of humic ligands, and provide a summary of the key processes that could influence these parameters. If this relationship is globally representative, humics could impose a concentration threshold that buffers the deep ocean iron inventory. This study highlights the dearth of humic data, and the immediate need to measure electroactive humics, dissolved iron and iron-binding ligands simultaneously from surface to depth, across different ocean basins.

Reduced sulfur species (RSS) are involved in essential biological and chemical processes, including metal complexation, yet little is known about their occurrence and behaviour in marine systems. Here, we present a quantitative and qualitative data set of species-specific RSS in open ocean samples collected during the GEOTRACES Tonga GPpr14 cruise. The cruise traversed differing biogeochemical provinces, from the mesotrophic Melanesian waters and the North Fiji Basin, through the hydrothermally active Lau Basin, eastward to the oligotrophic South Pacific Gyre. Using cathodic stripping voltammetry in acidified samples (pH 2), we measured the concentration of two RSS, with peak potentials of -0.18 and -0.09 V in equivalents of thioacetamide (TA) and glutathione (GSH) respectively. GSH-like compounds were only present in the upper 200 m at concentrations up to 6.2 nM eq. GSH, consistent with other cathodic stripping voltammetry as well as chromatography-based studies. In contrast, R S S − 0.18   V 2 compounds were detected at all depths at concentrations ranging from 48 nM to 980 nM eq. TA. Both R S S − 0.18   V 2 and GSH-like compounds were present at higher levels in the hydrothermally-impacted region of the Lau Basin relative to other stations. The highest levels, along with high sulfide concentrations, were detected in a hydrothermal plume sample, indicating that hydrothermal vents are a direct or indirect source of these compounds. Elevated levels of R S S − 0.18   V 2 compounds were detected throughout almost the entire water column at a station located in the North Fiji Basin. We also employed the qualitative technique of cathodic pseudopolarography on unbuffered samples (pH ~ 8.5). Pseudopolarograms of marine RSS were compared to sulfide, GSH and TA standards. Pseudopolarography supports the presence of GSH in marine samples. However, while a compound that is electrochemically similar to TA is often detected in marine samples, TA itself is not thought to be naturally present. This is supported by our pseudopolarograms of R S S − 0.52   V 8.5 which often lacked the characteristic TA reduction wave but suggested the presence of other unidentified RSS compounds.

Indoor hydroponic farming enables pesticide‐free food production with significantly lower water use than conventional agriculture. However, standard practices often involve frequent replacement of nutrient solutions to manage nutrient imbalances and microbial contamination, generating large volumes of nutrient‐rich waste with both environmental and economic costs. We investigated the temporal dynamics of nutrient composition under two contrasting nutrient management strategies, focusing on the impact on cost per yield and sustainability on Red Batavia lettuce production. In Treatment 1, half‐strength Hoagland's solution was replaced weekly to maintain an electrical conductivity (EC) of 1.0–1.2 mS/cm−1 as per typical industry practice to maintain target nutrient ratios. In Treatment 2, nutrient solution was not replaced, and instead topped up only to maintain sump tank volume, irrespective of EC. Macro and micronutrient concentrations were monitored across both treatments to evaluate EC as a proxy for nutrient availability. While Treatment 1 achieved significantly higher yields (> 20%; p < 0.05), the cost per gram of fresh and dry weight was substantially higher than in Treatment 2, by 63% and 66%, respectively. Nutrient depletion accelerated at later growth stages, particularly in Treatment 2, suggesting increased uptake efficiency under limited availability. Over both treatments, EC was found to be a poor proxy for actual nutrient changes. These findings highlight a trade‐off between yield maximisation and nutrient‐use efficiency, emphasising the need for more strategic nutrient management. Our results support the advancement of circular, resource‐efficient hydroponic systems aligned with long‐term sustainability goals. Specifically, the integration of solution reuse, targeted nutrient dosing, and non‐disruptive pathogen control could improve environmental performance and economic viability in indoor food production.

