Increased microbial and substrate complexity result in higher molecular diversity of the dissolved organic matter (DOM) pool

For solid phase extracted DOM (SPE-DOM) isolation, one-liter filtered (0.7-μm GF/F) and acidified (pH 2) seawater was extracted using cartridges (500 mg, Agilent Bond Elut PPL, USA) as described by Dittmar <i>et al.</i>. Mass spectrometry analysis was performed according to He <i>et al.</i>. Individual assigned molecular formula signal intensity was normalized to the total identified molecular formulas intensities with S/N threshold greater than 6. Double bond equivalents (DBE) and modified aromaticity indices (AImod) of identified molecular formulas were calculated as previously described.