N-acyl-homoserine lactones (AHLs) in intertidal marsh: diversity and potential role in nitrogen cycling

Aims: N-acyl-L-homoserine lactones (AHLs) based quorum sensing (QS) phenomenon is recognized as an effective agent for regulating bacterial growth and metabolism. However, diversity and biological role of AHLs in natural environments remain largely unknown. This study focuses on compositions of AHLs and their potential role in nitrogen transformation in intertidal marshes. Methods: We investigated the levels of AHLs in rhizosphere (Phragmites australis, Spartina alterniflora and Scirpus mariqueter) and non-rhizosphere soils from Chongming eastern intertidal wetland of the Yangtze Estuary using gas chromatography-mass spectrometry (GC/MS). Molecular techniques were employed to investigate the compositions and structure of bacterial community. Soil-slurry experiments in combination with nitrogen isotope tracing technique were conducted to examine the effects of AHLs on nitrogen transformation processes. Results: AHLs were higher in rhizosphere than non-rhizosphere soils, and correlated significantly with soil organic carbon and pH. C6-, C8-, C10- and C12-HSLs were identified as the probably universal QS signal molecules in intertidal marsh soils. The bacterial biodiversity might be regulated by AHLs, and 12 of the 50 most abundant microorganisms in intertidal marsh soils were related significantly with AHL concentrations. Furthermore, C6-, C8- and C12-HSLs might play an important role in mediating nitrogen transformation processes including nitrification, denitrification and anaerobic ammonium oxidation (anammox). Conclusions: AHLs are favored to accumulate in rhizospheric environments and thus further mediate the bacterial diversity and soil nitrogen cycling. This study highlights the associations of QS signal molecules with the dynamics and activity of nitrogen cycling bacteria in intertidal marshes.