Mar. Drugs 2014, 12, 352-367; doi:10.3390/md12010352 OPEN ACCESS marine drugs ISSN 1660-3397 www.mdpi.com/journal/marinedrugs Article Haloperoxidase Mediated Quorum Quenching by Nitzschia cf pellucida: Study of the Metabolization of N-Acyl Homoserine Lactones by a Benthic Diatom Michail Syrpas 1, Ewout Ruysbergh 1, Lander Blommaert 2, Bart Vanelslander 2, Koen Sabbe 2, Wim Vyverman 2,*, Norbert De Kimpe 1 and Sven Mangelinckx 1,* 1 Department of Sustainable Organic Chemistry and Technology, Faculty of Bioscience Engineering, Ghent University, Coupure Links 653, Ghent B-9000, Belgium; E-Mails: [email protected] (M.S.); [email protected] (E.R.); [email protected] (N.D.K.) 2 Laboratory of Protistology and Aquatic Ecology, Department of Biology, Ghent University, Krijgslaan 281-S8, Ghent B-9000, Belgium; E-Mails: [email protected] (L.B.); [email protected] (B.V.); [email protected] (K.S.) * Authors to whom correspondence should be addressed; E-Mails: [email protected] (S.M.); [email protected] (W.V.); Tel.: +32-(0)9-264-59-51 (S.M.); Fax: +32-(0)9-264-62-21 (S.M.); Tel.: +32-(0)9-264-85-01 (W.V.); Fax: +32-(0)9-264-85-99 (W.V.). Received: 5 November 2013; in revised form: 16 December 2013 / Accepted: 23 December 2013 / Published: 17 January 2014 Abstract: Diatoms are known to produce a variety of halogenated compounds, which were recently shown to have a role in allelopathic interactions between competing species. The production of these compounds is linked to haloperoxidase activity. This research, has shown that this system may also be involved in diatom-bacteria interactions via the H2O2 dependent inactivation of a type of quorum sensing (QS) molecule, i.e., N-β-ketoacylated homoserine lactones (AHLs), by a natural haloperoxidase system from the benthic diatom Nitzschia cf pellucida. The AHL degradation pathway towards corresponding halogenated derivatives was elucidated via HPLC-MS analysis and the synthesis of a broad series of novel halogenated AHL analogues as reference compounds. Furthermore, their biological activity as quorum sensing modulators was directly compared and evaluated against a series of naturally occurring β-keto-AHLs. It has been demonstrated that the loss of the QS activity results from the final cleavage of the halogenated N-acyl chain of the signal molecules. Mar. Drugs 2014, 12 353 Keywords: quorum sensing; haloperoxidase; degradation pathway; AHL; reference compounds; diatom-bacteria interactions 1. Introduction Algae are known as an important source of bioactive compounds with potential agrochemical and pharmaceutical applications [1]. Some of these bioactive metabolites include halogenated compounds [2]. These types of compound often demonstrate interesting biological properties including antifungal, anti-inflammatory, antifeedant, cytotoxic, insecticidal, and antibacterial activity [3]. Biohalogenation in marine organisms usually occurs via the action of haloperoxidases [4], which are enzymes that mediate the oxidation of halides by hydrogen peroxide via various mechanisms [5]. Until now, screening for bioactive halogenated compounds in marine environments has mainly been focused on macroalgae rather than on microalgae [2]. Research on diatoms, one of the most common type of marine microalgae, is even less widespread. Many bacteria use signal molecules, i.e., autoinducers, to control the expression of specific genes important for phenotypes, such as biofilm formation, bioluminescence, virulence expression, motility, conjugation, and symbiosis and this in a population density dependent manner [6]. This interbacterial signaling is widely known as quorum sensing (QS). N-Acylated homoserine lactones (AHLs) represent a major class of these signal molecules, and are commonly produced by Gram negative bacteria [7]. The QS system in these bacteria usually consists of two components: a LuxI homologue that synthesizes the AHL autoinducer, and a LuxR homologue which is an autoinducer-dependent transcriptional activator [7]. Different bacterial species have been shown to produce different AHLs [8]. The basic structure of AHLs consists of a homoserine lactone ring adjoined with an N-acyl chain. The acyl chain can vary in length from four to 14 carbon atoms, as can the degree of saturation and the chain may or may not contain an oxo- or hydroxy-group at the 3-carbon position [9]. A wide variety of AHLs, including various saturated and β-keto-AHLs, are found in marine stromatolite mats but also in intertidal surfaces colonized by algae and in intertidal rock pools [10,11]. AHLs are also known to modulate the settlement rate of green alga Ulva intestinalis [12]. The AHL producing Alphaproteobacteria, which are dominant in benthic diatom biofilms, were shown to have an influence