Quorum-sensing and the carbon cycle: identifying cell-density dependent organic carbon degradation among marine bacteria in sinking particles

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.  We 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) and we have developed new preconcentration and mass spectrometry methods for analyzing AHLs in sinking particles.

We will also test the hypothesis that that genes regulated by AHL-based QS in sinking particles encode enzymes for organic matter degradation. To test this hypothesis we will construct libraries of genomic DNA from sinking particles and screen these libraries for AHL production. This will 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.  We will then apply a “functional gene expression” strategy to definitively constrain whether hydrolytic enzymes are indeed the products of these AHL-regulated genes.  We have found that some marine bacteria also secrete enzymes to degrade AHLs, and we will examine whether this is occurring on sinking particles using both functional gene expression assays and incubation-based experiments.

The outcome of the chemical war between AHL-producing and AHL–degrading bacteria will only affect the degradation of sinking particles if AHL concentrations become sufficiently high in the sinking particles to induce organic-carbon degrading metabolisms (i.e. if a quorum is sensed).  We hypothesize that concentrations of AHLs reach sufficient levels to induce these metabolisms, and this will be tested by via physiological experiments with AHL-producing bacteria isolated from sinking particles.

Intellectual merit. Our proposed study of quorum sensing in sinking particles has the potential to reveal previously uncharacterized linkages between bacterial community composition and particle flux attenuation.   Our central 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.