Microbial consortia are exquisitely sensitive to chemical changes in their surroundings and the diversity of microbial communities evolves with the composition of available growth substrates and nutrients. In aquatic systems, a significant pool of growth substrates resides within dissolved organic matter (DOM), one of the largest reservoirs of reduced carbon on the Earth’s surface. The molecular complexity of DOM and the processes by which it is modified parallel the complexity of the marine microbial web: a consortium of autotrophic, heterotrophic and mixotrophic microbes and viruses linked by interactions with DOM. Although genome sequences can offer an initial glimpse into DOM production by microbes, the composition of DOM released by protozoan grazing or viral lysis cannot be reliably predicted (or estimated) with genome‐enabled metabolic reconstructions. Emerging techniques in chromatography-based mass spectrometry are now capable of providing molecular‐level compositions of complex mixtures such as mortality‐driven DOM and microbial exudates. Because DOM composition is intimately linked to microbial metabolism, the molecular insights derived here will explicitly inform our understanding of microbial connectivity in the surface ocean.
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