Bottom-up and top-down controls on Alteromonas macleodii lead to different dissolved organic matter compositions

Bottom-up (e.g. substrate) and top-down (e.g. viral lysis) controls affect the abundance and diversity of marine bacterial communities. However, the effects of bottom-up and top-down controls on the molecular composition of dissolved organic matter (DOM) have not been evaluated in parallel. In this study, we investigated the DOM composition of the model bacterium <em>Alteromonas macleodii</em> ATCC 27126 growing on different substrates (glucose, laminarin, extracts from a <em>Synechococcus</em> culture, oligotrophic seawater and eutrophic seawater). We also characterized the DOM when <em>A. macleodii</em> was exposed to a lytic phage. The ultra-high resolution mass spectrometry showed that when growing on different substrates <em>A. macleodii </em>prefer to use reduced, saturated organic molecules (i.e. nitrogen-containing molecules (O4 formular species)) and released or preserved oxidized, unsaturated molecules (i.e. sulfur-containing formulas (O6 formular species)). When infected with the lytic phage, <em>A. macleodii</em> produced organic molecules with higher hydrogen saturation, and more nitrogen- or sulfur-containing molecules (N2O, N3O and NOS formular species). Our results provide molecular evidence that bottom-up (i.e. varying substrates) and top-down (i.e. viral lysis) controls leave different molecular fingerprints in the produced organic matter, which lead to different biogeochemical impacts in the ocean.