N-acyl homoserine lactones (AHLs) are molecules produced by many Gram-negative bacteria as mediators of cell-cell signaling in a mechanism known as quorum sensing (QS). QS is widespread in marine bacteria regulating diverse processes, such as virulence or excretion of polymers that mediate biofilm formation. Associated eukaryotes, such as microalgae, respond to these cues as well, leading to an intricate signaling network. To date, only very few studies attempted to measure AHL concentrations

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... phototrophic microbial communities, which are hot spots for bacteria-bacteria as well as microalgae-bacteria interactions. AHL quantification in environmental samples is challenging and requires a robust and reproducible sampling strategy. However, knowing about AHL concentrations opens up multiple perspectives from answering fundamental ecological questions to deriving guidelines for manipulation and control of biofilms. Here, we present a method for sampling and AHL identification and quantification from marine intertidal sediments. The use of contact cores for sediment sampling ensures reproducible sample surface area and volume at each location. Flash-freezing of the samples with liquid nitrogen prevents enzymatic AHL degradation between sampling and extraction. After solvent extraction, samples were analyzed with an ultra-high performance liquid chromatography-high resolution mass spectrometry (UHPLC-HRMS) method that allows to baseline-separate 16 different AHLs in less than 10 min. The sensitivity of the method is sufficient for detection and quantification of AHLs in environmental samples of less than 16 cm 3 . N-acyl homoserine lactones (AHLs) are a class of molecules which are produced by many Gram-negative bacteria during a signaling process called quorum sensing (QS) (Waters and Bassler 2005) . Structurally, the lactone of the amino acid homoserine is functionalized via an amide bond with acyl chains that differ in their degree of saturation, length, and chemical functionalization, such as hydroxyl-and carbonylgroups (Waters and Bassler 2005). AHLs can be synthesized by This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.