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Abstract Mercury (Hg) is a bioaccumulative neurotoxin that can concentrate to potentially harmful levels in higher levels of marine food webs following conversion to methylmercury (MeHg). This is of public health concern as seafood is a main protein source for many in the Pacific region. To better understand Hg partitioning and transformations in the Pacific Ocean, Hg species and phases were measured along a meridional section from Alaska to Tahiti in 2018. This allowed the description of Hg concentrations and speciation under a variety of biogeochemical conditions such as the Alaskan shelf, the oligotrophic North Pacific gyre, and near the hydrothermally active Loihi seamount. Filtered HgT concentrations were elevated below 1,000 m near the Loihi Seamount with an average concentration of 1.45 pM, possibly indicating enrichment from hydrothermal venting. Filtered MeHg concentrations were notably higher at depth at the equator and generally lower south of the equator. Total Hg in suspended particles was greatest in the upper 1,000 m near the Alaskan Shelf and decreased in concentration southward. Suspended particle MeHg was greatest in the surface ocean in the upper 300 m near the Intertropical Convergence Zone (ITCZ). For both HgT and MeHg, particle‐associated concentrations appear to be related to organic fraction, and concentrations decreased southward. In general, all measured Hg species had greater concentrations in the northern than southern Pacific Ocean consistent with prior measurements. 

Biological nitrogen fixation is the conversion of dinitrogen (N2) gas into bioavailable nitrogen by microorganisms with consequences for primary production, ecosystem function, and global climate. Here we present a compiled dataset of 4793 nitrogen fixation (N2-fixation) rates measured in the water column and benthos of inland and coastal systems via the acetylene reduction assay, 15N2 labeling, or N2/Ar technique. While the data are distributed across seven continents, most observations (88%) are from the northern hemisphere. 15N2 labeling accounted for 67% of water column measurements, while the acetylene reduction assay accounted for 81% of benthic N2-fixation observations. Dataset median area-, volume-, and mass-normalized N2-fixation rates are 7.1 μmol N2-N m−2 h−1, 2.3 × 10−4 μmol N2-N L−1 h−1, and 4.8 × 10−4 μmol N2-N g−1 h−1, respectively. This dataset will facilitate future efforts to study and scale N2-fixation contributions across inland and coastal aquatic environments. 

Methylmercury (MeHg) is a neurotoxin that bioaccumulates to potentially harmful concentrations in Arctic and Subarctic marine predators and those that consume them. Monitoring and modeling MeHg bioaccumulation and biogeochemical cycling in the ocean requires an understanding of the mechanisms behind net mercury (Hg) methylation. The key functional gene pair for Hg methylation,hgcAB, is widely distributed throughout ocean basins and spans multiple microbial phyla. While multiple microbially mediated anaerobic pathways for Hg methylation in the ocean are known, the majority ofhgcAhomologs have been found in oxic subsurface waters, in contrast to other ecosystems. In particular, microaerophilicNitrospina, a genera of nitrite-oxidizing bacteria containing ahgcA-like sequence, have been proposed as a potentially important Hg methylator in the upper ocean. The objective of this work was therefore to examine the potential of nitrifiers as Hg methylators and quantify total Hg and MeHg across three Arctic and Subarctic seas (the Gulf of Alaska, the Bering Sea and the Chukchi Sea) in regions whereNitrospinaare likely present. In Spring 2021, samples for Hg analysis were obtained with a trace metal clean rosette across these seas. Mercury methylation rates were quantified in concert with nitrification rates using onboard incubation experiments with additions of stable isotope-labeled Hg and NH₄⁺. A significant correlation between Hg methylation and nitrification was observed across all sites (R²= 0.34,p< 0.05), with the strongest correlation in the Chukchi Sea (R²= 0.99,p< 0.001).Nitrospina-specifichgcA-like genes were detected at all sites. This study, linking Hg methylation and nitrification in oxic seawater, furthers understanding of MeHg cycling in these high latitude waters, and the ocean in general. Furthermore, these studies inform predictions of how climate and human interactions could influence MeHg concentrations across the Arctic in the future.