The high N 2 fixation rate observed in the Lau Basin of the western tropical South Pacific Ocean (WTSP) is fueled by iron (Fe) released from shallow hydrothermal systems. Understanding Fe bioavailability is crucial but the controls on the stability and bioavailability of hydrothermal Fe inputs are still poorly understood. Here, we provide new data on the spatial and vertical distribution of the soluble ubiquitous humic-like ligands (L FeHS ) and their associated dissolved Fe (DFe) in the WTSP, including in samples near hydrothermal vents. Our data show that L FeHS are heterogenous ligands with binding sites of both strong and intermediate strengths. These ligands are primarily produced in surface waters and partially mineralized in mesopelagic waters. A substantial fraction of DFe was complexed by L FeHS (mean ~30%). The DFe complexed by L FeHS is likely bioavailable to phytoplankton and L FeHS stabilized Fe released by the mineralization of sinking biomass. However, unsaturation of L FeHS by Fe suggest that part of DFe is not available for complexation with L FeHS . Possible reasons are competition between DFe and other metals, such as dissolved copper, or the inability of L FeHS to access colloidal DFe. The study of two volcanic sites indicates that L FeHS were not produced in these hydrothermal systems. At the active site (DFe ~50 nmol L -1 ), L FeHS can only partially solubilize the hydrothermal DFe released in this area (1~5.5% of the total DFe). We performed controlled laboratory experiments which show that the observed low solubilization yield result from the inability of L FeHS to solubilize aged Fe oxyhydroxides (FeOx - a kinetically mediated process) and to form stable complexes with Fe(II) species. Our study provides new understanding of the role of L FeHS on the bioavailability and stabilization of hydrothermal DFe.

Variation in the microbial cycling of nutrients and carbon in the ocean is an emergent property of complex planktonic communities. While recent findings have considerably expanded our understanding of the diversity and distribution of nitrogen (N 2 ) fixing marine diazotrophs, knowledge gaps remain regarding ecological interactions between diazotrophs and other community members. Using quantitative 16S and 18S V4 rDNA amplicon sequencing, we surveyed eukaryotic and prokaryotic microbial communities from samples collected in August 2016 and 2017 across the Western North Atlantic. Leveraging and significantly expanding an earlier published 2015 molecular dataset, we examined microbial community structure and ecological co-occurrence relationships associated with intense hotspots of N 2 fixation previously reported at sites off the Southern New England Shelf and Mid-Atlantic Bight. Overall, we observed a negative relationship between eukaryotic diversity and both N 2 fixation and net community production (NCP). Maximum N 2 fixation rates occurred at sites with high abundances of mixotrophic stramenopiles, notably Chrysophyceae . Network analysis revealed such stramenopiles to be keystone taxa alongside the haptophyte diazotroph host Braarudosphaera bigelowii and chlorophytes. Our findings highlight an intriguing relationship between marine stramenopiles and high N 2 fixation coastal sites.

Iron (Fe) is an essential micronutrient for phytoplankton growth, but its scarcity in seawater limits primary productivity across much of the ocean. Most dissolved Fe (DFe) in seawater is complexed with Fe-binding organic ligands, a poorly constrained fraction of dissolved organic matter (DOM), which increase Fe residence time and impact Fe bioavailability. Here, we present the conditional concentration (L Fe ) and binding-strength (log K F e ' L c o n d ) of Fe-binding ligands in the Western Tropical South Pacific (WTSP) Ocean during the GEOTRACES TONGA cruise (GPpr14). The transect crossed the Lau basin, a region subject to shallow hydrothermal Fe inputs that fuel intense diazotrophic activity, the oligotrophic South Pacific gyre, and the Melanesian basin. Organic speciation was analyzed by competitive ligand exchange adsorptive cathodic stripping voltammetry (CLE-AdCSV) using salicylaldoxime at 25 µM. We found a high mean L Fe of 5.2 ± 1.2 nMeqFe (n = 103) across the entire transect, predominantly consisting of intermediate strength L2 ligands (84%; mean log K F e ' L c o n d of 11.6 ± 0.4), consistent with humic-like substances. DFe correlated with the humic-like component of the fluorescent DOM (HS-like FDOM), yet the electroactive Fe-binding humic-like substances (L FeHS ) accounted for only 20 ± 13% of L Fe in the mixed layer and 8 ± 6% in deep waters. Ligands were in large excess compared to DFe (mean excess ligand eL Fe = 4.6 ± 1.1 nMeqFe), suggesting poor stabilization of DFe inputs. High L Fe (up to 9 nMeqFe) in samples close to hydrothermal sites could be due to detoxification strategies from plankton communities toward hydrothermally-fueled toxic trace metals other than Fe, with an apparent dilution of the DOM from the Lau basin into neighboring regions. We also observed a different peak potential of the Fe salicylaldoxime complex detected by CLE-AdCSV between the Lau and Melanesian basins, and between surface and deep waters. To our knowledge, this change in potential has not previously been reported; whether this represents a novel detection of specificities in DOM composition merits further investigation. Competition between Fe and competing metals for ligand binding sites could favor DFe oxidation and precipitation near hydrothermal vents and explain the absence of strong Fe stabilization in the WTSP.

