Assessing Microbial Nitrogen Fixation in Ancient Euxinic Basins

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EAR-0525464Scientific Basis. Organic nitrogen and bulk nitrogen in ancient black shales is characteristically depleted in 15N. This could indicate that ecosystems fueled by nitrogen fixation were widespread during times of enhanced organic carbon preservation in ocean basins. The association of organic-carbon rich black shales with this isotopic signature for nitrogen fixation poses a paradox: the most productive waters (at least as measured by carbon preservation) in the ancient ocean were possibly starved for fixed nitrogen to fuel normal ocean photosynthesis. Indeed, oxygen depletion in oceanic deep waters would have facilitated microbial removal of nitrogen by denitrification and ammonia oxidation, yet, also favoring the release of phosphate from sediments. We postulate that significant nitrogen fixation is, in fact, to be expected within oxygen-deficient environments with excess dissolved phosphate and low dissolved N/P ratios. We seek to elucidate the relative significance of nitrogen fixation in euxinic (anoxic and sulfidic)environments, particularly at times of near global deep-sea oxygen depletion (so-called "Oceanic Anoxic Events").We propose to ascertain the isotopic signatures and relative contributions of nitrogen derived from cyanobacteria in oxic surface waters and phototrophic sulfur bacteria at or below shallow chemoclines reflected in preserved sedimentary records. We will target isotopic analyses of N and C in biomarkers derived from ancient chlorophyll structures and carotenoid pigment compounds. Isotopic and chemical studies of pigments in modern ecosystems and cultures will form a critical framework and inform our interpretation of ancient signatures. Notably, postdepositional overprints (water-column and sediment diagenesis) can obscure primary nitrogen isotopic signatures in bulk sediment nitrogen and organic nitrogen. Separation and analysis of the nitrogen in geoporphyins provides a powerful way of obtaining primary nitrogen isotope values for biomass produced in ancient euxinic settings.We will analyze sample suites from Lower and middle Cretaceous (Atlantic ODP/DSDP Sites, Italy, Western Interior US), Upper Permian (Japan and China), Upper Devonian (NY) and Proterozoic (AZ) for nitrogen isotope and specific biomarker compounds indicative of important microbial groups. The proposed research will also require sampling and analysis of modern euxinic water columns and sediments (Fayetteville-Green Lake, NY and the Black Sea) and selected culture experiments in order to further constrain the relative nitrogen isotope discrimination between porphyrins and cell biomass for important microbial groups.Broader Impacts. The proposed research will constitute Ph.D. dissertation work for Jamey Fulton and Chris Junium. It will facilitate international opportunities for these students in the laboratory of Dr. Brendan Keely, at the University of York in Great Britain. Two undergraduate summer interns will be co-sponsored by this project and mentored by the PIs. Support will be provided to an existing and successful internship program (currently funded by an IGERT award) that provides students with rich experiences in biogeochemical research at Penn State. This internship program has a strong track record in recruiting minority (27.9%) and female students (76.7%), and it provides scientific, professional and social mentoring for all participants.Eutrophic conditions currently promote seasonal hypoxia or anoxia in coastal regions of the U.S. (e.g., Long Island Sound, Chesapeake Bay, Louisiana shelf) and, in some cases, these conditions are worsening. As a result, the balance between N and P availability to primary producers is being altered, due in part to phosphate liberation from sediments and denitrification accompanying low dissolved oxygen concentrations in the water column (Carpenter and Capone, 1998). Our proposed research on the response of microbial systems to development of anoxia in deep time should bring a greater understanding of such ecosystems, allowing better predictions to be made for future impacts.