Abstract
Cyanobacteria are recognized producers of toxic or otherwise bioactive metabolite associated, in particular, with so-called “harmful algal blooms” (HABs) and eutrophication of freshwater systems. In the present study, two apparently teratogenic indole alkaloids from a freshwater strain of the widespread cyanobacterial genus, Fischerella (Stigonemataceae), were isolated by bioassay-guided fractionation, specifically using the zebrafish (Danio rerio) embryo, as a model of vertebrate development. The two alkaloids include the previously known 12-epi-hapalindole H isonitrile (1), and a new nitrile-containing variant, 12-epi-ambiguine B nitrile (2). Although both compounds were toxic to developing embryos, the former compound was shown to be relatively more potent, and to correlate best with the observed embryo toxicity. Related indole alkaloids from Fischerella, and other genera in the Stigonemataceae, have been widely reported as antimicrobial compounds, specifically in association with apparent allelopathy. However, this is the first report of their vertebrate toxicity, and the observed teratogenicity of these alkaloids supports a possible contribution to the toxicity of this widespread cyanobacterial family, particularly in relation to freshwater HABs and eutrophication.
Highlights
IntroductionCyanobacteria (or “blue-green algae”) are recognized to produce a multitude of bioactive secondary metabolites
Cyanobacteria are recognized to produce a multitude of bioactive secondary metabolites
Recent taxonomic analyses [19] generally suggest that unambiguous assignments of species epiphets within the genus, and the Stigonemataceae more generally, including those associated with 16s rDNA sequence analyses submitted to GenBank, remain to be adequately resolved
Summary
Cyanobacteria (or “blue-green algae”) are recognized to produce a multitude of bioactive secondary metabolites. As an aquatic animal model, the zebrafish embryo has proven to be a relevant toxicological model for understanding a range of environmental contaminants, including the toxic secondary metabolites from cyanobacteria and other HAB species [5] In this regard, the zebrafish embryo has been employed in studies ranging from general toxicological characterization [4,5,6,9,10,11,12,13,14,15] of established cyanobacterial toxins, as well as otherwise uncharacterized cyanobacterial metabolites (e.g., components of extracts/mixtures), to studies, such as the present one, which employ the zebrafish embryo model toward isolation and subsequent chemical characterization, of bioactive cyanobacterial metabolites [15,16,17,18]. Two indole alkaloids— belonging to a class of compounds characteristic of the genus, and family (Stigonemataceae) more generally—including one previously known variant (1), and one apparently novel, nitrile containing congener (2), were identified and characterized
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