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Bacteria play a key role in sustaining the chemodiversity of marine dissolved organic matter (DOM), yet there is limited direct evidence of a major contribution of bacterial exometabolites to the DOM pool. This study tests whether molecular formulae of intact exometabolites can be detected in natural DOM via untargeted Fourier‐transform ion cyclotron resonance mass spectrometry (FT‐ICR‐MS). We analyzed a series of quantitative mixtures of solid‐phase extracted DOM from the deep ocean, of a natural microbial community and selected model strains of marine bacteria. Under standard instrument settings (200 broadband scans, mass range 92–1000 Da), 77% of molecular formulae were shared between the mesocosm and marine DOM. However, there was, Deutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/501100001659

ASLO 2021 Aquatic Sciences Meeting, Aquatic Sciences for a Sustainable Future: Nurturing Cooperation, 22–27 June 2021, Knowledge of dynamic interactions between organic matter (both dissolved and particulate) and microbial communities is critical to draw the routes of organic matter degradation or transformation, but also to understand the evolution and succession of microbial communities in ecosystems. Microorganisms are key mediators in the formation, transformation, and storage of organic matter in all aquatic environments such as marine, freshwater, and sediments. At the same time, organic matter provides the energetic foundation for the growth of heterotrophic microorganisms, affecting their community structure and metabolic potential. The combination of ”omic”-based approaches with novel analytical methods has revolutionized our ability to probe the interactions between marine microbial communities and organic matter. However, new interdisciplinary approaches are needed to move from this descriptive level to a better quantitative, process‐level understanding of the roles of microbes in biogeochemical cycles. In this session, we will explore and discuss recent advances and limitations of how organic matter (dissolved molecules, colloids, gels and particles) and microorganisms (bacteria, archaea, protozoa, fungi) can be integrated into our understanding of biogeochemical cycles to better predict functional diversity patterns, fluxes of matter (C/N/S/P…), and the impact of environmental change across microbial aquatic systems. We need to nurture cooperation between organic biogeochemists and microbial ecologists, between freshwater and marine scientists, between lab bench and computational data analysts, and include scientists from different fields who are interested in advancing the progress at the intersection of microbiology and organic matter biogeochemistry

Dissolved organic matter (DOM) in the ocean is one of the largest carbon pools on earth, similar in size to atmospheric CO2. Due to its richness in energy and nutrients it is fundamental for marine food webs and for microbial life. The microbial loop is an essential compartment in the global carbon cycle and is important for the transformation and recycling of organic matter and nutrients in the oceans. Microbial communities shape the molecular composition of DOM and vice versa. Earlier studies have shown that seasonal dynamics in DOM composition and microbial communities exist. A central aim of the study was to explore and characterize variations in composition of bacterial communities and DOM over much shorter periods of time, ranging from hours to days. Results revealed that there was no relation between the composition of DOM and the bacterial community neither in daily sampling nor in hourly sampling. Two significantly different sample groups were distinguishable in both daily and hourly sets of samples of bacterial communities and molecular DOM composition. In contrast DOM samples taken hourly did not group significantly. Multivariate statistics (dbRDA) reveal that variations in bacterial community and DOM composition are mainly driven by salinity changes. we therfore conclude that salinity and molecules typical for marine and terrestrial organic matter are significantly correlated. Further information on bacterial community structure will be available through sequencing of bacterial DNA.

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The western tropical South Pacific (WTSP) Ocean has been recognized as a global hot spot of dinitrogen (N₂) fixation. Here, as in other marine environments across the oceans, N₂ fixation studies have focused on the sunlit layer. However, studies have confirmed the importance of aphotic N₂ fixation activity, although until now only one had been performed in the WTSP. In order to increase our knowledge of aphotic N₂ fixation in the WTSP, we measured N₂ fixation rates and identified diazotrophic phylotypes in the mesopelagic layer along a transect spanning from New Caledonia to French Polynesia. Because non-cyanobacterial diazotrophs presumably need external dissolved organic matter (DOM) sources for their nutrition, we also identified DOM compounds using Fourier transform ion cyclotron resonance mass spectrometry (FTICRMS) with the aim of searching for relationships between the composition of DOM and non-cyanobacterial N₂ fixation in the aphotic ocean. N₂ fixation rates were low (average 0.63 ± 0.07 nmol N L⁻¹ d⁻¹) but consistently detected across all depths and stations, representing ∼ 6–88 % of photic N₂ fixation. N₂ fixation rates were not significantly correlated with DOM compounds. The analysis of nifH gene amplicons revealed a wide diversity of non-cyanobacterial diazotrophs, mostly matching clusters 1 and 3. Interestingly, a distinct phylotype from the major nifH subcluster 1G dominated at 650 dbar, coinciding with the oxygenated Subantarctic Mode Water (SAMW). This consistent pattern suggests that the distribution of aphotic diazotroph communities is to some extent controlled by water mass structure. While the data available are still too scarce to elucidate the distribution and controls of mesopelagic non-cyanobacterial diazotrophs in the WTSP, their prevalence in the mesopelagic layer and the consistent detection of active N₂ fixation activity at all depths sampled during our study suggest that aphotic N₂ fixation may contribute significantly to fixed nitrogen inputs in this area and/or areas downstream of water mass circulation.