Award: DEB-1542240

Marine Oxygen deficient zones (ODZs) are waters that are functionally devoid of oxygen. Without oxygen, some microbes are capable of converting nitrogen in the water into N2 gas which is then lost from marine waters. This loss of "fertilizer" from the ocean has impacts on phytoplankton growth and marine food webs, ultimately impacting fish and fisheries. Also, oxygen concentration itself is a control on fish distribution and changes in oxygen could further fish/fisheries distribution. While Oxygen deficient zones occupy a very small percentage of the ocean, they account for as much as half of the oceanic loss of N as N2. Moreover, the size of these regions are predicted to expand due to climate change during this century. The microbes that are capable of producing N2 gas are extremely diverse, and use several different biochemical pathways to carry out this process. They may occur both free-floating in the water and attached to small particles that are sinking from the surface waters and providing them a carbon source. We conducted three cruises to the Eastern Tropical North Pacific ODZ and made measurements of nitrogen and N2gas in the water, nitrogen and carbon in sinking particles, and studied the diversity and activity of the microbes responsible for carbon and nitrogen cycling. This grant work has contributed new oceanographic knowledge about changes in the eastern tropical North Pacific. We mapped the westward boundary of this ODZ and found the secondary nitrite peak that forms within the anoxic water extends continuously from the western Mexican coast out to 126 West longitude, a distance of about 3000 km. Our results suggest that there has been a rapid strengthening of the oxygen deficient zone. Oxygen concentrations in the eastern tropical North Pacific have decreased over the past 50 years, and the area where N loss occurs has expanded/intensified. This is likely due to global climate change. We developed new ocean technology, free floating incubating sediment trap systems, which have generated a series of ‘first ever’ measurements including measurements of sulfur transformations within the ODZ that were made within the waters where they occurred. We have studied the microbes in the ODZ by looking at their genes, their proteins and the small carbon compounds they make. One focus has been the phytoplankton Prochlorococcus which is abundant at the top of the ODZ at depths where there is enough light for it to conduct photosynthesis. We have shown that Prochlorococcus is expressing proteins to make organic carbon and produce oxygen in these waters, even though there is little to no measurable oxygen. This suggests that this oxygen is rapidly used by other organisms and Prochlorococcus supports a community of microbes that need small amounts of oxygen and would otherwise not be there. The sediment trap systems also allowed us to examine the content of particles deep in the ocean (1000m) and we detected protein fragments from Prochlorococcus at this depth, which are likely in the process of being broken down as they sink from more shallow waters. This project has provided research experience at sea to 28 undergraduate students, 18 graduate students and 5 post-docs. Of the 62 total scientists participating in the three cruises 38 were female and 24 were male. A total of 19 research groups from 8 institutions in the US and Mexico have been involved. In the summers of 2016-2019 we hosted a paid summer internship program that introduced 27 high school students to organic geochemistry. Last Modified: 03/11/2022 Submitted by: Gabrielle Rocap