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Algal
toxins are a group of strongly toxic substances occuring in the cells of a
number of Cyanophyceae (blue-green microalgae). Lysis of the cells allows the
release of the substances into the water so that also cell free water can have
a toxic effect. Many toxic cyanophyceae, however, develop next to the toxins
also substances with an extremely strong odor which functions as a warning so
that poisoning of humans is relatively rare (while cattle poisoning is not
uncommon). In fresh water the following Algal toxin forming genera occur:
Microcystis, Anabaena, Planktothrix, Nostoc. Aphanizomenon and
Cylindrospermopsis. Nodularia is found predominantly in seawater (ZECH, 2001). Algal
toxins are either hepatotoxic (Microcystin, Nodularin) oder nerve toxins
(Anatoxin, Saxitoxin). The hepatotoxins are generally cyclic oligopeptides
which inhibit protein phosphatases. They may enter liver cells, but not other
cell types, which explains their specific liver toxicity. About 100 variants of
this class of substances have been identified. All have in common that they
contain the rare C20 amino acid "Adda". The toxicity of the algal nerve toxin is
based on the fact that they either funtion as analogon of acetyl cholin
(anatoxin-a), as cholin esterase inhibitor (anatoxin-a(S)) or that they block
sodium channels (Saxitoxin) (WELLER, 2002).
Publications on algal toxins have caused a concern about potential Algal toxin
contamination of products used among others as supplement food for health
reasons. In this context the following has to be considered: Algal toxins occur
either in the intact cyanophycea cells or after lysis of the cells in the
surounding water. In products of low moisture content a contamination could
only be caused by the presence of algal cells.
In case of the cyanophycea Aphanizomenon flos-aquae (blue green alga, Klamath
alga) such a contamination can be imagined, although the actual hazard may be
insignificant. Aphanizomenon flos aquae is not cultivated but is harvested
"from the wild", that is, normally from the Upper Klamath Lake in
Oregon. In the phytoplacton of this lake Microcystin (as, however, the only
algal toxin) was found, probably caused by the occurrence of Microcystis.
Toxic strains (of Microcystis aeruginosa, Anabaena flos aquae) can often morphologically
not be distinguished from the non-toxic ones. However, the former common
assumption that Aphanizomenon flos aquae itself has a toxic variant
(CARMICHEAL, 1997; MAHMOOD, 1087; RAPALA, 1993) has recently been
questioned (LI, 2000).
At any rate, manufacturers of Aphanizmenon have established an extensive
control program (Mouse bioassay, enzyme linked immuno sorbent assay (ELISA)
etc) in order to assure that commercial Aphanizomenon flos aquae is free of
toxins. In line with a recommendation of the World Health Organization the
Department of Agriculture of Oregon has fixed the upper allowable limit of
Microcystin to be 1 microgram per gram of algal product. (CHORUS, 1999;
CARMICHAEL, 2000).
In another commercially used algal product, Spirulina platensis (actually
Arthrospira) Phycotoxins cannot occur for the simple reason that Spirulina
grows in an alcaline medium of a pH of around 9 which prevents the growth of
other organisms. There are no toxic cyanophyceae known which would survive
under such circumstances and analyses of commercial samples of Spirulina have
always found to toxin free (KUIPER-GOODMAN, 2001; LAWRENCE, 2001). When methods
for the mass culture of micralgae (especially Spirulina, Scenedesmus and
Clorella) were developped a few decades ago much attention was paid to
the possibility of occurrence of toxic substances. The following substances
were searched for: Aflatoxin, Ochratoxin A, Sterigmatocystin, Citrinin,
Patulin, Penicillins„ure, Zearalenon, T2-Toxin, Diacetoxysciroenol, Trichothecene,
Fumitremorgen and Verucolugen. None of the listed toxins was ever
discovered in algal samples, all biological tests had a negative result
(BECKER,1984(I), BECKER 1984(II)).
Also in case of the kelp species Lithothamnium used as Calcium and Magnesium
source a Algal toxin contamination can be excluded. The partly fossilized
calcareous Lithothamnium leaves are collected from the sea bottom where
cyanophyceae never occur in noticeable concentrations (toxic cyanophyceae form
rather floating accumulations). Be it only for the difference in size the
microscopic cyanophycea cannot by chance be harvested together with
Lithothamnium leaves. The same applies also to various other kelp species (as
Laminaaria and Fucus) used on account of their Iodine content.
(Questions concerning algal toxins are answered by Dr. E.W. Becker,
Tübingen,
email: wolfgang.becker@med.uni-tuebingen.de)
BECKER, E.W.(I), VENKATARAMAN, L.V. 1984: Production and utilization of the
blue-green alga Spirulina in India. Biomass, 4, 105-125
BECKER, E.W.(II), 1984: Biotechnology and exploitation of the green alga
Scendesmus obliquus in India, Biomass, 1-19
CARMICHAEL, W.W., 1997: The cyanotoxins. In Advances in Botanical Research,
Vol. 37 (Ed. by J.A. Callow), pp 211-256. Academic Press, London
CARMICHAEL, W.W.; DRAPEAU, C.; ANDERSON, D.M., 2000: Harvesting of
Aphanizomenon flos aquae Ralfs ex Born. & Flah var. flos aquae
(Cyanobacteria) from Klamath Lake for human dietary use. Journmal of Applied
Phycology, 12, 585-595
CHORUS, i., BARTRAM J. (eds). 1999: Toxic Cyanobacteria in water - A guide to
their public health consequences. Monitoring and Management. E & FN Spon,
London
KUIPER-GOODMAN, T., et al., 2001: Risk Assessment of microcystins in blue-green
algal health food products, in: Mycotoxins and phycotoxins in perspective at
the turn of the millenium. Proceedings of the Xth International IUPAC symposium
on mycotoxins and phycotoxins 21-25 May, 2000, Guarauja (Brazil), eds. W.J. de
Koe, R.A. Samson, H.P. van Egmond, J. Gilbert, M. Sabino, IUPAC and AOAC
International, Ponsen and Looyen, Wageningen, The Netherlands, pp.
549-556
LAWRENCE, J., et al., 2001: Comparison of Liquid Chromatography/Mass
spectrometry, ELISA, and Phosphatase assay for the determination of
microcystins in blue-green algae products. J. AOAC Int. vol. 84, no. 4,
1035-1044 LI,
R.H., CARMICHAEL, W.W., LIU, Y.D., WATANABE, M.M., 2000: Taxonomic
re-evaluation of Aphanizomenon flos aquae NH-5 based on morphology and 16S rRNA
gene sequences. Hydrobiologia, 438, 99-100
MAHMOOD, N.A. CARMICHEAL, W.W., 1987: Annatoxin-a(s), an anticholinesterase
from the cyanobacterium Anabaena flos aquae NRC-525-17. Toxicon, 25, 1221-1227
RAPALA, J.; SIVONEN, K., LYRA, C., NIEMELˇ S.I., 1993: Anatoxin-a concentration
in Anabaena and Aphanizomenon flos-aquae at different environmental conditions
and comparison of growth by toxic and non-toxic Anabaena strains, a laboratory
study. Applied Environmental Microbiology, 64, 99-105
WELLER, M.G., 2002: Algengifte im Wasser. Nachrichten aus der Chemie, 06/2002,
S. 700-7005
ZECH; CH., 2001: Entwicklung von immunanalytischen, chromatographischen und
massenspektrometrischen Methoden zur Bestimmung cyanobakterieller Hepatotoxine
(Microcystine und Nodularine). Dissertation, TU München, Nov. 2001, S.3