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(viz., 10-15 ppt), but increased with increasing salinity (Lim and Ogata 2005 ).
Parkhill and Cembella ( 1999 ) demonstrated a positive correlation between cellular
toxicity and salinity in A. tamarense , where the highest cellular toxicity occurred at
30 ppt. Toxin content in A. minutum (Grzebyk et al. 2003 ) and A. tamarense (Parkhill
and Cembella 1999 ) decreased with increasing salinity (Lim and Ogata 2005 ). This
resulted from the nutrients allocated for growth being reduced at higher salinity,
which limited nutrient available for toxin production (Lim and Ogata 2005 ).
The growth rate of cultured Alexandrium sp. increased as light increased up to a
saturation point of 150 ʼ mol m −2 s −1 at 20 °C (Parkhill and Cembella 1999 ). However,
the maximum growth rates could be reached at lower irradiances if temperatures were
higher (Ogata et al. 1990 ). A Malaysian A. tamiyavanichii strain achieved optimum
growth when cultured under a light intensity of 40 ʼ mol photons m −2 s −1 and at an opti-
mum temperature of 25 °C (Lim et al. 2006 ). In other HAB species, blooms of Karenia
have been associated with warmer and constant water temperature conditions (Dahl
and Tangen 1993 ). In one Tunisian lagoon, a Gymnodinium aureolum bloom was asso-
ciated with decreasing day length (Romdhane et al. 1998 ). Some study results sug-
gested that the toxin content in Alexandrium sp. (Hamasaki et al. 2001 ; Ogata et al.
1989 ) and Gymnodinium catenatum (Ogata et al. 1989 ) was inversely proportional to
the growth rate; slow growth at low temperature and under low light conditions
stimulated toxin synthesis. The results of other studies indicated that the toxin pro-
duced by A. tamarense and A. minutum was not infl uenced by growth of the organ-
ism, but rather by exogenous factors (Kodama 1990 ). Parkhill and Cembella ( 1999 )
reported that the cellular toxicity of the toxin produced by A. tamarense was at a
maximum under intermediate light conditions of 60 ʼ mol m −2 s −1 (Parkhill and
Cembella 1999 ). However, toxin production is suppressed in Pyrodinium bahamense
var. compressum (Usup et al. 1995 ) and A. minitum (Lim et al. 2006 ) when they
are grown under low light and low temperature conditions, due to the inhibition of the
enzyme Ribulose-1, 5-bisphosphate carboxylase/oxygenase (RUBISCO).
In general, the PST composition produced by Alexandrium sp. is not infl uenced
by the brightness of light to which the organism is exposed (Parkhill and Cembella
1999 ). However, temperature is an important factor in altering the PST composition
in A. tamiyavanichii , where the composition of GTX4 increased gradually as cells
were grown at temperatures higher than the optimum needed for growth (Lim et al.
2006 ). Transformation of GTX3 and GTX2 into GTX4 by enzymatic oxidation is
enhanced by increased temperatures until the optimum temperature of 30 °C is
reached (Kodama 2000 ). Salinity is another factor that infl uences the composition
of PST in HAB cells. Low salinity stimulated Alexandrium minimum to produce a
higher concentration of GTX1, which is the major toxin produced during periods of
contamination (Hwang and Lu 2000 ). PSTs are stable structurally under acidic con-
dition, but are easily oxidized under alkaline conditions; indeed, alkaline treatment
reduces overall toxicity (WHO 1984 ). However, saxitoxin has a greater effect at
neutral pH than under acidic or basic conditions (Baden and Trainer 1993 ). Both the
guanidinium and hydroxyl groups of the toxins are required for sodium channel
recognition. Changing the pH value of water affects these groups and can decrease
or eliminates the biological effects of many PST toxins.
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