Interestingly, the influence of light on toxicity associated
with P. parvum is less defined than other chemical and physical
factors. When Larsen and Bryant (Larsen and Bryant,
1998) exposed cultures of different strains of P. parvum to
light, they observed an increased growth rate with increasing
light (saturation occurred at about 200 mmol
photons/m2/s), but the magnitude of toxicity to Artemia
sp. was not affected. Similarly, Baker et al. (Baker et al.,
2007) observed that optimal growth was reached at
200 mmol photons/m2/s and the magnitude of toxicity to
fish did not respond to light intensity. Rahat and Jahn
(Rahat and Jahn, 1965) found that P. parvum growth was
possible in the dark when glycerol was added to the
culture medium, and these cultures were more toxic than
those grown with alternating dark and light periods, indicating
that light may not be a requirement for toxin synthesis
and production. They also concluded that any
toxicity assays performed from cultures grown in alternating
light–dark periods only reveal the net results of apparent
production and “inactivation’’ of the toxin (Rahat and
Jahn, 1965). Another study also demonstrated that constant
illumination with a fluorescent lamp reduced toxicity
and “inactivated’’ the toxins, whereas toxicity was only
observed with light and dark periods (Reich and Parnas 1962). Similar observations were reported by Fistarol et al.
(2003).