on the diatom growth and the release of extracellular polymeric substances [13]. Across different kingdoms these kinds of interactions may exert wider ecological effects [14]. In aquatic environments, bacteria interact in various ways with other organisms as pathogens, symbionts or competitors in the search for resources [14–16]. Many marine organisms have developed defense or deterrence mechanisms, which are often involved in the production of small diffusible signal molecules that can interfere with the QS systems [17,18]. A well-studied example is the production of halogenated furanones by the red alga Delisea pulchra [19,20]. In addition, QS antagonistic activity has been demonstrated in algae from Caulerpaceae (green algae), Rhodomelaceae and Galaxauraceae families (red algae) [21], red macroalga Asparagopsis taxiformis [22], green microalga Chlamydomonas reinhardtii [23], freshwater green microalga Chlorella sacharophilla [24], gorgonian corals from the Caribbean reefs [25] and the marine sponge Luffariella variabilis [26,27]. Mar. Drugs 2014, 12 354 Furthermore, another way of QS disruption in nature occurs via the action of enzymes. Enzymatic degradation of QS signal molecules by acylases and lactonases of bacterial origin has been demonstrated [28]. In another pathway, haloperoxidases from Delisea pulchra and Laminaria digitata were shown to mediate the deactivation of β-keto-AHLs by electrophilic halogenation with bromine [29,30]. Several diatoms are linked with the release of reactive bromine and iodine which is influenced by temperature, H2O2 concentration, bromide (and iodide) concentration, light, and pH [31]. The release leads to the formation of a wide range of halocarbons, formed via rapid and aspecific reactions of these reactive halogen intermediates with many organic molecules. Furthermore, this event plays a substantial role in oceanic bromoform production via the haloform reaction [32]. Interestingly, the production of volatile halocarbons by several Nitzschia species has been linked to haloperoxidase activity [33,34]. Nevertheless, not much is known about QS antagonistic compounds derived from diatoms. To the best of our knowledge there is no data which demonstrates the release of reactive bromine (or iodine) from diatoms as a possible defense mechanism against bacterial fouling. In the present study, it was envisioned that the natural haloperoxidase system from the benthic diatom N. cf pellucida, besides allelopathic interactions between competing species, may also be involved in the deactivation of bacterial QS molecules. Therefore, the goal of this study was to analyze the effect of haloperoxidase activity by the diatom N. cf pellucida on bacterial QS. During this research, the degradation pathway of the QS molecules was elucidated and a variety of halogenated AHL degradation derivatives were synthesized and evaluated for their ability to either induce or inhibit QS activity. 2. Results and Discussion 2.1. Nitzschia cf pellucida Deactivation of β-Keto-AHLs A first series of experiments was performed in order to determine whether the haloperoxidase system within Nitzschia cf pellucida is capable of mediating the deactivation of AHLs. In a previous study, the presence of a haloperoxidase in this benthic diatom was detected via the phenol red assay [33]. In the present study, haloperoxidase activity was verified via the formation of bromophenol blue after addition of phenol red and H2O2 (to ensure sufficient production of reactive halogen intermediates [29,30]) to the cultures (data not shown) [33]. Furthermore, the activity of N-3-oxohexanoyl homoserine lactone (OHHL) and N-hexanoyl homoserine lactone (HHL) was monitored over time with the Chromobacterium violaceum CV026 bioassay in the absence or presence of N. cf pellucida (Table 1). These compounds were selected, because OHHL is a known biofilm enhancer. For the control sample (synthetic seawater + H2O2 + OHHL) and the sample in which the N. cf pellucida culture was only supplemented with OHHL, the QS activity was retained throughout the monitoring period. The decrease in QS activity that was observed for these samples after 180 min of incubation is most probably due to pH-dependent AHL lactonolysis [35]. The addition of synthetic homoserine lactone bearing a β-keto moiety (OHHL) in combination with H2O2 to a N. cf pellucida culture resulted in the deactivation of this type of AHL which is demonstrated in Figure 1. This is in accordance with previous studies in which natural haloperoxidase systems of Laminaria digitata and Delisea pulchra mediated the deactivation of these AHLs in C. violaceum Mar. Drugs 2014, 12 355 CV026 [30] and A. tumefaciens NT1 bioassays [29]. Activity of β-keto-AHL was lost after 60