Are permeable coastal sediments hotspots for the formation of non-volatile dissolved organic sulfur (DOS) in the ocean?
Final Report Abstract
Organic sulfur compounds are ubiquitous in marine sediments. The main formation pathway of these compounds is the abiotic reaction of inorganic sulfur species with common biomolecules. Because of the importance of this process for global biogeochemical cycles, for oil field exploration, and for the preservation of the molecular paleorecord, there is a wealth of studies on the topic. Very little attention, however, has been paid to water-soluble compounds that may be formed during the process of sulfurization and that may eventually enter the oceanic water column as dissolved organic sulfur (DOS). Some sulfur compounds, in particular thiols, determine the mobility of hazardous elements and essential trace elements. Important climate-relevant volatile sulfur compounds originate from DOS. Thus, the marine cycling of DOS has both atmospheric and oceanic consequences. Despite this important role, the sources of marine DOS, its cycling in the ocean and its function for marine life are unknown. In this project, we sought to answer fundamental questions with respect to the formation and distribution of DOS in the ocean. We hypothesized that coastal sediments are prime sites for the formation of DOS. Joining the strength of the two partners in Brazil and Germany, we applied a combination of field sampling, laboratory experiments and elemental, isotopic and molecular analysis. Sampling included sharply contrasting regions in the German Wadden Sea and Amazonian mangroves, including the adjacent shelf regions on a large scale. We also studied the largest sulfidic basin on Earth, the Black Sea. In laboratory experiments, we simulated the conditions in coastal marine sediments and exposed natural DOM from the North Sea and from plankton communities to sulfidic conditions. Already after one hour at room temperature and more so after four weeks, new sulfur-containing compounds had been formed through spontaneous, abiotic reactions. We molecularly identified several thousand different DOS compounds. The same set of compounds occurred also in coastal marine sediments from various regions in the world, including tidal flats, mangroves, salt marshes and continental shelf sediments. We estimate that globally 30–200 Tg DOS are annually transported from sulfidic sediments to the oceans. Despite large uncertainties, this first estimate illustrates that benthic DOS flux is potentially one order of magnitude larger than riverine flux. We found that DOS from sediments quickly oxidizes when exposed to traces of air, and under the influence of sunlight, part of DOS is completely removed due to photochemical reactions. The remainder is surprisingly stable in oceanic waters and we could trace it over large distances across continental shelved towards the open ocean. In Earth’s history, large-scale oceanic anoxic events have caused dramatic fluctuations of life and global climate. The Black Sea is a model system in this context because it is the largest anoxic basin in the world, with sulfidic waters below 150 m depth and dissolved organic carbon (DOC) concentration ca. 2.5 times higher than in the oxygenated oceanic basins. We showed that, similar to coastal and marine sediments, DOM abiotically reacts with reduced sulfur species in the deep Black Sea. The resulting sulfurized compounds are likely resistant to microbial decomposition, which might be the reason for the accumulation of DOC in the Black Sea. In Earth’s history, enhanced carbon storage due to sulfurization of DOM likely exhibited an influence on global climate, because even today’s oceans store as much carbon in DOM as there is CO2 in the atmosphere.
Publications
- (2017) Experimental evidence for abiotic sulfurization of marine dissolved organic matter. Frontiers in Marine Science 4, 362, 1-11
Pohlabeln A, Gomez-Saez GV, Noriega-Ortega BE and Dittmar T
(See online at https://doi.org/10.3389/fmars.2017.00364) - (2017) Photochemical alteration of dissolved organic sulfur from sulfidic porewater. Environmental Science & Technology 51, 24, 14144- 14154
Gomez-Saez GV, Pohlabeln A, Stubbins A, Marsay CM and Dittmar T
(See online at https://doi.org/10.1021/acs.est.7b03713) - (2020) Metabolic activity analyses demonstrate that Lokiarchaeon exhibits homoacetogenesis in sulfidic marine sediments. Nature Microbiology 5, 248-255
Orsi WD, Vuillemin A, Rodriguez P, Coskun ÖK, Gomez-Saez GV, Lavik G, Morholz V, Ferdelman TG
(See online at https://doi.org/10.1038/s41564-019-0630-3)
