Phycotoxin

List of known phycotoxins and mechanisms of action

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

Types of toxins

Excreted toxins

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.

Contact toxins

Contact toxins are effective if they impact the grazer or harmful bacterium immediately after contact with the phytoplankton producer. These toxins are located at the cell surface and are typically classified as glycoproteins, glycolipids, or polypeptides. These toxins would have to be highly specific to their target receptors to be effective.

Post-ingestion toxins

In order for these types of toxins to take effect, post-ingestion toxin producers have to be consumed by a grazer. Post-ingestion toxins, also known as suicide toxins, are not beneficial to individual cells because unlike terrestrial plants, phytoplankton do not have sacrificial tissue. 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. Community defense is most beneficial in a clonal population where toxigenic species are abundant, for example during a monospecific phytoplankton bloom.

Chemical defense signal mechanisms

Table modified from Wolfe (2000)

Detection methods

It is technically difficult to identify and characterize a metabolite that is produced in low concentrations and is secreted into a fluid that contains a diversity of other metabolites. Allelopathy is very difficult to observe in the field (with the exception of harmful algal blooms) because phycotoxin production may be induced by a variety of environmental factors and may create a cascade of biotic and physical events, which are difficult to separate from direct allelopathic effects of one species on another. There are six points (similar in logic to Koch's postulates) that must be established to rigorously prove that one species is chemically inhibiting another in an ecological system

1. a pattern of inhibition of one species [...] by another must be shown
2. the putative aggressor [species] must produce a toxin
3. there must be a mode of toxin release from the [species] into the environment
4. there must be a mode of toxin transport and/or accumulation in the environment
5. the afflicted [species] must have some means of toxin uptake
6. the observed pattern of inhibition cannot be explained solely by physical factors or other biotic factors, especially competition and herbivory:
7. concentrations which impact the target species must be environmentally realistic given rates of transport and diffusion in the aquatic environment

Few (if any) studies on phytoplankton toxins have attempted to rigorously meet all of these criteria. All methods of detecting phycotoxins involve extraction of the candidate toxin from a phytoplankton culture; therefore, it's important to determine whether the toxin is secreted into the media or stored in the phytoplankton cell. It's also important to know whether the target organism must be present to induce toxin synthesis.

Most commonly, the presence of a phycotoxin is verified by bioassay-guided fractionation. The sample must be fractionated, or separated from the other metabolites and chemicals in the media using chromatography. These different fractions may be then tested on the target species to determine which sample causes the expected allelopathic symptom(s). This approach is useful for rapidly isolating an allelochemical whose structure is not known. However, bioassays have the potential to generate false positives. This may occur if the bioassay is not controlled properly. For example, in a mixed batch culture the target species may die or have reduced growth rates due to competition for nutrients, dissolved inorganic carbon, or pH levels which are too low for the target species.

Developments in genomics, transcriptomics, proteomics, and metabolomics are now yielding large volumes of biochemical data. "Metabolic profiling" allows for comparison between biologically active and inactive samples and identification of compounds present at low concentrations using mass spectrometry. These samples may then be compared by principal component analysis. Characterization of the compounds present in the active sample (but not in the inactive sample) may then be identified and characterized using standard methods in mass spectroscopy. Isotope labeling may also be used to identify the pathways used in phycotoxin biosynthesis.