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. More simply, these are toxic chemicals synthesized by photosynthetic organisms. These metabolites are (in most cases) not harmful to the producer but may be toxic to either one or many members of the marine food web. This page focuses on phycotoxins produced by marine microalgae; however, freshwater algae and macroalgae are known phycotoxin producers and may exhibit analogous ecological dynamics. In the pelagic marine food web, phytoplankton are subjected to grazing by macro- and micro-zooplankton as well as competition for nutrients with other phytoplankton species. Marine bacteria try to obtain a share of organic carbon by maintaining symbiotic, parasitic, commensal, or predatory interactions with phytoplankton. Other bacteria will degrade dead phytoplankton or consume organic carbon released by viral lysis. The production of toxins is one strategy that phytoplankton use to deal with this broad range of predators, competitors, and parasites. Smetacek suggested that "planktonic evolution is ruled by protection and not competition. The many shapes of plankton reflect defense responses to specific attack systems". Indeed, phytoplankton retain an abundance of mechanical and chemical defense mechanisms including cell walls, spines, chain/colony formation, and toxic chemical production. These morphological and physiological features have been cited as evidence for strong predatory pressure in the marine environment. However, the importance of competition is also demonstrated by the production of phycotoxins that negatively impact other phytoplankton species. Flagellates (especially dinoflagellates) are the principle producers of phycotoxins; however, there are known toxigenic diatoms, cyanobacteria, prymnesiophytes, and raphidophytes. Because many of these allelochemicals are large and energetically expensive to produce, they are synthesized in small quantities. However, phycotoxins are known to accumulate in other organisms and can reach high concentrations during algal blooms. Additionally, as biologically active metabolites, phycotoxins may produce ecological effects at low concentrations. These effects may be subtle, but have the potential to impact the biogeographic distributions of phytoplankton and bloom dynamics.

Phycotoxins may prevent grazing by several mechanisms: grazer death, infertility, or deterrence. Some evidence of anti-grazing effects:

Phycotoxins production may be useful to ward off parasitic or algicidal heterotrophic bacteria. Some evidence of anti-microbial effects:

Since many different species of phytoplankton compete for a limited number of nutrients (see Paradox of the Plankton), it's possible that phycotoxin production is used as a method to either kill competitors or to keep other phytoplankton out of the producer's nutrients space. Some evidence of competitive effects:

Most characterized phycotoxins have some economic or health impact on humans. Other well-studied phycotoxins are potential or existing pharmaceuticals or have some use in cellular research. Therefore, our level of knowledge on individual toxins does not necessarily reflect their ecological relevance. Additionally, the mode of action and level of toxicity are effects that have been documented in macroorganisms (typically mice). These modes of action may be different in the pelagic marine environment. 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.

The phytoplankton species listed below do not encompass the entire range of known toxigenic species. There exists experimental evidence for phytoplankton species that have inhibitory effects on grazers or other phytoplankton species, but their toxins have not been identified.

Table generated using information from Cembella, Shimizu

Excreted toxins may help deter predators and bacteria which are drawn in by phytoplankton waste products. Phytoplankton are known to excrete waste metabolites into the surrounding environment. 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. However, continuous toxin production and excretion carries a metabolic cost. 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. Excreted phycotoxins may act as repellents if their signal registers at the same speed as other signals that potential grazers can detect (kairomones), assuming both are encountered by a predator at the same time. Additionally, excreted toxins may be effective method of keeping harmful bacteria and other phytoplankton competitors outside of the phycotoxin producer's microzone of nutrients.