Biology Reference
In-Depth Information
reported to be toxic (Sivonen
et al
., 1990a). Oksanen
et al.
(2004) concluded that
Nostoc
sp. strain-IO-
102I is distantly related to
Nostoc
sp. strain-152 but closely related to
Nostoc punctiforme
PCC 73102
and other symbiotic strains. It is quite intriguing to note that 95% of the genera of cyanobacteria
belonging to the fi ve taxonomic groups revealed a common neurotoxic amino acid termed as β-N-
methylamino-L-alanine (Cox
et al
., 2005).
Classifi cation:
The cyanobacterial toxins are classifi ed either on the basis of their chemical properties
or symptoms of toxicity (Gorham and Carmichael, 1988; Codd, 1995, 2000; Briand
et al
., 2003; Haider
et al
., 2003). On the basis of their chemical structure cyanobacterial toxins are divided into (1) cyclic
peptides (2) alkaloids and (3) lipopolysaccharides in order of their decreasing toxicity. On the basis
of their toxicity symptoms, the cyanobacterial toxins are classifi ed as (1) hepatotoxins (2) neurotoxins
and (3) dermatotoxins. The latter classifi cation is followed in this text.
1) Hepatotoxins
:
Microcystins (MCs), nodularin and cylindrospermopsin (CYN) are the three
hepatotoxins produced by different cyanobacteria. MCs are produced by the freshwater bloom-
forming, unicellular and colonial
Microcystis
species, mainly
M
.
aeruginosa
that is worldwide
in its distribution (Fig. 1 A). Three other species of
Microcystis
that produce MCs are
M
.
botrys
,
M
.
viridis
and
M
.
wesenbergii
(Fig. 1 B, C and D). Filamentous forms of
Anabaena
(Fig. 1 E),
Anabaenopsis
,
Aphanizomenon
(Fig. 2 A-E),
Phormidium
,
Planktothrix
(Fig. 1 F) and
Nostoc
also produce
MCs, besides the terrestrial genus
Hapalosiphon
. As many as 90 variants of MCs have so far been
recorded which show variations in the degree of methylation, hydroxylation, epimerization, peptide
sequence and toxicity (Rinehart
et al
., 1988; Codd, 1995, 2000; Sivonen, 1996; Dow and Swoboda, 2000;
Kaeberinck and Neilan, 2001; Pearson
et al.
, 2010). Nodularins are produced by
Nodularia spumigena
(Fig. 3) whereas CYN is produced by
Cylindrospermopsis
(Fig. 4 A),
Raphidiopsis
(Fig. 4B) and certain
other fi lamentous forms. MCs are cyclic heptapeptides while nodularins are cyclic pentapeptides.
CYN is an alkaloid.
The target tissue in animals including man is liver. The symptoms of poisoning by MCs in
human beings range from weakness, loss of appetite, vomiting, diarrhea and cancer. It is because
of their carcinogenic nature, a lot of attention has been given to MC related research (Ueno
et al
.,
1996; Zhou
et al
., 2002).
2) Neurotoxins
:
Anatoxins and STXs that are alkaloid in nature are included here. Several freshwater
bloom-forming algae
Anabaena fl os-aquae
(anatoxins a,b,c) and
Aphanizomenon fl os-aquae
(Fig. 2 B),
Oscillatoria
(
O
.
mougeotii
) and
Cylindrospermopsis
are known to produce anatoxins (Mitrovic
et al
.,
2004). STXs are toxins generally found in the dinofl agellates that cause paralytic shellfi sh poisoning
and these toxins fi nd their way into human beings when the shellfi sh is consumed. More than 30
STXs are produced by the freshwater cyanobacteria. Other cyanobacteria such as
Lyngbya majuscula
and
O
.
mougeotii
are also known to produce STXs (Ferreira
et al
., 2001). Anatoxins and STXs mainly
act upon the nervous system and also affect skin, liver and gastro-intestinal region. The occurrence
of cyanobacterial neurotoxins, their chemical properties, mode of action and biosynthetic pathways
have been reviewed (Aráoz
et al
., 2010).
3) Dermatotoxins
:
Species of
Lyngbya
(Fig. 5),
Oscillatoria
and
Schizothrix
have been reported
to be responsible for the commonly observed severe dermatitis in swimmers. Severe oral and
gastrointestinal infl ammation also has been noted in certain cases of ingestion. The toxins of
Lyngbya,
i.e. aplysiotoxins and debromoaplysiotoxins are tumor promoters
. O. nigroviridis
and
Schizothrix
calcicola
also produce debromoaplysiotoxins along with other two such toxins (Chorus and Bartram,
1999).
