Molecular weight of dissolved organic matter in sediments of the Baltic Sea

Pore water dissolved organic matter average weighted apparent molecular weight (AMW) and abundancies of molecular weight (MW) fractions obtained by Gaussian decomposition of size exclusion chromatograms from pore water samples collected using Rhizon samplers from the water 2 cm above the water-sediment interface and the top 10 cm of the sediment cores at 2cm interval. The sediment cores were recovered using a GeMax sampler. These samples were collected between 2020 and 2021 during cruises onboard RV Oceania (Institute of Oceanology of the Polish Academy of Sciences), which took place in spring, autumn, and winter, to the Central and Southern Baltic Sea, i.e., Eastern Gotland Basin, Bornholm Basin, and Gdansk Basin. The pore water dissolved organic matter molecular weight was used for further evaluation of the sediment flux of dissolved organic carbon. The molecular weight fractionation was used for characterizing pore-water dissolved organic matter in the Baltic Sea sediments. AMW (Da) - averaged weighted apparent molecular weight. Molecular weight was measured by size exclusion chromatography (SEC) using a Shimadzu Nexera X2 UFLC system equipped with a TSKgel SuperAWM-H column. Dissolved organic matter was eluted with 0.15 M ammonium acetate (pH 7), and two sequential detectors were used. Absorbance was measured between 240 and 600 nm at 2 nm intervals using a Shimadzu SPD-M30. Retention times were converted to apparent molecular size based on a calibration curve using polystyrene sulfonate standards. For AMW, a spectral slope value between 275 and 295 nm (S275-295) was evaluated for each absorbance spectrum using log-transformed linear regression during SEC analyses at every second of retention time. The retention time was then converted into apparent MW after calibration and weighted average molecular weight (MW) was inferred. Abundances of low molecular weight (LMW), medium molecular weight (MMW), and high molecular weight (HMW) in the Dalton*Absorbance Unit were calculated by fractionation of MW chromatograms using absorption at 350nm signal. For this, each SEC-absorbtion chromatogram was fitted with three Gaussian peaks using "peakfit" function in MATLAB (MathWorks®). The three peaks were referenced as LMW for peaks centered at retention times equivalent to MW ~100-500 Da, MMW for the peaks centered at retention times equivalent to MW of ~500Da -1 kDa, and HMW for peaks centered at retention times equivalent to MW larger than 1 kDa for further analyses. The areas under the Gaussian peaks were referred to as abundancies of corresponding MW fractions.