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. 2022 Aug 5;8(31):eabm2249.
doi: 10.1126/sciadv.abm2249. Epub 2022 Aug 5.

Trace metal stoichiometry of dissolved organic matter in the Amazon plume

Martha Gledhill  1 , Adrienne Hollister  2 , Michael Seidel  3 , Kechen Zhu  1 , Eric P Achterberg  1 , Thorsten Dittmar  3   4 , Andrea Koschinsky  3
Affiliations

Trace metal stoichiometry of dissolved organic matter in the Amazon plume

Martha Gledhill et al. Sci Adv. .

Abstract

Dissolved organic matter (DOM) is a distinct component of Earth's hydrosphere and provides a link between the biogeochemical cycles of carbon, nutrients, and trace metals (TMs). Binding of TMs to DOM is thought to result in a TM pool with DOM-like biogeochemistry. Here, we determined elemental stoichiometries of aluminum, iron, copper, nickel, zinc, cobalt, and manganese associated with a fraction of the DOM pool isolated by solid-phase extraction at ambient pH (DOMSPE-amb) from the Amazon plume. We found that the rank order of TM stoichiometry within the DOMSPE-amb fraction was underpinned by the chemical periodicity of the TM. Furthermore, the removal of the TMSPE-amb pool at low salinity was related to the chemical hardness of the TM ion. Thus, the biogeochemistry of TMs bound to the DOMSPE-amb component in the Amazon plume was determined by the chemical nature of the TM and not by that of the DOMSPE-amb.

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Figures

Fig. 1.
Fig. 1.. Map of the study area showing sampling sites.
Depth contours are for 25, 200, and 1000 m. Symbol colors indicate samples collected in the estuary (red), southwest of the estuary off a belt of mangrove forests (orange), and in the North Brazil Current (yellow). Labels show station numbers. Salinity was interpolated from data recorded using the ship’s underway system. For cruise track, see fig. S1.
Fig. 2.
Fig. 2.. Biogeochemical characteristics of the study area.
Variation in (A) DIP, (B) pH (expressed on the total scale), (C) chlorophyll a (chl a), and (D) DOC with salinity in the study area. Colors show samples collected in the estuary (red), southwest of the estuary off a belt of mangrove forests (orange), and in the North Brazil Current (yellow).
Fig. 3.
Fig. 3.. Relationship between DOMSPE-amb components and salinity in the study area.
Salinity-property plots are shown for ultraviolet absorbance at 254 nm (A254), ion abundances in positive and negative ionization modes (EICpos and EICneg, respectively), and total concentrations of elements determined in DOMSPE-amb samples collected in our study area (parameter identity is indicated in the facet heading). Results shown here were calculated from the sum of the peak areas observed in SEC data. Vertical bars represent the analytical uncertainty associated with element concentrations. Colors show samples collected in the estuary (red), southwest of the estuary off a belt of mangrove forests (orange), and in the North Brazil Current (yellow). μAU, micro–arbitrary units.
Fig. 4.
Fig. 4.. Estuarine behavior of TMs associated with DOMSPE-amb compared to observed dissolved (<0.2 μm) TM concentrations.
TMSPE-amb is shown as filled circles colored according to sample origin, and dissolved TM concentrations are shown as open triangles. Here, we assume that TMs bound to DOM were extracted with the same efficiency as determined for DOC (8 ± 2%). Concentrations of TMs determined in DOMSPE-amb were therefore multiplied by a factor of 12.5 for this comparison. Dissolved TM concentrations are those for samples collected from surface waters in close proximity to DOMSPE-amb samples (see fig. S1). We show TMSPE-amb values obtained on analysis by SEC. Vertical bars represent analytical uncertainties associated with dissolved trace element concentrations. Uncertainties associated with the TMSPE-amb fraction are omitted for clarity.
Fig. 5.
Fig. 5.. Elemental stoichiometry observed in MMW and Hphob fractions.
Elements are ranked on the x axis according to their stoichiometry in the MMW fraction. C:element ratios are plotted as log10 values using a quasi-random density plot. “Total dissolved” points show the ratio calculated for dissolved elements to DOC using concentrations of S as sulphate (calculated from salinity), DIP, and total dissolved TM concentrations. Predicted values were obtained from calculations of TM speciation at ambient salinity, pH, and dissolved TM and DOC concentrations, assuming that binding sites in DOM scale to DOC. We assumed that TM-DOM binding could be represented by the NICA model combined with the Donnan model for electrostatic interactions. For calculations of C:Fe ratios, we assumed competition between NICA binding sites and the formation of Fe(OH)3(s) (19).
Fig. 6.
Fig. 6.. Influence of chemical periodicity on TMSPE-DOM biogeochemistry in the Amazon estuary.
(A) The decrease in C:TM ratios (circles show log10 values for the MMW fraction as a quasi-random density plot) is consistent with trends in other thermodynamic stability constants of metal complexes as illustrated here for first hydrolysis constant (log10KMeOH). Cu, Co, and Mn were assumed to be present in the +2 oxidation state, while Fe was assumed to be in the +3 oxidation state. (B) The proportion of TMSPE-amb fraction that is removed at low salinity increased with polarizing power of the TM ion as expressed by the square of the oxidation state (z2) divided by Shannon’s ionic radii. Diamond symbols show the percentage removed for the total TMSPE-amb concentrations determined by SEC, squares removal of the MMW fraction, and circles removal of the Hphob fraction. For Mn and Co, the dashed arrow represents the increase in polarizing power that results from oxidation from the +2 oxidation state to the +3 oxidation state. Note the log10 scale for polarizing power.
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