Low molecular weight reduced sulfur substances: a major component of non-volatile dissolved organic sulfur in the Pacific Ocean

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 D..., 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.
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[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. |
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