A wide range of marine organisms produce a toxin of some kind. In many cases, a particular species may produce a cocktail of toxins with different biological actions. The exact nature or mix of toxins can vary considerably, even between closely related species. Toxins are most commonly used to subdue prey, as in the case of jelly fish, cone shells and sea snakes, for example. Some species, such as toad fish and stingrays, use toxins primarily for defensive purposes. In the case of dinoflagellates, toxins seem to serve both functions to varying degrees.
Toxins exist in a bewildering variety of molecular forms. Some are synthesised by relatively minor variations of other metabolic processes. Others require complex biochemical pathways for their formation. Some species, such as toad fish, do not synthesise their toxins but acquire them from other organisms, in this case, bacteria that reside in their digestive tract.
Neurotoxins target specific molecules necessary for the transmission of electrical impulses by nerve fibres to other nerves or to the muscles the nerves are controlling. Some toxins directly target molecules on the muscles themselves. Most of these neurotoxins are very resistant to metabolic breakdown and can persist in the water or animal tissues for long periods without losing any potency.
Cytotoxins mostly target the cell membrane that forms a barrier between the contents of a cell and the very different environment outside the cell. Damage to the membrane breaks this barrier leading to the death of the cell. Some cytotoxins disrupt the membrane by displacing key elements of its structure; others actively breakdown some components of the membrane; yet other cytotoxins insert themselves into the membrane and assemble leaky pores within it.
Dinoflagellates are single-celled microalgae that possess chloroplasts and are therefore capable of photosynthesis. However, many of them, including the dominant species in the current SA bloom, can be mixotrophic, meaning that they also gain nutrients from other living organisms. At least some species use cytotoxins or neurotoxins to immobilise their prey prior to ingesting it.
One of the dominant dinoflagellate species in the current bloom plaguing South Australian waters is Karenia mikimotoi. It is cytotoxic, producing lethal effects on an incredibly wide range of other organisms. This species probably also makes a specific toxin, gymnocin, chemically related to the neurotoxin, brevetoxin, but which has mainly cytotoxic actions. However, laboratory tests have shown that gymnocins have very limited solubility in water and, in practice, show only minimal toxicity to fish. It is highly likely that all Karenia species are cytotoxic. Other dinoflagellates detected in the bloom, such as Karlodinium, also mostly produce cytotoxins.
In November, 2025, a related species, Karenia cristata, was identified as another dominant organism in the South Australian bloom. It is the most likely source of brevetoxin, a neurotoxin which has been detected in some water samples during the bloom. Karenia papilionacea, present in low numbers in the current bloom, also can produce brevetoxin. These species probably also produce cytotoxins.
This table shows a summary of the main species found in the bloom and the toxins they produce. Species other than Karenia mikimotoi and Karenia cristata are present in much lower numbers through most of the bloom.
| dinoflagellate species | main toxin class | toxin name | toxin target |
|---|---|---|---|
| Karenia mikimotoi | cytotoxin | unknown | damages cell membranes |
| Karenia cristata | neurotoxin cytotoxin* | brevetoxin unknown | activates sodium channels damages cell membranes |
| Karenia papilionacea | neurotoxin cytotoxin* | brevetoxin unknown | activates sodium channels damages cell membranes |
| Karenia brevisulcata | neurotoxin cytotoxin | brevisulcatic acid brevisulcenal | activates sodium channels damages cell membranes |
| Karenia longicanalis | neurotoxin cytotoxin* | gymnodimine unknown | blocks nicotinic receptors damages cell membranes |
| Karlodinium veneficum | cytotoxin | karlotoxin | damages cell membranes |
| Gymnodinium spp. | neurotoxin | saxitoxin | blocks sodium channels |
The following sections explain how these toxins work in more detail, along with the relevant references from the scientific literature.
Click on the headings or images to go the relevant section.
< back to overview and topic list