• domoic acid;
  • electron microscopy;
  • irradiance;
  • Irish waters;
  • mass spectrometry;
  • Pseudo-nitzschia australis

A nonaxenic isolate of the potentially toxic diatom Pseudo-nitzschia australis (Frenguelli) from Irish waters was tested in two separate batch culture experiments. When grown under a low irradiance (∼12 μmol photons·m2·s1; 16:8-h light:dark cycle) for up to 40 days, the culture produced only trace amounts of the neurotoxin domoic acid (DA) during late stationary phase. Growth at a higher irradiance (∼115 μmol photons·m2·s1; 12:12-h light:dark cycle) resulted in DA production starting during late exponential phase and reaching a maximum concentration of 26 pg DA·cell1 during late stationary phase. Liquid chromatography coupled to mass spectrometry was used to confirm the identity of DA in the culture. Irradiance and photoperiod could be important factors that contribute directly or indirectly to the control of DA production in P. australis. This is the first record of a DA-producing diatom in Irish waters, and results indicate P. australis may have been the source of DA that has recently contaminated shellfisheries in this area.