Red tide WORLD OF MICROBIOLOGY AND IMMUNOLOGY482•
algae can also clog or irritate the gills of fish and can cause
stress or mortality by this physical effect.
Saxitoxin is a natural but potent neurotoxin that is syn-
thesized by certain species of marine dinoflagellates.
Saxitoxin causes paralytic shellfish poisoning, a toxic syn-
drome that affects humans who consume contaminated shell-
fish. Other biochemicals synthesized by dinoflagellates are
responsible for diarrhetic shellfish poisoning, another toxic
syndrome. Some red tide dinoflagellates produce reactive
forms of oxygen—superoxide, hydrogen peroxide, and
hydroxyl radical—which may be responsible for toxic effects.
A few other types of marine algae also produce toxic
chemicals. Diatomsin the genus Nitzchiasynthesize domoic
acid, a chemical responsible for amnesic shellfish poisoning
in humans.
Marine animals can also be poisoned by toxic chemicals
synthesized during blooms. For example, in 1991, a bloom in
Monterey Bay, California, of the diatom Nitzchia occidentalis
resulted in the accumulation of domoic acid in filter-feeding
zooplankton. These small animals were eaten by small fish,
which also accumulated the toxic chemical and then poisoned
fish-eating cormorants and pelicans that died in large num-bers. In addition, some humans who ate shellfish contami-
nated by domoic acid were made ill.
In another case, a 1988 bloom of the planktonic alga
Chrysochromulina polylepisin the Baltic Sea caused exten-
sive mortalities of various species of seaweeds, invertebrates,
and fish. A bloom in 1991 of a closely related species of alga
in Norwegian waters killed large numbers of salmon that were
kept in aquaculture cages. In 1996, a red tide killed 149 endan-
gered manatees in the coastal waters of Florida.
Even large whales can be poisoned by algal toxins. In
1985, 14 humpback whales died in Cape Cod Bay,
Massachusetts, during a five-week period. This unusual mor-
tality was caused by the whales eating mackerel that were con-
taminated by saxitoxin synthesized during a dinoflagellate
bloom. In one observed death, a whale was seen to be behav-
ing in an apparently normal fashion, but only 90 minutes later,
it had died. The symptoms of the whale deaths were typical of
the mammalian neurotoxicity that is associated with saxitoxin,
and fish collected in the area had large concentrations of this
poisonous chemical in their bodies.See alsoPhotosynthetic microorganisms; Plankton and plank-
tonic bacteriaRed tide caused by the growth of algae in the sea.womi_R 5/7/03 8:17 AM Page 482