Sources and transformations of organic sulfur in the ocean

The flux of carbon from the surface of the ocean to the seafloor links CO2 in the atmosphere to long-term reservoirs of carbon in the deep ocean, sediments and rocks. Most of the organic carbon that sinks out of the photic zone is respired, but relatively large proportions of organic carbon may reach the deep ocean when the water column is anoxic, which means that microorganisms rely on nitrate, sulfate, and other electron acceptors for their metabolism. Anoxic zones of the ocean are expanding in response to global climate change, but the mechanisms by which they affect the sinking flux of organic carbon are still not understood.

We collected sinking particle samples from the anoxic zone off of Mexico (the Eastern Tropical North Pacific or ETNP) to test the hypothesis from genomic and other data that sinking particles support an active microbial sulfur cycle in internal particle microenvironments despite the presence of more energetically favorable electron acceptors in the water column. We also investigated how the production of sulfide during microbial sulfate reduction impacts the properties and preservation potential of organic matter within the particles. We find that both microbial sulfate reduction and organic matter sulfurization are active in ETNP particles, and the sulfide reacts rapidly (1-2 days) to form both organic sulfur and iron monosulfides. Both of these products form concurrently, which demonstrates that organic matter sulfurization and iron sulfide formation are kinetically competitive reactions that may sample the same pool of sulfide. Rapid sulfurization therefore has the potential to enhance the preservation potential of sinking OM in particles and may therefore drive enhanced OM burial in sediments underlying anoxic water columns. In future years, we will be returning to the ETNP to better understand the scale and drivers of this process, and to quantify the potential contribution of particle-hosted sulfuriation in expanded anoxic zones to global rates of marine carbon storage in coming decades.

About this series: The PAOC Colloquium is a weekly interdisciplinary seminar series that brings together the whole PAOC community. Seminar topics include all research concerning the physics, chemistry, and biology of the atmospheres, oceans and climate, but also talks about e.g. societal impacts of climatic processes. The seminars take place on Monday from 12-1pm. Contact paoc-colloquium-comm@mit.edu for more information and Zoom password.