International Journal on Algae, 2014, 16(1): 95–105 95 The Growth Response of Prorocentrum minimum Pavill. (Dinophyta) to Karlotoxin Exposure G. Ozbay 1 , Sh.S. Chambliss 1 , G.H. Wikfors 2 , J.E. Adolf 3 , L.K. Chintapenta 1 & A.R. Place 4 1 Delaware State University, Department of Agriculture and Natural Resources, 1200 North Dupont Highway, Dover, Delaware 19901, USA e-mail: gozbay@desu.edu 2 National Oceanic and Atmospheric Administrations (NOAA), National Marine Fisheries Service Center, Milford Laboratory, Milford, Connecticut 06460, USA e-mail: gary.wikfors@noaa.gov 3 University of Hawaii Hilo, Hilo, Hawaii 96729, USA e-mail: jadolf@hawaii.edu 4 University of Maryland Biotechnology Institute, Institute of Marine and Environmental Technology, Columbus Center, Baltimore, MD 21202, USA e-mail: place@umces.edu ABSTRACT Extracellular metabolites produced by harmful algae can act as growth-inhibiting agents for other phytoplankton species, influencing species competition and succession and hence affecting structure of the plankton community. Karlodinium veneficum Ballant., a cosmopolitan estuarine dinoflagellate, produces toxic compounds known as karlotoxins that exhibit sterol-dependent, cytotoxic activity and are frequently associated with fish kills. Karlotoxin-sensitive cells tend to have desmethyl sterols as predominant cellular sterols, and karlotoxin-resistant cells have 4-methyl sterols as dominant sterols. The allelopathic effects of karlotoxins on other algae have been described, but the question of whether or not K. veneficum is allelopathic against Prorocentrum minimum, a common co-occurring dinoflagellate, is unknown. We determined the sterol profiles of two different Chesapeake Bay strains (RR4B1 and IB) of P. minimum and also exposed them to different concentrations of karlotoxin extracted from K. veneficum cells. The strains, RR4B1 and IB, experienced mortality at high toxin concentrations, i.e., 256 ng mL -1 . After 24 hours of exposure, cell counts declined resulting in calculated “negative growth rates” of -1.17 (d -1 ) for RR4B1 and Originally published in Algologia, 2013, 23(3), pp. 268–269 ISSN 1521–9429 ©Begell House Inc., 2014