Molecular level characterization of the biolability of rainwater dissolved organic matter

The atmospheric wet deposition has been recognized as a significant allochthonous source of dissolved organic carbon (DOC) to the ocean. However, few studies have examined the biolability of rainwater dissolved organic matter (DOM) at the molecular level. Rainwater samples were collected and incubated with ambient microbes. DOC, UV-vis spectroscopy, formic acid (FA), acetic acid (AA), and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICRMS) were applied. Approximately 50 ± 16 % of rainwater DOC and ~90 % of FA and AA were bioconsumed within 28 days. The contribution of FA and AA to the total BDOC was ~30 %, which was the largest known biolabile fraction in rainwater DOC. In contrast, only approximately 15 % of formulae identified by FT-ICRMS were consumed, which were characterized by higher saturation, higher heteroatom content and lower modified aromaticity. Among the major high molecular weight secondary organic carbon (HWW-SOC)-like compounds, organosulfate contained the largest fraction of consumed formulae, while biogenic volatile organic-derived CHO compounds had the lowest. Our study for the first time provided both quantitative and qualitative understanding of the bioavailability of rainwater DOM, which is essential for understanding their effects on the biogeochemical cycles and the environmental health in the receiving waters.