Synergistic allelochemicals from a freshwater cyanobacterium

Abstract

The ability of cyanobacteria to produce complex secondary metabolites with potent biological activities has gathered considerable attention due to their potential therapeutic and agrochemical applications. However, the precise physiological or ecological roles played by a majority of these metabolites have remained elusive. Several studies have shown that cyanobacteria are able to interfere with other organisms in their communities through the release of compounds into the surrounding medium, a phenomenon usually referred to as allelopathy. Exudates from the freshwater cyanobacterium Oscillatoria sp. had previously been shown to inhibit the green microalga Chlorella vulgaris. In this study, we observed that maximal allelopathic activity is highest in early growth stages of the cyanobacterium, and this provided sufficient material for isolation and chemical characterization of active compounds that inhibited the growth of C. vulgaris. Using a bioassay-guided approach, we isolated and structurally characterized these metabolites as cyclic peptides containing several unusually modified amino acids that are found both in the cells and in the spent media of Oscillatoria sp. cultures. Strikingly, only the mixture of the two most abundant metabolites in the cells was active toward C. vulgaris. Synergism was also observed in a lung cancer cell cytotoxicity assay. The binary mixture inhibited other phytoplanktonic organisms, supporting a natural function of this synergistic mixture of metabolites as allelochemicals.allelopathy | chemical ecology | Oscillatoria | cyclic peptides | synergism C yanobacteria are a prolific source of nearly 800 diverse bioactive secondary metabolites, originating mainly from nonribosomal peptide synthetase (NRPS) or mixed polyketide synthase (PKS)-NRPS biosynthesis (1, 2). Efforts to isolate and characterize cyanobacterial metabolites have usually been motivated from a desire to describe either their natural toxicity toward animals in natural settings (e.g., ref.3) or promising activities from in vitro biomedical screening programs (e.g., ref. 4, 5). However, in general the ecological role played by the majority of these metabolites is not well known (6, 7). Functions established to date for cyanobacterial secondary metabolites include nitrogen storage (8), UV protection (9), metal chelation (10), defense against predation (11), and quorum sensing (12).Allelopathy refers to the chemically mediated interaction between plants or microorganisms (13). These interactions are characterized by the release of allelopathic compounds (allelochemicals) into the surrounding medium, eliciting either a positive or deleterious response in a target organism (13). In aquatic ecosystems, allelopathy is regarded as an important process influencing the shaping of microbial communities (14-16). Toxic properties have been attributed to cyanobacteria over the last 130 yr (e.g., ref. 17), and the allelopathic potential of these organisms was described through field-derived observations in the 1970s (18,19). Since th...

Growth inhibitory effect of Microcystis on Aphanizomenon flos-aquae isolated from cyanobacteria bloom in Lake Dianchi, China

“…Indeed, the continuous application of filtrate has been found to amplify the allelopathic effect of monoculture filtrate (Suikkanen et al, 2004). In addition, studies have also shown that the phase of growth can influence allelopathic effects (Leão et al, 2010;Suikkanen et al, 2004). Our results support this suggestion as the allelopathic effect we observed was stronger in the late exponential and stationary phases than in earlier phases (concentrated filtrate from day 21 and day 28 greatly inhibited A. flos-aquae growth, while those from day 7 and day 14 did not).…”

Marine and Freshwater Microalgae as a Potential Source of Novel Herbicides

“…Specific mixtures of the most abundant congeners, portoamides A and B, in the ratios of 2:1 and 1:2.6 inhibited C. vulgaris at 30 ppm, but not in other ratios (e.g. 4.4:1) tested (Leão et al, 2010). This finding is particularly stunning given the rather minimal difference between the structure of the two (differing only in the presence of a methoxy group), and is further underscored by the apparent lack of activity associated with other chemically related congeners (e.g.…”

“…Interestingly, allelopathic activity was found to specifically peak during the early stages of exponential growth of the cyanobacterial culture, suggesting an ecological role (i.e. to "open" a niche for colonization), and consequently providing, in this case, an opportunity to purify a sufficient quantity of the otherwise low concentration metabolite for chemical characterization (Leão et al, 2010). Even more interesting, the investigators documented an apparent synergism of these metabolites.…”

“…Specifically, it was found (Christiansen et al, 2001) using sequence-specific primers that nearly all (97% and 100%, respectively) of the cyanobacteria in these two sections were positive for NRPS genes (compared to only 52%, 80% and 64%, respectively, for Sections I, II and III). In a very recent study, Leão et al (2010) used bioassay-guided fractionation, specifically based on inhibition of the green alga, Chlorella vulgaris, to isolate a series of cyclic peptides, the portoamides (Fig. 4) as apparent allelopathic agents from of the blue-green alga, Oscillatoria.…”

“…4. Portoamides A and B isolated from a species of Oscillatoria as synergistic allelopathic metabolites (Leão et al, 2010).…”