Algal and cyanobacterial secondary metabolites in freshwaters: a comparison of allelopathic compounds and toxins

1. The photoautotrophic micro-organisms collectively termed 'micro-algae' (including micro-eukaryotes and cyanobacteria) are known to produce a wide range of secondary metabolites with various biological actions. A small subset of these compounds has been identified. Some of them, termed allelopathic compounds, have been shown to play a role in allelopathy, defined here as inhibitory effects of secondary metabolites against either competitors or predators. Freshwater cyanobacteria also produce some secondary metabolites, termed toxins, which are highly toxic for animals. 2. While allelopathic compounds play a role in the interactions between the emitter organisms and their direct competitors or predators, toxins are categorised according to their toxic effect on several organisms, including some that may not be present in their immediate environment. However, these two definitions are not mutually exclusive. This review considers the evolutionary, ecological and physiological aspects of the production of allelopathic compounds by micro-algae in freshwaters, and compares the characteristics of allelopathic compounds with those of toxins. 3. Allelopathic compounds include alkaloids, cyclic peptides, terpens and volatile organic compounds. Toxins include alkaloids, cyclic peptides and lipopolysaccharides. No allelopathic compound type is associated with a particular phylogenetic group of algae. In contrast, freshwater toxins are only produced by cyanobacteria belonging to a restricted number of genera. Allelopathic compounds have various modes of action, from inhibition of photosynthesis to oxidative stress or cellular paralysis. Toxins are often enzyme inhibitors, or interfere with cell membrane receptors. 4. The ecological roles of allelopathic compounds have been well identified in several cases, but those of toxins are still debated. In the light of descriptions of negative effects of toxins on both micro-invertebrates and photoautotrophic organisms, we suggest that at least some toxins should actually be considered as allelopathic compounds. Further research on toxic secondary metabolites in freshwaters is now needed, with emphasis on the ecological effects of the compounds in the immediate environment of the emitter algae.