Kinetic constraints on acylated homoserine lactone-based quorum sensing in marine environments

Abstract

Quorum sensing (QS) via acylated homoserine lactones (AHLs) was discovered in the ocean, yet AHLs are expected to be very short-lived at seawater pH due to rapid abiotic degradation. Quorum quenching, the enzymatic degradation of AHLs, is also likely. To better understand the potential for QS to regulate behaviors of marine bacteria, we investigated the degradation of a variety of AHL molecules in several types of seawater media. We did this by incubating AHLs and tracking their concentration using HPLC/electrospray-ionization mass-spectrometry (HPLC/ESI-MS). AHL concentrations decreased with time, and degradation rate coefficients were calculated by applying a first-order rate law. The rate of abiotic degradation showed strong dependence on acyl chain length and the presence of 3-oxo substitutions on the acyl chain. We found that the rate of abiotic degradation of AHLs in artificial seawater was much slower than that predicted by an oft-cited equation for non-marine media that takes only pH into account. However, AHLs degraded more rapidly in natural seawater than in artificial seawater, an observation we found to be due to quorum quenching enzyme activity. By applying calculated degradation rates in a simple steady-state calculation, we suggest that despite the observed quorum quenching activity, AHLs are likely to be viable signals in organic particles and in other microbial ‘hotpsots’ in marine environments.