Li
Jingxuan
Li
Jingxuan
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ArticleStrong margin influence on the Arctic Ocean Barium Cycle revealed by pan‐Arctic synthesis(American Geophysical Union, 2022-03-22) Whitmore, Laura M. ; Shiller, Alan M. ; Horner, Tristan J. ; Xiang, Yang ; Auro, Maureen E. ; Bauch, Dorothea ; Dehairs, Frank ; Lam, Phoebe J. ; Li, Jingxuan ; Maldonado, Maria T. ; Mears, Chantal ; Newton, Robert ; Pasqualini, Angelica ; Planquette, Helene ; Rember, Robert ; Thomas, HelmuthEarly studies revealed relationships between barium (Ba), particulate organic carbon and silicate, suggesting applications for Ba as a paleoproductivity tracer and as a tracer of modern ocean circulation. But, what controls the distribution of barium (Ba) in the oceans? Here, we investigated the Arctic Ocean Ba cycle through a one-of-a-kind data set containing dissolved (dBa), particulate (pBa), and stable isotope Ba ratio (δ138Ba) data from four Arctic GEOTRACES expeditions conducted in 2015. We hypothesized that margins would be a substantial source of Ba to the Arctic Ocean water column. The dBa, pBa, and δ138Ba distributions all suggest significant modification of inflowing Pacific seawater over the shelves, and the dBa mass balance implies that ∼50% of the dBa inventory (upper 500 m of the Arctic water column) was supplied by nonconservative inputs. Calculated areal dBa fluxes are up to 10 μmol m−2 day−1 on the margin, which is comparable to fluxes described in other regions. Applying this approach to dBa data from the 1994 Arctic Ocean Survey yields similar results. The Canadian Arctic Archipelago did not appear to have a similar margin source; rather, the dBa distribution in this section is consistent with mixing of Arctic Ocean-derived waters and Baffin Bay-derived waters. Although we lack enough information to identify the specifics of the shelf sediment Ba source, we suspect that a sedimentary remineralization and terrigenous sources (e.g., submarine groundwater discharge or fluvial particles) are contributors.
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ArticleCanadian Arctic Archipelago shelf-ocean interactions: a major iron source to Pacific derived waters transiting to the Atlantic(American Geophysical Union, 2021-09-20) Colombo, Manuel ; Rogalla, Birgit ; Li, Jingxuan ; Allen, Susan E. ; Orians, Kristin J. ; Maldonado, Maria T.Continental shelves are important sources of iron (Fe) in the land-dominated Arctic Ocean. To understand the export of Fe from the Arctic to Baffin Bay (BB) and the North Atlantic, we studied the alteration of the Fe signature in waters transiting the Canadian Arctic Archipelago (CAA). During its transit through the CAA, inflowing Arctic Waters from the Canada Basin become enriched in Fe as result of strong sediment resuspension and enhanced sediment-water interactions (non-reductive dissolution). These high Fe waters are exported to BB, where approximately 10.7 kt of Fe are delivered yearly from Lancaster Sound. Furthermore, if the two remaining main CAA pathways (Jones Sound and Nares Strait) are included, this shelf environment would be a dominant source term of Fe (dFe + pFe: 26–90 kt y−1) to Baffin Bay. The conservative Fe flux estimate (26 kt y−1) is 1.7–38 times greater than atmospheric inputs, and may be crucial in supporting primary production and nitrogen fixation in BB and beyond.
