Chemical cues: Why basic peptides are signal molecules in marine environments

Waterborne chemical cues are critical in mediating interactions among marine organisms, yet mechanisms controlling the extinction of these signals have not been described. Exogenous factors, such as uptake by bacteria or adsorption to suspended clays or biofilms, could influence the concentrations of environmental signal molecules. In this study, we chose glycyl-glycyl-L-arginine (GGR) to examine the rates and mechanisms of peptide uptake by a natural assemblage of marine bacteria. GGR is a potent synthetic analog of the natural cue-inducing settlement by oyster larvae. Kinetic parameters for the microbial uptake and colloidal adsorption of glycine, L-arginine, and GGR were determined from the decay of (14)C-radiolabeled substrates and from high-performance liquid chromatography (HPLC) analyses. The bacterial uptake rate constant (k(substrate)) was 3.5-fold smaller for GGR (0.019 h(-1) for 40-1,000 nM substrate concentration) than for its component amino acids. Nonspecific adsorption of GGR onto colloids had a similar rate constant (0.021 h(-1)). HPLC analyses of peptide solutions did not indicate any build-up of glycine, L-arginine, glycyl-L-arginine, or glycyl-glycine, the potential proteolytic products of GGR, during experiments. Thus, it is unlikely that the nonradiolabeled portion of GGR was cleaved extracellularly before internalization. Our results show that the tripeptide GGR is taken up by marine bacteria at significantly slower rates than are its component amino acids. The slow utilization of GGR by bacteria and low adsorption to colloids could provide at least one rationale-signal persistence-for the selection of basic peptides as chemical cues in marine environments.