Award: OCE-1635414

In the small regions of the world?s ocean where oxygen is absent (anoxic zones), microbes undertake anaerobic respiration to fuel their metabolism. In turn, this removes the important nutrient nitrogen from the ocean in a process called denitrification, placing limits on phytoplankton growth and biological carbon storage in the ocean. Before this project, evidence had been mounting that anaerobic respiration also occurs much more extensively throughout low-oxygen (but not anoxic) waters that surround the anoxic zones, harbored inside sinking organic particles that contain tiny anoxic zones (microenvironments) in their cores. However, the conditions that result in microenvironment formation and the global impact they have on carbon and nutrient cycling were not well understood. This project developed numerical models of sinking organic particles that resolve internal particle chemistry, and combined them with a range of observations to quantify and elucidate microenvironment processes. These models revealed that anaerobic respiration can occur inside particles wherever low oxygen waters and intense respiration rates overlap ? a condition that often occurs in shallow subsurface waters directly above and surrounding the anoxic zones. We showed that denitrification under these conditions (when summed over the large volume of low oxygen waters) likely rivals the denitrification occurring in anoxic zones themselves, thus changing our understanding of where nitrogen removal occurs in the ocean. Our model-data synthesis showed that an additional consequence of microenvironment formation in particles is to dramatically slow down the decomposition of organic matter they carry. This may explain previous observations showing that particles are efficiently transferred through low oxygen waters, enhancing carbon delivery to the deep ocean where it is sequestered for hundreds of years. Finally, our work demonstrated that anoxic particle microenvironments may allow rapid production of the important greenhouse gas nitrous oxide to occur broadly through low oxygen waters, rather than only in anoxic waters as previously thought. This would process would help reconcile the observed distribution of nitrous oxide, which suggests widespread production, with microbial culture studies that indicate rapid production only occurs under anoxic conditions. Our work sheds new light on how ocean nutrient and greenhouse gas cycles will be impacted over the next century, when warming is expected to significantly reduce oxygen concentrations through much of the ocean interior, because gases are less soluble in warmer water. Based on our findings, this perturbation will result in more widespread denitrificiation, more efficient carbon transfer to depth, and enhanced nitrous oxide production in vast low oxygen regions, rather than just in the small anoxic zones of the ocean. Results from this project have been disseminated in six peer-reviewed publications (with at least two more in the pipeline) and presented at international scientific conferences. The project provided research opportunities to two undergraduate students, and the results it generated were incorporated into two different classes taught at the University of Rochester to illustrate feedbacks between climate change and ocean chemistry. Finally, the project supported the development of an outreach workshop focused on climate change and the ocean carbon cycle aimed at high school students from a low income school district in Rochester, NY. The 3-day workshop was offered twice to around 10 students each time, and introduced the students to scientific concepts and methods that are not accessible at their school. Last Modified: 02/20/2022 Submitted by: Thomas S Weber