Experimental Evidence for Abiotic Sulfurization of Marine Dissolved Organic Matter

Abstract

Dissolved organic sulfur (DOS) is the largest pool of organic sulfur in the oceans, and as such it is an important component of the global sulfur cycle. DOS in the ocean is resistant against microbial degradation and turns over on a millennium time scale. However, sources and mechanisms behind its stability are largely unknown. Here, we hypothesize that in sulfate-reducing sediments sulfur is abiotically incorporated into dissolved organic matter (DOM) and released to the ocean. We exposed natural seawater and the filtrate of a plankton culture to sulfidic conditions. Already after 1-h at 20 • C, DOS concentrations had increased 4-fold in these experiments, and 14-fold after 4 weeks at 50 • C, indicating that organic matter does not need long residence times in natural sulfidic environments to be affected by sulfurization. Molecular analysis via ultrahigh-resolution mass spectrometry showed that sulfur was covalently and unselectively bound to DOM. Experimentally produced and natural DOS from sediments were highly similar on a molecular and structural level. By combining our data with published benthic DOC fluxes we estimate that 30-200 Tg DOS are annually transported from anaerobic and sulfate reducing sediments to the oceans. Uncertainties in this first speculative assessment are large. However, this first attempt illustrates that benthic DOS flux is potentially one order of magnitude larger than that via rivers indicating that this could balance the estimated global net removal of refractory DOS.

Abiotic formation of dissolved organic sulfur in anoxic sediments of Santa Barbara Basin

“…It also highlights the fact that sulfurization reactions in SBB pore waters are not limited to CHO formulas and also involve a wide variety of compounds including nitrogen-containing DOM compounds. This latter point agrees with laboratory sulfurization experiments which show that abiotic sulfur incorporation reactions are non-selective and occur with a wide variety of DOM compounds including DON compounds (Pohlabeln et al, 2017;Gomez-Saez et al, 2017).…”

“…To identify DOS formulas that are potentially formed in shallow hydrothermal systems through abiotic sulfurization reactions, Gomez-Saez et al (2016) used FTICR-MS to track nine possible sulfur addition reactions that involve addition or removal of hydrogen and/or oxygen atoms (a total of 27 potential sulfurization reactions). The same approach has been used to verify experimentally the occurrence of abiotic sulfurization under sulfidic conditions of natural DOM, and DOM directly derived from algal cultures (Pohlabeln et al, 2017). This approach has also been used to follow the photochemical alteration of DOS from sulfidic pore water (Gomez-Saez et al, 2017).…”

Chemical Diversity and Biochemical Transformation of Biogenic Organic Sulfur in the Ocean

“…Although sulfonated organics generally account for 20-40% of all organic sulfur in marine sediments (Vairavamurthy et al, 1994), their sources and fates remain unclear. Both biotic and abiotic processes of organic matter diagenetic sulfurization may occur in the sulfidic sediments (Pohlabeln et al, 2017), whereby sulfate-reducing microorganisms might substantially contribute to sulfide incorporation into organic matter through the dissimilatory sulfur reduction pathway. Such diagenetically formed organic sulfur compounds might be buried during carbon sequestration as they may not be recognized by microbial enzymes (Wasmund et al, 2017).…”

Non-conservative Behavior of Dissolved Organic Matter and Trace Metals (Mn, Fe, Ba) Driven by Porewater Exchange in a Subtropical Mangrove-Estuary

“…These DOM compounds are indicative for biotransformation of terrestrial DOM and input of autochthonous microbial DOM (Kujawinski et al, 2004;Sleighter and Hatcher, 2008;Medeiros et al, 2015;Osterholz et al, 2016) as well as porewater-exchange-driven input of sulfurcontaining DOM from sulfidic porewaters (Schmidt et al, 2009;Seidel et al, 2014;Sleighter et al, 2014). Sulfur-enriched DOM is generally observed in sulfidic enviroments and can be the result of abiotic chemical reactions of sulfide with DOM (Schmidt et al, 2009;Gomez-Saez et al, 2016;Pohlabeln et al, 2017). As the respective DOM molecular formulae from the mangrove cluster were also related to the higher dissolved Ba, Mn, and TDN concentrations (Figure 6), we suggest that the mangrovefringed zone is strongly influenced by the input of microbially transformed mangrove DOM from the mangrove porewaters.…”

“…However, transformation processes leave characteristic imprints in the DOM molecular composition. For instance, the decrease of aromatic terrestrial DOM-compounds along a salinity gradient or the enrichment of sulfur-containing compounds along different redox-regimes (Seidel et al, 2014;Medeiros et al, 2015;Osterholz et al, 2016;Pohlabeln et al, 2017).…”