In the Western Tropical South Pacific (WTSP) Ocean, a hotspot of dinitrogen fixation has been identified. The survival of diazotrophs depends, among others, on the availability of dissolved iron (DFe) largely originating, as recently revealed, from shallow hydrothermal sources located along the Tonga-Kermadec arc that fertilize the Lau Basin with this element. On the opposite, these fluids, released directly close to the photic layer, can introduce numerous trace metals at concentrations that can be toxic to surface communities. Here, we performed an innovative 9-day experiment in 300 L reactors onboard the TONGA expedition, to examine the effects of hydrothermal fluids on natural plankton communities in the WTSP Ocean. Different volumes of fluids were mixed with non-hydrothermally influenced surface waters (mixing ratio from 0 to 14.5%) and the response of the communities was studied by monitoring numerous stocks and fluxes (phytoplankton biomass, community composition, net community production, N 2 fixation, thiol production, organic carbon and metal concentrations in exported material). Despite an initial toxic effect of hydrothermal fluids on phytoplankton communities, these inputs led to higher net community production and N 2 fixation rates, as well as elevated export of organic matter relative to control. This fertilizing effect was achieved through detoxification of the environment, rich in potentially toxic elements (e.g., Cu, Cd, Hg), likely by resistant Synechococcus ecotypes able to produce strong binding ligands, especially thiols (thioacetamide-like and glutathione-like compounds). The striking increase of thiols quickly after fluid addition likely detoxified the environment, rendering it more favorable for phytoplankton growth. Indeed, phytoplankton groups stressed by the addition of fluids were then able to recover important growth rates, probably favored by the supply of numerous fertilizing trace metals (notably Fe) from hydrothermal fluids and new nitrogen provided by N 2 fixation. These experimental results are in good agreement with in-situ observations, proving the causal link between the supply of hydrothermal fluids emitted at shallow depth into the surface layer and the intense biological productivity largely supported by diazotrophs in the WTSP Ocean. This study highlights the importance of considering shallow hydrothermal systems for a better understanding of the biological carbon pump.

The concentrations of dissolved copper (Cud), copper-binding ligands, thiourea-type thiols and humic substances (HSCu) were measured in estuarine waters adjacent to Sapelo Island, Georgia, USA, on a monthly basis from April to December 2014. Here we present the seasonal cycle of copper speciation within the estuary and compare it to the development of an annually occurring bloom of Ammonia Oxidising Archaea (AOA), which require copper for many enzymes. Two types of complexing ligands (L1 and L2) were found to dominate with mean complex stabilities (log K′CuL) of 14.5 and 12.8. Strong complexation resulted in lowering the concentration of free cupric ion (Cu2+) to femtomolar (fM) levels throughout the study and to sub-fM levels during the summer months. A Thaumarchaeota bloom during this period suggests that this organism manages to grow at very low Cu2+ concentrations. Correlation of the concentration of the L1 ligand class with a thiourea-type thiol and the L2 ligand class with HSCu provide an interesting dimension to the identity of the ligand classes. Due to the stronger complex stability, 82 - 99% of the copper was bound to L1. Thiourea-type thiols typically form Cu(I) species, which would suggest that up to ~90% copper could be present as Cu(I) in this region. In view of the very low concentration of free copper (pCu >15 at the onset and during the bloom) and a reputedly high requirement for copper, it is likely that the Thaumarchaeota are able to access thiol-bound copper directly.

While hydrothermal vents are a recognized source of trace elements to the ocean inventory, the contribution of slow-spreading ridges remains poorly resolved. To address this, high-resolution dissolved (<0.45µm) iron (dFe) and manganese (dMn) samples were collected during the GEOTRACES HERMINE GApr07 process study at the Mid Atlantic Ridge. Samples were collected at nine stations, from the TAG vent site to 75 km south-southwest following the neutrally buoyant plume. Concentrations of dMn and dFe ranged from 71 ± 6 and 51 ± 2 nmol kg-1 right above the vent site to 0.43 ± 0.01 and 1.56 ± 0.02 nmol kg-1 at the most distal station, respectively. Using a 5-box model coupled with our data, we show that as the plume travelled away from the vent, aggregation processes controlled dFe concentrations in the first 2 km, with an aggregation rate averaging between 8.0 ± 0.6 and 0.11 ± 0.04 nmol L-1 d-1, respectively in the first and second kilometer. Aggregation likely of small colloidal particles, led to partitioning of the size fractionated Fe pool, as 6% of the dFe was moved into the particulate size fraction. Further away, disaggregation processes became more prevalent, with rates ranging from 0.27 ± 0.02 to 0.008 ± 0.001 nmol L-1 d-1, enriching the dFe pool by 10 %. The computed decrease of hydrothermal Fe within the neutrally buoyant plume was likely caused by flocculation of small Fe oxyhydroxide particles. This process resulted in Fe aggregate formation with radii estimated to range from 18 ± 1 µm in the first km from TAG to 2.5 ± 0.4 and 6 ± 2 µm between 1 and 30 km from the vent site.