Quorum-sensing and the Carbon Cycle: Identifying Cell-density Dependent Organic Carbon Degradation among Marine Bacteria in Sinking Particles

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The vast majority of sinking organic carbon formed within the surface layer of the ocean is degraded in transit through the water column, and heterotrophic bacteria on sinking particles are important agents of this process. Recent work suggests that these bacteria have the ability to regulate organic carbon degrading metabolisms by communicating with one another via quorum sensing (QS). QS is a process where bacteria use cell-cell signaling to gauge the density of related cells in their environment for the purpose of coordinating metabolic responses among these related cells. In this study, researchers at the Woods Hole Oceanography Institution and the Marine Biological Laboratory will test the hypothesis that AHL-based QS systems are active in sinking marine particles by trapping sinking particles, and analyzing them for cell-cell signaling molecules that are diagnostic for active QS. Sinking marine particles contain abundant Proteobacteria. This group of bacteria utilizes a class of QS molecules called acylated homoserine lactones (AHL) for which the team has previously developed new pre-concentration and mass spectrometry methods for analyzing AHLs in sinking particles. Additionally, the team will also test the hypothesis that that genes regulated by AHL-based QS in sinking particles encode enzymes for organic matter degradation by constructing libraries of genomic DNA from sinking particles and screening these libraries for AHL production. This should enable sections of genomic DNA that contain AHL-regulated genes to be singled out and analyzed further for genes encoding hydrolytic enzymes without relying on sequence database searches. Finally, they will apply a "functional gene expression" strategy to definitively constrain whether hydrolytic enzymes are indeed the products of these AHL-regulated genes. In previous work, the research team found that some marine bacteria also secrete enzymes to degrade AHLs; consequently, they will will examine whether this is occurring on sinking particles using both functional gene expression assays and incubation-based experiments. This study of quorum sensing in sinking particles has the potential to reveal previously uncharacterized linkages between bacterial community composition and particle flux attenuation. The primary justification for the proposed study is that quorum sensing is one such connection. QS has been well-characterized in the biomedical literature, and, as such, is ripe for exploration in marine environments. Broader impacts: The project will support two graduate students, one of whom is specifically interested in pursuing a postdoctoral career path that will depend heavily on research conducted by undergraduates.