Page 302 - Algae

Algae and Men 285

supplement due to its content of highly nutritious proteins and PUFAs, and to the simultaneous
production of antioxidant compounds, such as b-carotene, vitamin C, and vitamin E.
Table 7.7 summarizes commercially exploited algae and the corresponding nutraceutical.

TOXIN
Toxins are all compounds that are either synthesized by the algae or formed by the composition of
metabolic products and hence represent an intrinsic characteristic of the organism. Of the millions
of species of microalgae those that produce specific toxins scarcely exceed a hundred. These occur
in both salt and freshwaters, and while most are planktonics some are benthic or floating at water
surface. Toxins can attract particular attention when they cause the death of livestock that has
drunk water containing them or fish and shellfish in the sea, or humans that consume these.
Toxins have been divided into different classes based on the syndromes associated with exposure
to them, such as paralytic shellfish poisoning (PSP), diarrheic shellfish poisoning (DSP), neuro-
toxic shellfish poisoning (NSP), ciguatera fish poisoning (CFP), and amnesic shellfish poisoning
(ASP).
Algae which seem to be directly producer of toxic substances mostly belong to three taxonomic
groups: Cyanophyta, Haptophyta, and Dinophyta. In addition to these there are some groups which
include one or two toxic members. Species of Chattonella and Heterosygma, belonging to the
Raphidophyceae, form toxic red tides in Japanese waters and a few diatoms of the genus
Peusdonitzschia produce domoic acid, a low molecular amino acid causing amnesic shellfish
poisoning.
Among the 50 freshwater existing cyanobacteria genera, 12 are capable of producing toxins.
While blue-green algae have significant taste and odor constituents, representing a moldy smell,
their toxic metabolites have no taste, odor, or color. The risk of exposure to algal toxins may
come from drinking water, recreational water, dietary supplements, or residue on produce irrigated
with contaminated water and consumption of animal tissue. Avoiding cyanobacteria toxins is not as
easy as avoiding a harmful algal bloom as toxins may be present in fish, shellfish and water even
after the bloom has dissipated. Cyanobacterial toxins are responsible for a variety of health effects
such as skin irritations, respiratory ailments, neurological effects, and carcinogenic effects.
The three major classes of these compounds are:

. Cyclic peptides (nodularins, microcystins). Nodularia, a well-known cyanobacterium,
produce nodularins that are primarily a concern in marine and brackish waters thus creating
a risk to recreational swimmers. The 65 variants of microcystins, however, are isolated
from freshwaters worldwide and are produced by Microcystis (the most commonly identified
cyanobacteria in human and animal poisonings), Anabaena, and other algae. They are very
stable in the environment and resistant to heat, hydrolysis, and oxidation. Both toxins have
an affinity for the liver. Other symptoms of exposure to microcystins may range from weak-
ness, loss of appetite, vomiting, and diarrhea to cancer.
. Alkaloids (anatoxin, saxitoxin). Anatoxins may affect the nervous system, skin, liver, or
gastrointestinal tract. These neurotoxins can cause symptoms of diarrhea, shortness of
breath, convulsions and death, in high doses, due to respiratory failure. Saxitoxins are
the cause of paralytic shellfish poisonings in humans consuming contaminated shellfish.
There are no reports of similar poisonings via the drinking water route.
. Lipopolysaccharides (endotoxins). A similar cell wall toxin as found in Salmonella
bacteria, but appears to be less toxic.
The physicochemical nature of the water source can have an effect on not only the growth but
also the toxicity of the algal bloom. For example, some algae increase in toxicity when blooms are