Pseudo-nitzschia australis (Frenguelli), a toxigenic pennate diatom capable of producing the neurotoxin domoic acid (DA), was examined in unialgal laboratory cultures to quantify its physiological response to ocean acidification (OA) – the decline in pH resulting from increasing partial pressure of CO2 (pCO2) in the oceans. Toxic blooms of P. australis are common in the coastal waters of eastern boundary