| Summary: | IMBeR Future Oceans 2, 17-21 June 2019, Brest, France Interactions between microbial communities and dissolved organic matter (DOM) include microbial uptake, transformation and release, affecting the molecular composition of the heterogeneous DOM pool. Conversely, DOM composition can select for specific microbial taxa. Here, we explore the diversity of microbial (both Bacteria and Archaea) communities (Illumina tag sequencing of 16S rRNA genes) and the chemodiversity of dissolved organic molecules (Fourier-transform ion cyclotron resonance mass spectrometry, FT-ICR-MS). Sampling was conducted in the Eastern North Atlantic Ocean off the Galician coast (43ºN, 9º-15ºW) from 100 m to >5000 m, thereby encompassing a wide variety of water masses with contrasting origins and different aging. Eastern North Atlantic Central Water (ENACW) displayed lower values of AOU (35.3-77.1 µmol/kg) than those measured in the Mediterranean Water (MW, 98.8 µmol/kg) and the underlying North East Atlantic Deep Water (NEADW, 95.7 µmol/kg) and the Lower Deep Water (LDW, 107.2 µmol/kg). FT-ICR-MS average molecular composition of Solid Phase Extracted (SPE) DOM samples was C871H1087O415N18S2P1, with a mean molecular mass of 432.1 Da. Highly unsaturated molecular formulae were the most abundant (90.1%) followed by unsaturated (5.1%) and aromatic molecular formulae (3.0%). Potential peptides, carbohydrates and polycyclic aromatics accounted, each, for less than 1% of the total composition. There was no significant relationship between the FT-ICR-MS elemental composition and molecular mass with depth or AOU values, except for the H/C ratio, which correlated negatively with AOU. Highly unsaturated molecular formulae correlated significantly and positively with AOU whereas unsaturated aliphatics and potential peptides also correlated significantly, but negatively, with AOU. Accordingly, the relative abundance of some bacterial phylotypes (such as Acidobacteria, SAR202 and Chlamydiales) increased their relative contribution to community ...
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