Biology Reference
In-Depth Information
glutathione levels has been suggested to be due to an inhibition in its synthesis. Cytochrome P450
appears to be involved in the metabolism of CYN and the metabolites formed might even be more
toxic and potential inhibitors of glutathione synthesis (Runnegar
et al
., 1995).
In relation to toxicity three important points are worth considering. Firstly, the linearization of MCs
or nodularin abolishes the toxic effects. Secondly, the overall shape of these molecules in relation to
Adda side chain is critical for interaction with PPs. Modifi cations in the Adda such as demethylation
of the C9 of Adda side chain have little effect on toxicity but the geometrical isomers in which C6
diene is changed from
trans
E confi guration to the
cis
Z confi guration possess decreased toxicity by
100-folds. Accordingly, the IC
50
of such variants towards PPs is increased (Nishiwaki-Matsushima
et
al
., 1991). Thirdly, the presence of carboxylic acid on the glutamic acid residue to MCs is important to
toxicity. MC variants with alterations in Xaa and Zaa at 2nd and 4th positions all possess almost the
same toxicity as MC-LR as evidenced by the IC
50
values (Craig
et al
., 1993; Rinehart
et al
., 1994).
F) Toxicity studies with Hepatotoxins:
Cyanobacteria have been known to cause animal and human
poisoning in different parts of the world for over last 100 years. Toxicity studies on plants and animals
(mussels, crayfi sh and fi shes used for human consumption) have shown that MCs accumulate in
body parts and are harmful to human beings on consumption (De Figueiredo
et al
., 2004). Toxicity
in relation to animal testing data is generally expressed as LD
50
value of the toxin that is required
to kill 50% of the population of the tested animals. In growth measurements, concentration of the
toxin inhibiting growth by 50% is expressed as EC
50
whereas concentration of the toxin inhibiting
enzyme activity or lethal to animalicules in solution by 50% is expressed as IC
50
(Table 5).
i) Zooplanktons:
The blooms of
M
.
aeruginosa
become predominant in natural waters by inhibiting the
growth of other microalgae which serve as food for the zooplankton. This results in an unfavourable
nutritive condition for the zooplanktons (Kurmayer and Juettner, 1999). Cyanobacterial toxicity to
Daphnia
has been reported as early as 1980s (Lampert, 1981; Fulton and Pearl, 1987; Benndorf and
Henning, 1989). Biomass of
M
.
aeruginosa
or
Aph. fl os-aquae
exerted considerable toxic effects on
the life cycle (Reinikainen
et al
., 1994), movements and feeding appendages of
Daphnia
(Haney
et
al
., 1995). Two alternatives have been suggested to explain the grazer resistance shown by certain
zooplanktons. The fi rst is the evolution of phenotypes with plasticity in feeding behavior through
natural selection (Hariston
et al
., 2001) and the second is the availability of phosphorus-defi cient
diets (DeMott and Dhawale, 1995).
Toxic cyanobacteria inhibit the feeding of zooplanktons and increase their mortality (Lampert,
1987; De Bernardi and Giussani, 1988, 1990). DeMott
et al
. (1991) compared the feeding habits of
four species of zooplanktons that differed in their physiological sensitivities to MC-LR, nodularin
and feeding behaviour in presence of toxic and non-toxic bloom algae.
Daphnia pulcaria
was less
sensitive to these toxins (LD
50
21.4 µg ml
-1
for 48 h) and stopped feeding in presence of toxic cells. On
the contrary,
D
.
pulex
exhibited greater sensitivity (LD
50
9.6 µg ml
-1
for 48 h) but showed uninhibited
feeding on toxic cells. The copepod
Diaptomus birgei
was highly sensitive (LD
50
0.45 to 1.0 µg ml
-1
for
48 h) but could survive better than the earlier two because of its capability in selecting its food and
uninterrupted feeding.
Daphnia hyalina
(LD
50
11.6 µg ml
-1
for 48 h) is moderately sensitive but showed
rapid inhibition in feeding in presence of toxic cyanobacteria. It means in the absence of alternative
food, the zooplanktons may become selective (so stop feeding) or may not be able to differentiate
between toxic and non-toxic food materials (so show continued feeding behaviour). Kurmayer
and Juettner (1999) confi rmed the fi ndings of DeMott
et al
. (1991) and suggested that copepods
avoid toxic cyanobacteria and are highly selective while daphnid cladocerans are less selective. It
is because of this reason they ingest both toxic and non-toxic strains of
Microcystis
(Rohrlack
et al
.,
