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Rivers carry large amounts of dissolved organic matter (DOM) to the oceans thereby connecting terrestrial and marine element cycles. Photo-degradation in conjunction with microbial turnover is considered a major pathway by which terrigenous DOM is decomposed. To reveal globally relevant patterns behind this process, we performed photo-degradation experiments and year-long bio-assays on DOM from ten of the largest world rivers that collectively account for more than one-third of the fresh water discharge to the global ocean. We furthermore tested the hypothesis that the terrigenous component in deep-sea DOM may be far higher than biomarker studies suggest, because of the selective photochemical destruction of characteristic biomolecules from vascular plants. DOM was molecularly characterized by a combination of non-targeted ultrahigh-resolution mass spectrometry and quantitative molecular tracer analyses. We show that the reactivity of DOM is globally related to broad catchment properties. Basins that are dominated by forest and grassland export more photo-degradable DOM than other rivers. Chromophoric compounds are mainly vascular plant-derived polyphenols, and partially carry a pyrogenic signature from vegetation fires. These forest and grassland dominated rivers lost up to 50% of dissolved organic carbon (DOC) during irradiation, and up to 85% of DOC was lost in total if subsequently bio-incubated for 1 year. Basins covered by cropland, on the other hand, export DOM with a higher proportion of photo-resistant and bio-available DOM which is enriched in nitrogen. In these rivers, 30% or less of DOC was photodegraded. Consistent with previous studies, we found that riverine DOM resembled marine DOM in its broad molecular composition after extensive degradation, mainly due to almost complete removal of aromatics. More detailed molecular fingerprinting analysis (based on the relative abundance of >4000 DOM molecular formulas), however, revealed clear differences betwee

We hypothesized that microbial and photochemical processing of dissolved organic matter (DOM) determines its molecular formula composition in aquatic systems to a greater degree than does the original source of the DOM. To test this hypothesis, we exposed DOM from a leachate of a wetland plant (Juncus effusus) to solar radiation or incubated it in the dark for 1.25 yr. Analysis of the extracted DOM of the leachates via Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) identified 2800 molecular formulae. Of the formulae in the initial DOM, 11% were lost during microbial decomposition in the dark and 54% under solar radiation. Solar radiation also produced a large number of formulae containing N, that were preferentially degraded by microorganisms (47% loss). We compared the “recalcitrant formulae”, i.e. those not degraded in the experiment, with those of DOM from the deep North Pacific Ocean. Of the deep sea DOM formulae, 18% were present in the recalcitrant fraction of the initial DOM. An additional 18% of the formulae in marine DOM were photoproduced and recalcitrant, and 8% were produced by microbes in the experiment. Consequently, 44% of the deep sea DOM shares identical molecular formulae with the recalcitrant DOM from the experiment, most of which were produced by the combined action of sunlight and microbes. This indicates that processes in the water column may be more important than the original source in determining the composition of bulk DOM.