Bloom forming Alexandrium ostenfeldii (Dinophyceae) in shallow waters of the A ˚ land Archipelago, Northern Baltic Sea Anke Kremp a, *, Tore Lindholm b , Nicole Dreßler c , Katrin Erler c , Gunnar Gerdts d , Sanna Eirtovaara e , Elina Leskinen e a Tva ¨rminne Zoological Station, University of Helsinki, 10900 Hanko, Finland b Department of Biology, A ˚ bo Akademi University, 20500 Turku, Finland c Department of Food Chemistry, Institute of Nutrition, University of Jena, Dornburger Strasse 25, D-07743 Jena, Germany d Biologische Anstalt Helgoland, Alfred Wegener Institute for Polar and Marine Research, 27483 Helgoland, Germany e Department of Biological and Environmental Sciences, 00014 University of Helsinki, Finland 1. Introduction Alexandrium ostenfeldii (Paulsen) Balech and Tangen is a gonyaulacoid dinoflagellate with a wide geographic distribution in temperate waters around the world. Originally described from the coast of Iceland (Paulsen, 1904) and thereafter mostly reported from high latitude waters of Europe (Braarud, 1945; Balech and Tangen, 1985; Moestrup and Hansen, 1988; John et al., 2003b) and North America (Balech, 1995; Levasseur et al., 1997; Cembella et al., 2000a; Gribble et al., 2005), the Russian Arctic (Okolodkov and Dodge, 1996) and the eastern Siberian Seas (Konovalova, 1991 and references therein), this species was long considered to be confined to northern cold-water environments. Records of A. ostenfeldii from New Zealand (MacKenzie et al., 1996) and the coasts of Spain (Fraga and Sa ´ nchez, 1985) as well as the Mediterranean (Balech, 1995) demonstrate that the species is apparently widely distributed around the globe. A. ostenfeldii has a long taxonomic history which includes allocation to Alexandrium tamarense as a variety (Braarud, 1945) and the repeated transfer to different genera (Balech, 1995). The taxonomy and identification of the species are complicated by the variability of distinctive characters, which separate A. ostenfeldii from the closely related Alexandrium peruvianum (Balech and de Harmful Algae 8 (2009) 318–328 ARTICLE INFO Article history: Received 6 February 2008 Accepted 7 July 2008 Keywords: Alexandrium ostenfeldii Baltic Sea Bioluminescent blooms LSU and ITS rDNA Morphology Salinity tolerance Toxicity ABSTRACT In the past years, late summer blooms of the bioluminescent dinoflagellate Alexandrium ostenfeldii have become a recurrent phenomenon in coastal waters of the central and Northern Baltic Sea. This paper reports exceptionally high cell concentrations (10 5 to 10 6 cells L À1 ) of the species found during bioluminescent blooms in 2003 and 2004 in a shallow embayment of the A ˚ land archipelago at the SW coast of Finland. Clonal cultures were established for morphological, molecular, toxicological and ecophysiological investigations to characterize the Finnish populations and compare them to other global A. ostenfeldii isolates. The Finnish isolates exhibited typical morphological features of A. ostenfeldii such as large size, a prominent ventral pore and an orthogonally bent first apical plate. However, unambiguous differentiation from closely related Alexandrium peruvianum was difficult due to considerable variation of sulcal anterior plate shapes. The Finnish strains were genetically distinct from other isolates of the species, but phylogenetic analyses revealed a close relationship to isolates from southern England and an A. peruvianum morphotype from the Spanish Mediterranean. Together these isolates formed a distinct clade which was separated from a clade containing other Northern European, North American and New Zealand populations. Toxin analyses confirmed the presence of the PSP toxins GTX2, GTX3 and STX in both Finnish isolates with GTX3 being the dominant toxin. Total relative PSP toxin contents were moderate, ranging from approximately 6 to 15 fmol cell À1 at local salinities of 5 and 10 psu, respectively. Spirolides were not detected. Salinity tolerance experiments showed that the Finnish isolates were well adapted to grow at the low salinities of the Baltic Sea. With a salinity range of approximately 6 to 20– 25 psu, Baltic populations are physiologically distinct from their marine relatives. Vigorous production of different cyst types in the cultures suggest that cysts may play a crucial role in the survival and retainment of A. ostenfeldii populations in the Baltic Sea. ß 2008 Elsevier B.V. All rights reserved. * Corresponding author at: Finnish Environment Institute, P.O. Box 140, 00251 Helsinki, Finland. Tel.: +358 400 148524; fax: +358 204 902291. E-mail address: anke.kremp@environment.fi (A. Kremp). Contents lists available at ScienceDirect Harmful Algae journal homepage: www.elsevier.com/locate/hal 1568-9883/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.hal.2008.07.004