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The low‐molecular‐weight (LMW) reduced sulfur substances (RSS) composition of dissolved organic matter (DOM) was examined along the GEOTRACES US‐GP15 section in the Pacific Ocean. We demonstrate that LMW RSS constitutes a significant fraction of nonvolatile dissolved organic sulfur (DOS). While thiols such as glutathione were below our detection limit (300 pM), RSS containing two carbon (C) sulfur (S) bonds were present at concentrations in the hundreds of nM range. RSS accumulation was observed in subtropical waters. The most likely source of these RSS is microbial alteration of sulfurized DOM with production of secondary thioamidated metabolites. RSS are initially produced by cyanobacteria to mitigate copper and oxidative stress induced by UV‐B irradiance. A preferential remineralization of RSS over dissolved organic carbon (DOC) in the upper 350 m suggests a partial lability of LMW DOS. Deeper, homogeneous concentrations and C : S ratio indicate increasing stability of this LMW DOS.

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.

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.

\"History is cruel,\" says Rodinson, and it may be guessed that even he has no idea of how pregnant his phrase is. For it is history--that great implacable amoral force--which has forced the fatal dilem-...

The genealogy of one's country, first refuge of the armchair patriot, is always pleasant to read. But it is far from this alone which makes TWO ROADS TO GUADALUPE by Robert Lewis Taylor (Doubleday: $5.95) a delight. It is, rather, the author's total under-...

Suppose, at some summer camp, a group of counselors should charge down to the dock where a learners' swimming class is in progress, and--whooping with joy-throw all the kids into...

\"History is cruel,\" says Rodinson, and it may be guessed that even he has no idea of how pregnant his phrase is. For it is history--that great implacable amoral force--which bas forced the fatal dilem-...

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Here is a primer of possibilities for the future, a handy textbook for every politician and planner.

Perhaps the most extraordinary part of this book is what it omits. Nowhere does it mention religion, the church or spiritual indoctrination of any kind. It deals solely with the more practical agencies--police, parole officers, courts, community interest or sheer humanism--as the only way to go...