DOM is organic material dissolved in seawater - containing mostly carbon, but also nitrogen, sulfur, oxygen, and other elements - that is smaller than 0.7 micrometers in size (tinier than a bacterium). The ocean contains as much carbon in DOM as there is carbon dioxide in the atmosphere and DOM can exist in the deep ocean for thousands of years, though no one is sure why it persists for so long.

New research suggests that the accumulation of DOM in the deep ocean occurs with minor input from organic sulfur compounds found in ocean sediments, and thus rejects one leading hypotheses for why it persists for so long. Michael Seidel, scientist at the ICBM at University of Oldenburg and co-author of the study explains, “Long-lived marine DOM compounds are so interesting because they bind carbon for a long time and can therefore no longer be released into the atmosphere as carbon dioxide.“ “Our paper helped address a lingering question in carbon sequestration, rejecting a theory of where some old carbon was coming from,” adds first author Alexandra Phillips, who conducted the research while at Caltech with Alex Sessions, professor of geobiology.

Phillips and Sessions led a team of researchers at Caltech, Scripps Institution of Oceanography, and the University of Oldenburg in Germany who tested the hypothesis that extremely long-lived DOM exists due to reactions with hydrogen sulfide in porewaters (the water that flows through sediments on the ocean floor), creating molecules that then leech from sediments into the ocean. Such reactions are known to make molecules more resistant to microbial degradation, and scientists have previously linked the creation of “sulfurized” organic matter in sediments to cooling events in Earth's history.

Alexandra Phillips and her colleagues used sulfur isotopes to test if these sulfurization reactions were also responsible for a significant portion of the deep ocean’s old carbon. They developed a way to analyze sulfur isotopes in organic material at very low levels, which in practical terms, reduced the volume to filtering only 10 liters of seawater per sample instead of 1,000 liters.

The team found sulfur isotope signals in marine DOM that did not match a sedimentary source. The samples, collected from the Pacific and Atlantic Oceans, had much more of the heavier sulfur-isotope than would be expected if the material originated from sulfurized organic matter. The researchers also noted that as the relative abundance of the heavy sulfur isotope decreased in their samples, the samples also showed an overall loss of sulfur relative to carbon. This further disproved the hypothesis that sulfurized organic matter was the missing source of the ocean’s oldest dissolved carbon.

Instead, the sulfur isotope ratios were nearly identical to those found in phytoplankton, the microscopic photosynthesizing organisms that comprise the base of the marine food web. The findings offer direct evidence that phytoplankton is the origin of the sulfur, and indirect evidence for the origin of DOM in general. However, it should be noted that sulfur-containing compounds comprise less than 10 percent of typical marine DOM. Alexandra Phillips says, “Marine organic sulfur is a lot more dynamic than we were expecting.” And Michael Seidel adds, “Now we have to find out how the sulfurized compounds from the porewater are broken down and why only a few percent survive in the deep ocean.” After all, researchers are looking for answers to these questions because it will help to understand the long-term fate of marine DOM in the context of carbon storage in the ocean.

Original publication
Alexandra A. Phillips, Margot E. White, Michael Seidel, Fenfang Wu, Frank F. Pavia, Preston C. Kemeny, Audrey C. Ma, Lihini I. Aluwihare, Thorsten Dittmar, and Alex L. Sessions (2022) Novel sulfur isotope analyses constrain sulfurized porewater fluxes as a minor component of marine dissolved organic matter. Proceedings of the National Academy of Sciences 119: e2209152119
https://www.pnas.org/doi/10.1073/pnas.2209152119

Contact
Dr Alexandra A. Phillips
Earth Sciences Department, University of California Santa Barbara, Santa Barbara, CA 93016
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Prof. Dr Alex L. Sessions
Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125
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Prof. Dr Thorsten Dittmar
ICBM, Institute for Chemistry and Biology of the Marine Environment
University of Oldenburg
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Dr Michael Seidel
ICBM, Institute for Chemistry and Biology of the Marine Environment
University of Oldenburg
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