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.针对固相萃取溶解态有机物（solid phase extracted DOM, SPE-DOM）的分离流程，取1升经0.7 μm GF/F滤膜过滤且酸化至pH 2的海水，采用500 mg规格的美国安捷伦Bond Elut PPL萃取小柱，按照Dittmar等人所述的方法进行萃取。质谱分析参照He等人的方法开展。将信噪比（S/N）阈值大于6的已鉴定分子式的总信号强度作为基准，对单个指定分子式的信号强度进行归一化处理。已鉴定分子式的双键当量（Double Bond Equivalents, DBE）与修正芳香性指数（Modified Aromaticity Index, AImod）均按照前文所述方法计算。