Allelopathy of aquatic autotrophs

Allelopathy in aquatic environments may provide a competitive advantage to angiosperms, algae, or cyanobacteria in their interaction with other primary producers. Allelopathy can influence the competition between different photoautotrophs for resources and change the succession of species, for example, in phytoplankton communities. Field evidence and laboratory studies indicate that allelopathy occurs in all aquatic habitats (marine and freshwater), and that all primary producing organisms (cyanobacteria, micro- and macroalgae as well as angiospenns) are capable of producing and releasing allelopathically active compounds. Although allelopathy also includes positive (stimulating) interactions, the majority of studies describe the inhibitory activity of allelopathically active compounds. Different mechanisms operate depending on whether allelopathy takes place in the open water (pelagic zone) or is substrate associated (benthic habitats). Allelopathical interactions are especially common in fully aquatic species, such as submersed macrophytes or benthic algae and cyanobacteria. The prevention of shading by epiphytic and planktonic primary producers and the competition for space may be the ultimate cause for allelopathical interactions. Aquatic allelochemicals often target multiple physiological processes. The inhibition of photosynthesis of competing primary producers seems to be a frequent mode of action. Multiple biotic and abiotic factors determine the strength of allelopathic interactions. Bacteria associated with the donor or target organism can metabolize excreted allelochemicals. Frequently, the impact of surplus or limiting nutrients has been shown to affect the overall production of allelochemicals and their effect on target species. Similarities and differences of allelopathic interactions in marine and freshwater habitats as well as between the different types of producing organisms are discussed.