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 et al.. Mass spectrometry analysis was performed according to He et al.. 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.