Phycotoxins (from Ancient Greek φῦκος (phûkos) 'seaweed' and τοξικόν (toxikón) 'poison, toxin') are complex allelopathic chemicals produced by eukaryotic and prokaryotic algal secondary metabolic pathways.
Marine bacteria try to obtain a share of organic carbon by maintaining symbiotic, parasitic, commensal, or predatory interactions with phytoplankton.
However, it is unlikely that the synthesis of complex and energetically expensive chemicals should be conserved over evolutionary time if they do not confer some advantage on the producer.
Even if we do not yet know the effect of many toxins in their natural environment, their mere presence and impressive diversity indicates that they do serve some ecological purpose.
This is a potential source of reduced nutrients and carbon for bacteria and may act as a signal for predators which can detect and follow kairomone gradients in their environment.
Excreted toxins would seem most advantageous to the individual cell in their ability to keep predators and/or parasitic and algicidal bacteria at a distance.
For excreted toxins to be effective, they must have a low molecular weight to rapidly diffuse in the marine environment and to be energetically cheap to produce.
However, excreted toxins may not actually repel larger motile predators because molecular diffusivity is slow and turbulence at the millimeter scale is large in water.
Additionally, excreted toxins may be effective method of keeping harmful bacteria and other phytoplankton competitors outside of the phycotoxin producer's microzone of nutrients.
However, if internal toxins do result in the death, decrease growth rate, infertility, or deterrence of a predator the remaining representatives of the plankton community may benefit.
[15] Community defense is most beneficial in a clonal population where toxigenic species are abundant, for example during a monospecific phytoplankton bloom.
"Metabolic profiling" allows for comparison between biologically active and inactive samples and identification of compounds present at low concentrations using mass spectrometry.