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Semeniuk, DM, Bundy RM, Payne CD, Barbeau KA, Maldonado MT.  2015.  Acquisition of organically complexed copper by marine phytoplankton and bacteria in the northeast subarctic Pacific Ocean. Marine Chemistry. 173:222-233.   10.1016/j.marchem.2015.01.005   AbstractWebsite

Copper (Cu) is an essential micronutrient for marine phytoplankton, but can cause toxicity at elevated intracellular concentrations. The majority of Cu (>99.9%) in oceanic surface waters is bound to strong organic ligands, presumably produced by prokaryotes to detoxify Cu. Although laboratory studies have demonstrated that organically complexed Cu may be bioavailable to marine eukaryotic phytoplankton, the bioavailability of Cu organic complexes to indigenous marine phytoplankton has not been examined in detail. Using the carrier free radioisotope Cu-67 at an iron limited station in the northeast subarctic Pacific Ocean, we performed size fractionated short-term Cu uptake assays with three Cu(II)-chelates, and Cu-67 bound to the strong in situ ligands, with or without additions of weak Cu(I) ligands. Estimates of the maximum supply of inorganic Cu (Cu') to the cell surface of eukaryotic phytoplankton were unable to account for the observed Cu uptake rates from the in situ ligands and two of the three added Cu(II)-chelates. Addition of 10 nM weak organic Cu(I) ligands enhanced uptake of Cu bound to the in situ ligands. Thus, Cu within the in situ and strong artificial Cu(II) organic ligands was accessible to the phytoplankton community via various possible Cu uptake strategies, including; cell surface enzymatically mediated reduction of Cu(II) to Cu(I), the substrate of the high-affinity Cu transport system in eukaryotes; or ligand exchange between weak Cu-binding ligands and the cellular Cu transporters. During a 14-hour uptake assay, particulate Cu concentrations reached a plateau in most treatments. Losses were observed in some treatments, especially in the small size fractions (<5 mu m), corresponding with faster initial Cu uptake rates. This may indicate that Cu cycling is rapid between particulate and dissolved phases due to cellular efflux or remineralization by micrograzers. The acquisition of Cu from the strong in situ ligands puts into question the historic role attributed to Cu binding ligands in decreasing Cu bioavailability. (C) 2015 Elsevier B.V. All rights reserved.

Hopkinson, BM, Barbeau KA.  2007.  Organic and redox speciation of iron in the eastern tropical North Pacific suboxic zone. Marine Chemistry. 106:2-17.   10.1016/j.marchem.2006.02.008   AbstractWebsite

The organic and redox speciation of iron was examined in the strongly layered upper water column of the eastern tropical North Pacific, including oxic and sub oxic waters, in a region 100- 1300 km offshore. Suboxic conditions ([O-2] < 5 mu M) were found to affect the organic speciation of iron, and reduced dissolved iron, Fc(II), was present in the suboxic zone, but conditions were not sufficiently reducing to convert all iron to Fe(II). Dissolved iron concentrations in the suboxic zone were similar to concentrations found in oxic regions. Using a competitive ligand exchange-adsorptive cathodic stripping voltammetry (CLE-ACSV) method, natural ligands were found to have distinct characteristics in the oxic and suboxic waters with stronger ligands found in the suboxic zone. It is unusual to find stronger ligands below the euphotic zone, but their strength, logK(Fe'L) = 12.1-12.8, is within the range determined for surface ligands in other regions. These strong ligands may be the result of the unique chemistry of the suboxic zone stabilizing reduced or labile compounds, or they may be actively produced by microbes to enhance iron uptake. No onshore-offshore trends in ligand strength or concentration were detected suggesting the ligands may result from the inherent chemistry of the suboxic zone or production from denitrifiers, rather than the resident suboxic zone population of Prochlorococcus which were more abundant nearshore. A luminol-chemiluminescence based flow injection analysis (FIA) technique capable of detecting picomolar concentrations of Fe(II) was used to assess the redox state of iron in the suboxic zone and overlying oxic waters at a station 1300 km offshore. An elevated signal equivalent to 0.12-0.15 nM Fe(II), 21-24% of dissolved iron, was found only in the suboxic waters. Oxidation kinetics suggest that this Fe(II) is most likely produced by an in-situ process, as opposed to being transported from shelf sediment. The luminol-chemiluminescence Fe(II) method was systematically tested for inferences from reduced species potentially present in the suboxic zone to validate our Fe(II) results. Several species, V(IV) and V(111), produced significant signals, but considerations of the reducing state of the suboxic zone make it unlikely that reduced V is present. With additional information on the identity of the suboxic zone species provided by analysis of signal decay rate, it was determined that Fe(II) was the most reasonable source of the signal, and at minimum the chemiluminescence data allows us to set limits on the Fe(II) concentration in the offshore suboxic water column. (C) 2006 Elsevier B.V. All rights reserved.