Growth dynamics of non-toxic Pseudo-nitzschia delicatissima and toxic P. seriata (Bacillariophyceae) under simulated spring and summer photoperiods Johanna Fehling a, * , Keith Davidson a , Stephen S. Bates b a Scottish Association for Marine Science, Dunstaffnage Marine Laboratory, Oban PA37 1QA, UK b Fisheries and Oceans Canada, Gulf Fisheries Centre, P.O. Box 5030, Moncton, NB, Canada, E1C 9B6 Received 19 October 2004; received in revised form 2 November 2004; accepted 12 November 2004 Abstract Marine planktonic diatoms of the genus Pseudo-nitzschia Peragallo have been responsible for amnesic shellfish poisoning (ASP) events worldwide through the production of the neurotoxin domoic acid (DA). The appearance and toxicity of Pseudo- nitzschia species is variable throughout the year and potentially linked to changes in environmental parameters; many ASP events occur in relatively high latitudes where day length is particularly variable with season. In UK waters, shellfish monitoring has prevented any impact on human health but has led to long-term closures of fisheries, with severe economic consequences. Laboratory experiments on two Pseudo-nitzschia species typically found in Scottish West Coast waters during spring (short photoperiod (SP)) and summer (long photoperiod (LP)) conditions were conducted to determine the influence of photoperiod on their growth and toxicity. Results indicated that non-toxic P. delicatissima (Cleve) Heiden achieved a greater cell density under SP (9-h light:15-h dark (L:D) cycle). For toxin-producing P. seriata (Cleve) H. Peragallo, a LP (18-h L:6-h D cycle) resulted in an enhanced growth rate, cell yield and total toxin production, but it decreased the toxin production per cell. A better understanding of the response of Pseudo-nitzschia species to photoperiod and other foreseeable environmental variables may help predict the appearance of toxic strains. # 2004 Elsevier B.V. All rights reserved. Keywords: Domoic acid; Growth rate; Photoperiod; Pseudo-nitzschia; P. delicatissima; P. seriata 1. Introduction Harmful algal blooms are perceived to be an increasing environmental, health and economic problem world wide. Most of the causative organisms are dinoflagellate species of marine phytoplankton. However, examples of harmful species have now also been identified within the Bacillariophyceae. Marine www.elsevier.com/locate/hal Harmful Algae 4 (2005) 763–769 * Corresponding author. Present address: Department of Biology, Area 3, University of York, P.O. Box 373, York, YO10 5YW, UK. Tel.: +44 1904 328633; fax: +44 1904 328505. E-mail address: jf510@york.ac.uk (J. Fehling). 1568-9883/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.hal.2004.11.002