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ArticleSubstantial oxygen consumption by aerobic nitrite oxidation in oceanic oxygen minimum zones(Nature Research, 2021-12-02) Beman, J. Michael ; Vargas, Sonia M. ; Wilson, Jesse M. ; Perez-Coronel, Elisabet ; Karolewski, Jennifer S. ; Vazquez, Samantha ; Yu, Angela ; Cairo, Ariadna E. ; White, Margot E. ; Koester, Irina ; Aluwihare, Lihini I. ; Wankel, Scott D.Oceanic oxygen minimum zones (OMZs) are globally significant sites of biogeochemical cycling where microorganisms deplete dissolved oxygen (DO) to concentrations <20 µM. Amid intense competition for DO in these metabolically challenging environments, aerobic nitrite oxidation may consume significant amounts of DO and help maintain low DO concentrations, but this remains unquantified. Using parallel measurements of oxygen consumption rates and 15N-nitrite oxidation rates applied to both water column profiles and oxygen manipulation experiments, we show that the contribution of nitrite oxidation to overall DO consumption systematically increases as DO declines below 2 µM. Nitrite oxidation can account for all DO consumption only under DO concentrations <393 nM found in and below the secondary chlorophyll maximum. These patterns are consistent across sampling stations and experiments, reflecting coupling between nitrate reduction and nitrite-oxidizing Nitrospina with high oxygen affinity (based on isotopic and omic data). Collectively our results demonstrate that nitrite oxidation plays a pivotal role in the maintenance and biogeochemical dynamics of OMZs.
ArticleRefractory dissolved organic matter has similar chemical characteristics but different radiocarbon signatures with depth in the marine water column(American Geophysical Union, 2023-04-04) White, Margot E. ; Nguyen, Tran B. ; Koester, Irina ; Lardie Gaylord, Mary C. ; Beman, J. Michael ; Smith, Kenneth L. ; McNichol, Ann P. ; Beaupré, Steven R. ; Aluwihare, Lihini I.The >5,000‐year radiocarbon age (14C‐age) of much of the 630 ± 30 Pg C oceanic dissolved organic carbon (DOC) reservoir remains an enigma in the marine carbon cycle. The fact that DOC is significantly older than dissolved inorganic carbon at every depth in the ocean forms the basis of our current framing of the marine DOC cycle, where some component persists over multiple cycles of ocean mixing. As a result, 14C‐depleted, aged DOC is hypothesized to be present as a uniform reservoir with a constant 14C signature and concentration throughout the water column. However, key requirements of this model, including direct observations of DOC with similar 14C signatures in the surface and deep ocean, have never been met. Despite decades of research, the distribution of Δ14C values in marine DOC remains a mystery. Here, we applied a thermal fractionation method to compare operationally defined refractory DOC (RDOC) from different depths in the North Pacific Ocean. We found that RDOC shares chemical characteristics (as recorded by OC bond strength) throughout the water column but does not share the same 14C signature. Our results support one part of the current paradigm—that RDOC is comprised of structurally related components throughout the ocean that form a “background” reservoir. However, in contrast to the current paradigm, our results are consistent with a vertical concentration gradient and a vertical and inter‐ocean Δ14C gradient for RDOC. The observed Δ14C gradient is compatible with the potential addition of pre‐aged DOC to the upper ocean.