Evaluation of intracellular and extracellular domoic acid content in Pseudo-nitzschia multiseries cell cultures under different light regimes.

Pseudo-nitzschia multiseries is a diatom species associated with the production of domoic acid (DA), a water soluble neurotoxin that is easily transferred up in the food web, causing devastating effects on top marine organisms and humans. Despite studies on Pseudo-nitzschia are relevant to human health safety, partitioning of marine toxins between intracellular and extracellular fractions are poorly documented. This study aimed to determine the growth rates and DA content, both intracellular and extracellular, of Pseudo-nitzschia multiseries cultures at three different light settings (15, 120 and 560 µmol m-2 s-1). The optimal conditions for cell growth were observed at 120 and 560 µmol m-2 s-1, whereas DA production was observed in P. multiseries at 15 and 120 µmol m-2 s-1, ranging between 0.18-2.56 and 0.16-3.5 pg DA cell-1, respectively. Higher intracellular DA concentrations were found during the senescence phase at low light intensity and during the exponential phase at medium light intensity, while higher concentrations of dissolved DA were found at low and medium light intensities in the senescence phase reaching 3 and 10 ng DA mL-1 respectively. The amount of toxin released into the culture medium represents the most important fraction ranging between 63 and 98% during the exponential phase and nearly 99% during the senescence phase. In contrast, under low light intensity, dissolved DA was detected in the culture medium only during the senescence phase. This study confirms the importance of light intensity on DA production and clearly shows that dissolved domoic acid is an important fraction in Pseudo-nitzschia cultures, suggesting with the careful assumptions of results from static cultures extrapolated to bloom situations that waterborne exposure of marine organism should be considered during blooms of Pseudo-nitzschia multiseries.