Research by marine biologists from Wageningen University has shown that feeding on zooplankton by scleractinian corals has been greatly underestimated.
|Chemical defense mechanisms|
|Written by Sander van der Weijden, M.Sc.|
The ocean can be a dangerous place even for corals. Sessile as they are, they have turned to chemical warfare in an attempt to prevent predation. This continued battle between predators and prey has led to some of the most lethal toxins found in nature today. In this review we will discuss a few of them.
Why do coral possess toxins?
Corals are sessile organisms; they remain stationary for a large part of their lifecycle. As they are attached to a surface, they cannot flee from predation. As a way to prevent predation, some corals produce toxins, or live in symbiosis with bacteria and protists that produce toxins. Also, most corals possess nematocytes. These stinging cells are used to capture small prey, and to kill of neighbouring corals in a continuous battle for space.
Figure 1: Nematocyst discharge: A dormant nematocyst discharges in response to nearby prey touching the cnidocil. The operculum flap opens and its stinging apparatus fires the barb into the prey leaving a hollow filament through which poisons are injected to immobilize the prey. Next, the tentacles maneuver the prey to the polyp mouth. Copyright the Wikimedia Foundation.
The toxins contained in these cells are relatively harmless to humans, although fire corals (though not technically a coral, but rather a member of the Hydrozoa class) do contain a cocktail of toxins which can cause severe pain and inflammatory effects.
Types of toxins and mode of action
Corals and organisms associated with them produce several types of toxins, most of them neurotoxic in origin. Neurotoxins interfere with signal transmission in the nervous system of animals. In this mini-review we will discuss three of the major neurotoxins produced by corals, and how they interfere with neuronal signal transmission.
Saxitoxin is a potent neurotoxin produced by dinoflagellates (LD50 oral 260 µg/kg in mice), which acts on Na+ (sodium) channels. This toxin effectively blocks these channels, preventing the influx of sodium ions, causing paralysis and respiratory failure. It acts fast; in a matter of minutes, and is classified as a chemical weapon by the United States military. Together with tetrodoxin (a toxin contained in pufferfish) this toxin is the major cause of paralytic shellfish poisoning (PSP). Interestingly, due to its high specifity this toxin is frequently used in laboratory studies to investigate the structure and mechanism of action of Na+ channels1.
Figure 2, right: Chemical structure of saxitoxin. Copyright the Wikimedia Foundation
Another very potent toxin is palytoxin. Although moderate concentrations of this toxin are present in several Zoanthid species, this toxin is thought to be produced by a dinoflagellate (Ostreopis siamensis). This complex toxin acts on Na+/K+ (sodium/potassium) antiporters, responsible for maintaining the resting potential of cells. It disables the ion pump, and allows free flow of Na+ and K+, thereby interfering with membrane potential and even inducing cell lysis. Maintaining this potential is essential for proper functioning of the kidneys, red blood cells and signal transduction. Exposure to high concentrations of this toxin can lead to hemolysis and kidney, respiratory and heart failure. This molecule is the longest non-peptide toxin found in nature3.
Figure 3: Structure of palytoxin. Notice the large structure of the molecule. Copyright the Wikimedia Foundation.
The LD50 of palytoxin is about 100 ng/kg, which means that lethal dose for an adult human is almost invisible to the naked eye.
Lophotoxin is a toxin produced by Lophogorgia sp. and Pseudopterogorgia sp. The toxin blocks nicotinic acetylcholine receptors, which are found at synapses and neuromuscular junctions, where nerves connect with muscles. Stimulation of these receptors causes muscle contractions. Not surprisingly, this toxin can cause paralysis and respiratory failure.
Toxins and medicines
Many coral toxins and symbiont toxins associated with corals are under investigation for their medicinal properties. One such compound called curacin A shows great promise in becoming the next anti-cancer drug. This protein, produced by a coral-associated bacterium called L. majuscula, binds to tubulin. This is a cytoskeletal protein, important for cell division and vesicle transport. Scientists are currently trying to optimize this protein for drug usage4.
Another well-documented compound under investigation for its use in medicine is bryostatin, produced by certain Bryozoa sp. Bryostatins display potent anti-tumour activity. Extensive clinical research has established that bryostatin 1 inhibits protein kinase C (PKC) , a protein which promotes cancer when overactive. Next to this, recent studies using a mouse model system demonstrated that bryostatin 1 has the potential te counter the effects of depression and dementia, making it a promising therapeutic agent (medicine) for Alzheimer's disease and other central nervous system disorders in humans5.
The toxins described in this articles are some of the most toxic found in nature. The use of rubber gloves when working in the aquarium has been discussed over the years, and has been laughed at. In the light of this information, wearing gloves doesn't seem funny anymore.
The toxins and other compounds produced by corals which are currently being studied might reveal promising drugs. In the future these may be used to combat current diseases like cancer, atherosclerosis, heart failure and other ilnesses. Next time you take a peak in your aquarium you may wonder about its inhabitants, producing deadly toxins and life-giving drugs at the same time...
van Apeldoorn ME, van Egmond HP, Speijers GJ, Bakker GJ. Toxins of cyanobacteria. Mol Nutr Food Res. 2007 Jan;51(1):7-60.
Sorenson E. M., Culver P., Chiappinelli V. A. .Lophotoxin: selective blockade of nicotinic transmission in autonomic ganglia by a coral neurotoxin. Neuroscience. 1987 Mar; 20(3): 875-84
Habermann E. Palytoxin acts through Na+,K+-ATPase. Toxicon. 1989;27(11):1171-87. Review.
Wipf P, Reeves JT, Day BW. Chemistry and biology of curacin A. Curr Pharm Des. 2004;10(12):1417-37.
Paul V.J, Arthur K.E., Ritson-Williams R., Ross C., Sharp K. Chemical Defenses: From Compounds to Communities. Biol. Bull. 213: 226-251. (December 2007)