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 Alteromonas macleodii ATCC 27126 growing on different substrates (glucose, laminarin, extracts from a Synechococcus culture, oligotrophic seawater and eutrophic seawater). We also characterized the DOM when A. macleodii was exposed to a lytic phage. The ultra-high resolution mass spectrometry showed that when growing on different substrates A. macleodii 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, A. macleodii 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.