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

Waterborne chemical cues are critical in mediating interactions among marin e organisms, yet mechanisms controlling the extinction of these signals hav e not been described. Exogenous factors, such as uptake by bacteria or adso rption to suspended clays or biofilms, could influence the concentrations o f environmental signal molecules. In this study, we chose glycyl-glycyl-L-a rginine (GGR) to examine the rates and mechanisms of peptide uptake by a na tural assemblage of marine bacteria. GGR is a potent synthetic analog of th e natural cue-inducing settlement by oyster larvae. Kinetic parameters for the microbial uptake and colloidal adsorption of glycine, L-arginine, and G GR were determined from the decay of C-14-radiolabeled substrates and from high-performance liquid chromatography (HPLC) analyses. The bacterial uptak e rate constant (k(substrate)) was 3.5-fold smaller for GGR (0.019 h(-1) fo r 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-u p of glycine, L-arginine, glycyl-L-arginine, or glycyl-glycine, the potenti al proteolytic products of GGR, during experiments. Thus, it is unlikely th at the nonradiolabeled portion of GGR was cleaved extracellularly before in ternalization. Our results show that the tripeptide GGR is taken up by mari ne bacteria at significantly slower rates than are its component amino acid s. The slow utilization of GGR by bacteria and low adsorption to colloids c ould provide at least one rationale-signal persistence-for the selection of basic peptides as chemical cues in marine environments.