Several algal species produce potent toxins that cause Ciguatera Fish Poisoning, Neurotoxic, Paralytic, Diarrhetic, and Amnesic Shellfish Poisoning. Some of these toxins affect ion channels which are like gates in the membranes that allow ions to enter (see diagrams below). Commonly, toxic algae affect the sodium and calcium channels that regulate ion transport across the membranes of nerve and muscle cells. Other toxins affect proteins or act like amino acids. Each of these toxins affects humans in different ways. On the other hand, some harmful algae do not produce toxins but lead to unhealthy ecological conditions that can kill marine organisms: Click here to learn more.

TOXIC DINOFLAGELLATE BLOOMS: Toxins that affect ion channels
Below at left, the resting cell's interior has a slight negative charge (-) relative to its exterior. Changes in voltage across the membrane can open and close voltage gates to control the flow of positively-charged (+) ions toward the cell's interior. Such flow occurs to achieve equilibrium across the membrane in terms of electrical charge and ion concentration. Some algae-derived toxins bind to these channels, blocking ions from flowing into cells. Other toxins have the opposite effect: they bind to these channels, keeping them open and promoting higher-than-normal ion flux.
Ciguatoxin binds to the sodium channel and promotes sodium ion (Na+) influx by keeping sodium channel open. Maitotoxin keeps calcium channels open, promoting calcium ion influx (Ca++). Ciguatoxin behaves similarly to brevetoxin (below), although the molecules are structurally different.
Brevetoxin binds to the sodium channel, keeping it open and helping to promote sodium ion (Na+) influx. Although different in molecular structure from ciguatoxins (above), the effect on the sodium channel is similar. Also, the effect of brevetoxin on the sodium channel is opposite that of saxitoxins (below).
The positive charge on part of the saxitoxin molecule allows it bind to and block the sodium channel, inhibiting the passage of sodium ions (Na+); this causes muscles to relax and may lead to respiratory failure or death.
TOXIC DINOFLAGELLATE BLOOMS: Toxins that inhibit proteins
Inhibits the proteins (phosphatase 1 and 2A) that control sodium secretion by intestinal cells, thereby causing nausea, vomiting, abdominal pain, and diarrhea in humans. Chronic exposure to these toxins has also been suggested to promote the formaton of digestive tract tumors, although the mechanism hasn't been identified.
Domoic acid (DA) is similar in structure to amino acid which communicates messages among neurons in the central nervous system (glutamic acid). However, DA overstimulates the neurons in the brain's hypocampus until these cells start to die. DA also keeps the calcium channels open in nerve and muscle cells (as depicted above).