CYANOBACTERIAL TOXINS - Cyanobacteria - page 521

ii) Fishes : Species of mussel ( Mytilus edulis and M . galloprovincialis ) accumulated MCs upon feeding
toxic bloom of M . aeruginosa (Williams et al ., 1997; Amorim and Vasconcelos, 1999). Fish treated with
purifi ed MCs or fed on toxic cyanobacterial material exhibited pathological changes analogous to
those reported for mammals. The studies on rainbow trout, Salmo gairdneri (Phillips et al ., 1985),
common carp, Cyprinus carpio (Rabergh et al ., 1991) and rainbow trout Oncorhynchus mykiss (Tencalla
and Dietrich, 1997) highlight such pathological changes. O . mykiss that has been fed with acutely
toxic doses (5700 µg MC kg -1 body weight) of lyophilized culture of M . aeruginosa revealed that the
toxin accumulated in the liver in a time-dependent manner. Hepatocyte necrosis has been suggested
to be due to reversible and irreversible interaction of MC-LR with PP1 whereas apoptotic cell death
after 48 h was primarily due to the irreversible inhibition of PP2A (Fischer et al ., 2000). An increase in
liver mass (depicted by hepatosomatic index; HSI) was detected in O . mykiss exposed to Microcystis
cells or its extracts with heterotrophic bacterial lipopolysaccharide (Best et al ., 2003). Together with
an increase of HSI, the fi sh accumulated water in the gut. Cell contents of Microcystis stimulated
a drinking response that led to fl uid in the gut. Crayfi sh Procambarus clarkii has been reported to
accumulate MCs in the intestine and hepatopancreas (Vasconcelos et al ., 2001). In European carp,
Cyprinus carpio low concentrations of MCs cause damage to hepatopancreas and kidney (Fischer and
Dietrich, 2000). The targets of MC-LR toxicity in embryos and larvae of freshwater fi sh, Misguruns
mizolepis are liver and heart (Liu et al ., 2002). Developing embryos of fi sh are known to be more
sensitive than juvinile stages and adults to MCs (Oberemm, 2001a; Wiegand and Pfl ugmacher, 2001).
Oreochromis nilotieus , a freshwater fi sh accumulated MCs in the gut, liver and kidney (Mohamed et
al ., 2003). Fishes that accumulate MCs in their muscle tissue tend to be dangerous to human beings
when consumed (Magalhaes et al ., 2001). Employing Thamnocephalus platyurus bioassay, Blom et al .
(2001) determined that [D-Asp 3 , (E)-Dhb 7 ] MC-RR, produced by Planktothrix rubescens , has higher
grazer toxicity compared to MC-LR, MC-RR, MC-YR and nodularin.
A number of investigators considered oxidative stress as the marker for assessing the toxicity
of MCs. The variations in the levels of reduced gultathione (GSH) and malondialdehyde (MDA)
have been selected as biomarkers for assessing the toxicity of MCs in three groups of silver carp
( Hypopththalmichthys molitrix Val.) that were reared in aquarium, grown in store pond without
cyanobacterial bloom and fi shes exposed for 25 days to natural bloom populations of M . ichthyoblabe
(60% by composition) and M . aeruginosa (40%). The levels of GSH were signifi cantly elevated in
fi sh exposed to toxic blooms (MC concentration of nearly 513 µg g -1 dry weight) than the other
two groups of fi shes. The increase of MDA levels was slightly higher in the fi shes exposed to toxic
blooms. This signifi es that fi shes exposed to MCs experience oxidative stress that is a fundamental
initiating factor to disease development in living organisms (Blaha et al ., 2004). The levels of lipid
peroxidation (LPO), superoxide dismutase (SOD), glutathione peroxidase (GPx) and glutathione
reductase (GR) increased in a time-dependent manner in tilapia fi sh ( Oreochromis sp.) exposed to
60 µg MC-LR fi sh -1 day -1 . Signifi cant increase of LPO level in liver, kidney and gills of fi sh treated
with MC-LR for 21 days suggests that oxidative stress plays an important role in the toxicity to tilapia
fi sh (Jos et al ., 2005). Tilapia fi sh, administered with a single dose of MCs (500 µg of MC-LR or MC-
RR kg -1 body weight) showed enhanced actvities of SOD, catalase and LPO that is suggestive of an
adaptive response of the fi sh to the exposure of MCs. Since these enzymes are considered as good
oxidative stress biomarkers, they can be taken as valuable tools for the assessment of early responses
of fi sh to these toxins (Prieto et al ., 2006). The toxicity of P . rubescens was studied by oral feeding
of the cultures (80,000 cells 120 µl -1 equivalent to 0.6-0.9 µg MC-LR kg -1 body weight) in European
whitefi sh ( Coregonus lavaretus ). Histological studies revealed tissue damage to liver, gastro-intestinal
tract and kidney due to accumulation of MC (Ernst et al ., 2006).