Abstract
Despite of the major role ascribed to marine dissolved organic matter (DOM) in the global carbon cycle, the reactivity of this pool in the dark ocean is still poorly understood. Present hypotheses, posed within the size-reactivity continuum (SRC) and the microbial carbon pump (MCP) conceptual frameworks, need further empirical support. Here, we provide field evidence of the soundness of the SRC model. We sampled the high salinity core-of-flow of the Levantine Intermediate Water along its westward route through the entire Mediterranean Sea. At selected sites, DOM was size-fractionated in apparent high (aHMW) and low (aLMW) molecular weight fractions using an efficient ultrafiltration cell. A percentage decline of the aHMW DOM from 68–76% to 40–55% was observed from the Levantine Sea to the Strait of Gibraltar in parallel with increasing apparent oxygen utilization (AOU). DOM mineralization accounted for 30 ± 3% of the AOU, being the aHMW fraction solely responsible for this consumption, verifying the SRC model in the field. We also demonstrate that, in parallel to this aHMW DOM consumption, fluorescent humic-like substances accumulate in both fractions and protein-like substances decline in the aLMW fraction, thus indicating that not only size matters and providing field support to the MCP model.
Highlights
Most of the 662 Pg C of dissolved organic matter (DOM) accumulated in the oceans is resistant to microbial degradation and, stored for hundreds to thousands of years[1, 2]
Other works based on bacterial degradation experiments have found discrepancies with the size-reactivity continuum (SRC) model[17, 18], as they observed that low molecular weight (LMW) DOM is biologically more reactive than high molecular weight (HMW) DOM
Nine stations were occupied along the Mediterranean Sea (Fig. 1)[27] for DOM sampling at different depths covering the whole water column
Summary
Nine stations were occupied along the Mediterranean Sea (Fig. 1)[27] for DOM sampling at different depths covering the whole water column. For the protein-like fluorescence (peak T) the contribution is higher in the less aged waters (low AOU), while for the humic-like fluorescence (peak C) the two DOM fractions presented a significant increase with AOU (Fig. 3A,B) This is indicative of the production of humic-like fluorescence per unit of carbon in parallel to the general trend of DOC and aHMW DOC consumption. Peak T exhibited a significant decrease in both DOM fractions with AOU (Fig. 3C), indicative of the utilization of this type of compounds during mineralization processes This finding suggests that small organic compounds related to protein-like molecules were degraded when intermediate waters aged. It would be necessary to perform similar studies in the main water mass formation areas of the world ocean
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