Characterization of biologically reactive dissolved organic matter exuded by major reef primary producers

Marine primary producers release a portion of their fixed carbon budget into the surrounding environment as dissolved organic matter (DOM). Previous studies have shown that exuded DOM differs in both bulk quantity and quality between marine benthic producers, which influences the microbial community composition and metabolic function in reef waters. It was further demonstrated that DOM from reef-building constituents, stony corals and crustose coralline algae (CCA), engendered different microbial functional groups and smaller carbon removal rates than fleshy macro- or turf algae, which elicited higher rates of carbon removal and oxygen transformation. Characterization of this DOM produced by fleshy macroalgae and subsequent microbial remineralization has been limited to bulk quantification of organic carbon and the release and utilization of dissolved combined neutral sugars. Advances in analytical approaches and informatic tools to classify the diverse assemblages of compounds in natural samples has expanded our view of microbe-DOM interactions to a broader variety of chemical functional groups. The DOM exudates (exometabolites) were harvested from two corals (Pocillopora verrucosa and Porites lobata), two fleshy algae (Dictyota sp., and turfing algae), and CCA, then subsequently remineralized by ambient reef water microbes in 2-day dark batch culture experiments. High resolution liquid chromatography tandem mass spectrometry (LC-MS/MS) was used to characterize the DOM exudates before and after incubation with microbial communities. We identified 242 distinct chemical features that were biologically labile, including amino acids, facty acids and polyketides. This study represents one of the first comparisons of exometabolite composition and lability to improve our understanding of decomposition processes in coral reef ecosystems.