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ArticleElement-selective targeting of nutrient metabolites in environmental samples by inductively coupled plasma mass spectrometry and electrospray ionization mass spectrometry(Frontiers Media, 2021-03-13) Li, Jingxuan ; Boiteau, Rene M. ; Babcock-Adams, Lydia ; Song, Zhongchang ; McIlvin, Matthew R. ; Repeta, Daniel J.Metabolites that incorporate elements other than carbon, nitrogen, hydrogen and oxygen can be selectively detected by inductively coupled mass spectrometry (ICPMS). When used in parallel with chromatographic separations and conventional electrospray ionization mass spectrometry (ESIMS), ICPMS allows the analyst to quickly find, characterize and identify target metabolites that carry nutrient elements (P, S, trace metals; “nutrient metabolites”), which are of particular interest to investigations of microbial biogeochemical cycles. This approach has been applied to the study of siderophores and other trace metal organic ligands in the ocean. The original method used mass search algorithms that relied on the ratio of stable isotopologues of iron, copper and nickel to assign mass spectra collected by ESIMS to metabolites carrying these elements detected by ICPMS. However, while isotopologue-based mass assignment algorithms were highly successful in characterizing metabolites that incorporate some trace metals, they do not realize the whole potential of the ICPMS/ESIMS approach as they cannot be used to assign the molecular ions of metabolites with monoisotopic elements or elements for which the ratio of stable isotopes is not known. Here we report a revised ICPMS/ESIMS method that incorporates a number of changes to the configuration of instrument hardware that improves sensitivity of the method by a factor of 4–5, and allows for more accurate quantitation of metabolites. We also describe a new suite of mass search algorithms that can find and characterize metabolites that carry monoisotopic elements. We used the new method to identify siderophores in a laboratory culture of Vibrio cyclitrophicus and a seawater sample collected in the North Pacific Ocean, and to assign molecular ions to monoisotopic cobalt and iodine nutrient metabolites in extracts of a laboratory culture of the marine cyanobacterium Prochorococcus MIT9215.
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ThesisSiderophore cycling in the North Pacific Subtropical Gyre(Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 2023-06) Li, Jingxuan ; Repeta, Daniel J.Across large regions of the surface ocean, the essential micronutrient iron (Fe) limits the growth of microbes, including phytoplankton and heterotrophic bacteria. In order to adapt to chronic low Fe conditions, some heterotrophic bacteria are able to produce siderophores, which are small organic ligands with extraordinarily high stability constant for Fe(III), to facilitate Fe uptake. The recently developed mass spectrometry based techniques allow for the characterization and quantification of siderophores in seawater for the first time, and the use of siderophores as biomarker of Fe limitation on heterotrophic bacteria. However, there has been no depth resolved transect data on siderophores distribution, and the prevalence, turnover, and of controlling factor of siderophores in the ocean remain unknown. In this thesis, I report the distribution of siderophores in the upper 1000 m of the US GEOTRACES GP15 Pacific Meridional Transect. Siderophore concentration ranges from 0-70 pM, and there is no correlation between the concentration of siderophores and dissolved Fe (dFe, <0.2 μm). In contrast, most siderophore hotspots, defined by concentration that is higher than 20 pM, are associated with a dFe:NO3- ratio that is lower than 50 μmol/mol, suggesting that the distribution of siderophores which are N rich molecules might be controlled by dFe:NO3- ratio. In the North Pacific Subtropical Gyre, hotspots of siderophores are found at 200-400 m. In the upwelling regions where nitracline is lifted to shallower depths, the hotspots are also lifted, to up to 20 m. The transect is dominated by marinobactins, a suite of non polar siderophores with a peptidic head group for Fe binding and a fatty acid chain. In addition, we found that the same siderophore could bind either Fe or Al, which is the first time Al-siderophores are reported in seawater samples. The hotspots of siderophore could be explained by either a dynamic turnover, or slow turnover and accumulation. To investigate the turnover of siderophores, I report the measurement of 57Fe uptake from siderophores by heterotrophic bacteria at Station ALOHA, which is representative for the North Pacific Subtropical Gyre - using enclosed bottle incubations. Between 200 and 400 m, we found almost complete consumption of 57Fe-siderophores for all of the siderophores over a period of 5 days. At the end of the incubation, approximately 60% of the added siderophores were present as nonmetallated apo siderophores. The presence of apo siderophores demonstrates that the drawdown of 57Fe-siderophore was largely due to Fe uptake from siderophores. Therefore, heterotrophic bacteria are Fe limited at 200-400 m, which is in accordance with the high concentration of siderophores in the North Pacific Subtropical Gyre. Our results reveal that the zone of potential Fe limitation in the ocean extends well below the euphotic zone and includes oligotrophic gyres that are not classically Fe limited.