DOI: 10.5061/dryad.d51c5b0bq
title: Low molecular weight reduced sulfur substances: a major component of non-volatile dissolved organic sulfur in the Pacific Ocean

description: 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 non-volatile 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 meters suggests a
 partial lability of LMW DOS. Deeper, homogeneous concentrations and C:S
 ratio indicate increasing stability of this LMW DOS.
description: Samples were collected onboard the R/V Roger Revelle between
 September and November 2018). The seawater was collected using the
 Oceanographic Data Facility's (ODF, Scripps Institution of
 Oceanography) CTD rosette equipped with twelve 30 L Niskin bottles.
 Samples were filtered through 0.8/0.45 μm Acropak 500 filter cartridges
 and were stored in a -20°C freezer until analysis.  The
 concentrations of DOC in samples were measured by size exclusion
 chromatography coupled to an organic carbon detector (SEC-OCD)
 (DOC-Labor®, Karlsruhe, Germany) following the protocol of Huber et al.
 (2011) adapted for seawater (Dulaquais et al., 2018; Fourrier at al.,
 2022a,b). The analysis of thiols were performed by Ad-CSV on a mercury
 drop electrode with a deposition potential of 0 V and a deposition time of
 180 s following the recommendations of Pernet-Coudrier et al. (2013).
 Samples were thawed at 4°C and acidified at pH 2 (HCl, Suprapure, Merck©)
 immediately before analysis. All the solutions as well
 as the washing of the material was performed with ultrapure water from a
 purification system, Milli-Q Element (resistivity &gt; 18.2 MΩ) from
 Millipore®. All the chemicals (mobile/acid phases) for
 SEC-OCD were prepared following the protocol from Huber et al. (2011). The
 data treatment was also done in the same way as these authors. The
 calibration of the OCD was performed as Dulaquais et al. (2018) which
 adapt DOC measurement by SEC for marine waters. The SEC device, equipped
 with two chromatographic columns (250 mm × 20 mm, TSK HW-50S, 3000
 theoretical plates, Toso, Japan), permits the separation of DOM into five
 fractions of organic compounds with an optimal resolution.
 Reduced sulfur species (RSS) such as thioacemamide (TA) form
 strong complexes with metals and are thus detectable by Ad-CSV at the
 surface of a mercury drop electrode. The TA concentration was determined
 by the standard addition method (example for a surface sample in Figure
 S1) using commercial TA (ACS reagent, Sigma-Aldrich, Switzerland, ≥ 99%)
 in ultrapure water (132 – 140 µM). Before analysis, the sample was
 introduced into a glass voltammetric cell, washed beforehand with
 acidified ultrapure water (pH 2, HCl), to limit any risk of contamination.
 7.00 ± 0.01 g of water were weighed directly into the cell under laminar
 flow hood before lowering the pH to 2 by adding ~70 µL of a diluted HCl
 solution (~1 mol L-1). Measurements were performed using a three-electrode
 electrochemical device (Metrohm model 663 VA) connected to a
 potentiostat/galvanostat (µAutolab® Type III, Utrecht, Holland),
 controlled by GPES 4.9 software. The working electrode was a Static
 Mercury Drop Electrode (SMDE) with a drop size of ~ 0.52 mm². An Ag/AgCl
 electrode (salt bridge in KCl 3 mol L-1, Suprapur®, Merck) and a glassy
 carbon electrode were used as reference and counter (auxiliary) electrodes
 respectively. After a degassing step under stirring (60 s, N2 99.99%), the
 "Hg-(TA)2" complex adsorbed on the working electrode (Figure S2)
 under the application of a potential of 0 V for a duration depending on
 the TA concentration in the sample (60 – 120 s). The redissolution of the
 complexes deposited on the electrode occurs during the cathodic scan in
 differential pulse mode from 0 to -0.6 V. To validate
 DOC measurements, an intercomparison was performed between the DOC
 concentrations measured by our SEC-OCD system and data acquired using the
 « classical » TOC-V method. We compared our measurements to the historic
 dataset from the P16 CLIVAR expedition at a station located at the same
 latitude but different longitude (20°S, 150°W) and it showed a good
 agreement between the two datasets in surface and deep waters.
 All the DOC concentrations measured within each fraction lof each
 sample largely exceeded the limits of detection determined by Dulaquais et
 al. (2018) for marine waters. Deep seawater reference (DSR) samples used
 to validate the DOC measurements were provided by the Hansell research
 laboratory (DOC<sub>DSR</sub> = 43.2 ± 1.7 µMC; n = 5;
 consensus value of lot #10 – 18: 43 – 45 µMC). For measurements by Ad-CSV,
 a reproducibility of 12% (n = 8) and a repeatability of 3.3% (n = 11) were
 calculated. For deposition times of 60 s and 300 s, limits of detection
 were estimated to be 81 nN and 22 nM, respectively.
description: # Low molecular weight reduced sulfur substances: a major component of
 non-volatile dissolved organic sulfur in the Pacific Ocean
 [https://doi.org/10.5061/dryad.d51c5b0bq](https://doi.org/10.5061/dryad.d51c5b0bq) | The objective is to evaluate the spatial and vertical contribution of reduced sulfur substances (RSS) detected by adsorptive cathodic stripping voltammetry (Ad-CSV) to the non-volatile dissolved organic sulfur (DOS) pool in the Pacific. We study the sources and fate of thioacetamide-like compounds at large scale. A combination of Ad-CSV and size exclusion chromatography with organic carbon detection (SEC-OCD) datasets contributes to a better understanding of the role of low molecular weight (LMW) RSS in the oceanic DOS cycle. | | :-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | # Description of the data and file structure **Table 1.** Description of the variables (i.e., columns) in the dataset. | **Column name** | **Definition** | **Units** | | :-------------- | :---------------------------------------------------------------------------------------------- | :------------ | | Station | Station number in accordance with the common nomenclature for sampling during the campaign. |   | | ID | Name of the sample in accordance with the common nomenclature for sampling during the campaign. |   | | Position | GPS coordinates. |   | | Depth | Sampling depth by CTD. | Meters (m) | | TA (nM) | Concentration of thioacetamide-like compounds measured by Ad-CSV. | nmol eqTA L-1 | | SD TA | Standard deviation on the concentration of thioacetamide-like compounds measured by Ad-CSV. | nmol eqTA L-1 | | DOC (µMC) | Concentration of dissolved organic carbon measured by SEC-OCD. | µmolC L-1 | | SD DOC | Standard deviation on the concentration of dissolved organic carbon measured by SEC-OCD. | µmolC L-1 |