Changing diets – microbial remineralization of primary producer exudates from reefs during phase shifts

In oligotrophic areas such as coral reefs, there is a tight transfer of carbon between the organisms inhabiting these densely packed and biodiverse ecosystems. Benthic holobionts, such as corals and algae, exude part of their photosynthates into the water column as dissolved organic matter (DOM). This DOM serves as the main food source for free-living microbial communities influencing their abundance, community structure, and metabolism. Ongoing phase shifts from coral- to algae dominated reefs alter the bulk composition of primary producer derived DOM which contributes to the microbialization of reefs. DOM from different sources representing varying stages of coral-, macroalgae-, and turf algae dominated reefs were subjected to 28h microbial remineralization experiments. We used untargeted HR LC-MS/MS to characterize a portion of the DOM pool (~30%). We explored putative chemical changes by mining redundant mass shifts within the MS/MS networks and, if possible, annotated them with known chemical groups (e.g., methyl-, alkene-, ketones). Where previously this method was used to link different mass shift profiles to specific benthic holobionts (i.e. shifts within samples), we now expand on this approach to quantify the chemical modifications occurring during the microbial remineralization of DOM (i.e. shifts between samples). This method allows comparing chemical modifications occurring during the microbial remineralization of DOM mimicking reefs in varying stages of the phase shift. The combined MS/MS networks (from all remineralization experiments) yielded 6381 mass shifts between networked features, that comprised 135 distinct masses, of which 50 could be annotated as an addition or removal of known chemical groups (e.g., 18.0 is a gain or loss of H₂0). The novel approach sheds light onto the specific molecular transformations during remineralization of different forms of DOM of reef systems at different stages of degradation. The knowledge is pivotal to understand the molecular mechanisms behind algae induced, microbially mediated reef degradation that poses one of the biggest threats to coral reefs exposed to increasing anthropogenic pressure.