Proton Binding Characteristics of Dissolved Organic Matter Extracted from the North Atlantic

Marine dissolved organic matter (DOM) presents key thermodynamic properties that are not yet fully constrained. Here, we report the distribution of binding sites occupied by protons (i.e., proton affinity spectra) and parametrize the median intrinsic proton binding affinities (log K̅H) and heterogeneities (m), for DOM samples extracted from the North Atlantic. We estimate that 11.4 ± 0.6% of C atoms in the extracted marine DOM have a functional group with a binding site for ionic species. The log K̅H of the most acidic groups was larger (4.01–4.02 ± 0.02) than that observed in DOM from coastal waters (3.82 ± 0.02), while the chemical binding heterogeneity parameter increased with depth to values (m1= 0.666 ± 0.009) ca. 10% higher than those observed in surface open ocean or coastal samples. On the contrary, the log K̅H for the less acidic groups shows a difference between the surface (10.01 ± 0.08) and deep (9.22 ± 0.35) samples. The latter chemical groups were more heterogeneous for marine than for terrestrial DOM, and m2 decreased with depth to values of 0.28 ± 0.03. Binding heterogeneity reflects aromatic carbon compounds’ persistence and accumulation in diverse, low-abundance chemical forms, while easily degradable low-affinity groups accumulate more uniformly in the deep ocean.


Strategy for the derivation of NICA-Donnan model parameters
The experimental datasets of titrant volume and pH (free scale) were converted into pH and charge curves using mass and charge balance relationships, as detailed in the supporting information of Lodeiro et al. 5 .The optimization of the NICA-Donnan parameters was carried out by non-linear regression using MATLAB to minimize the root-mean square error (RMSE, in mol•kg DOM -1 ) in the DOM charge: where Qi and  �  are the experimental and fitted values of DOM charge, respectively; N is the number of data points and l is the number of model parameters.We used the MATLAB function "fminsearchbnd", which finds the minimum of a bound constrained multivariable function using derivative-free method.Q values in mol•kg DOM -1 were subsequently transformed to mmol•mol C -1 using the calculated carbon content of the DOM extracts.The number of DOM binding sites is not affected by electrostatic effects, and so we considered that the ratio QmaxH,2/ QmaxH,1 remains constant when calculated the specific binding parameters.Table S3.Optimized NICA−Donnan (PB−VD) parameter values for proton binding at 25ºC.QmaxH,j: total amount of proton binding sites within each distribution; log  � H,j : median value of the j th affinity distribution for protons (the proton binding affinity); mj: chemical binding heterogeneity; a: fitted parameter of the VD equation 6 : , where I is the ionic strength, Q is the surface charge density and z de valence of the symmetric background electrolyte; RMSE: root−mean square error.

S8
We calculated median values of the intrinsic proton binding affinity for the DOM1 mode (log  � H,1 ) that are larger than the previously reported for coastal and terrigenous DOM (Figure S3).The values for surface (3.61 ± 0.03) and deep (3.64 ± 0.01) samples were similar.On the other hand, and despite the larger uncertainties obtained, the median values for the intrinsic proton binding affinities for the DOM2 groups (log  � H,2 ) showed a clear difference between the surface (9.0 ± 0.3) and deep (7.9 ± 0.4) samples (Figure S3).Both log  � H,2 values were close to the average reported for a coastal sample (8.6) and a generic fulvic acid (8.6 ± 1.06) and within the natural variability between terrestrial samples (7.2−10.9).

1 .
Figure S1.Sampling map.The line that joins the two red dots represent the area where the surface sample was collected whilst the ship was underway.The blue points are the two locations where the deep water samples (ca.500 m) were collected.

S5 4 .
Intrinsic and effective proton binding parameters calculated using a NICA-Donnan model with VD consistent with the non-linear Poissson-Boltzman equation (PB-VD) and fits to proton titration data.

Figure S2 .
Figure S2.NICA−Donnan fits to proton titration data of North Atlantic SPE−DOM at

5 .
Potentiometric titration data fitted to a standard VD Donnan modelExperimental acid−base titration curves for the surface and deep North Atlantic DOM were also fitted to the NICA−Donnan model using a standard VD equation (FigureS3).

Figure S3 .
Figure S3.NICA−Donnan fits to proton titration data of North Atlantic SPE−DOM at 25 °C, and 0.007, 0.1, 0.7 and 1.0 M ionic strength, using the standard VD Donnan model: surface (left panel) and deep samples (right panel).Symbols: experimental values.Colored lines correspond to the model fits at each ionic strength.The uppermost black curve corresponds to the 'master curve' charge (QmaxH,tot -QH) vs. pHD.

Figure S4 .
Figure S4.Median values of the intrinsic proton binding affinity (log  � H,i ) parameters obtained from the fits of the standard VD Donnan model to proton binding data shown in Figure S2 for the DOM1 (green bars) and DOM2 (blue bars) distributions.Bar heights indicate the mean, and error bars indicate the standard deviation.The values Baltic Kiel Fjord are from Lodeiro et al. 5 Bar heights indicate the mean, and error bars indicate the standard deviation.The values FA (Generic) indicate the generic fulvic acid parameters from Milne et al. 7 ; the error bar indicates the range of values for the datasets analyzed.

S9
We also calculated the intrinsic ion binding heterogeneity for North Atlantic solid-phase extracted dissolved organic matter (SPE−DOM) based on the standard VD Donnan model and found values for the low proton affinity groups that are ca.10% more homogeneous than a coastal DOM, and 35 % more homogeneous than a generic fulvic acid of terrestrial origin (FigureS5).The DOM1 distribution of the deep open ocean SPE−DOM presents an intrinsic binding heterogeneity of 0.81 ± 0.01, which is close to the maximum value of 1 expected for a perfect homogeneous ligand.

Figure S5 .
Figure S5.Width of the affinity distribution function (chemical binding heterogeneity, mi) parameters obtained from the fits of the standard VD Donnan model to proton binding data shown in Figure S2 for the DOM1 (green bars) and DOM2 (blue bars) distributions.The values for the Baltic (Kiel Fjord) are from Lodeiro et al. 5 Bar heights indicate the mean, and error bars indicate the standard deviation.The values FA (Generic) indicate the generic fulvic acid parameters from Milne et al. 7 ; the error bar indicates the range of values for the datasets analyzed.

Table S2 .
North Atlantic seawater micro-and macronutrient concentrations.