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5th HAB US Conference Abstract Allelopathic effect of Cochlodinium polykrikoides isolates from North America on competing phytoplankton Ying Zhong Tang and Christopher J. Gobler* School of Marine and Atmospheric Sciences, Stony Brook University, Stony Brook, NY 117945000, USA *Corresponding author. E-mail address: [email protected]. Tel.: +1 631 632 5043, Fax: 631-632-5070 ABSTRACT The toxic dinoflagellate, Cochlodinium polykrikoides, forms annual harmful algal blooms (HAB) in estuaries on both coasts of the US, and its acute toxicity to multiple species of finfish, shellfish, and shellfish larvae has been recently reported by our lab group (Gobler et al 2008, Tang and Gobler 2009, Tang and Gobler, submitted). The nature of the toxicity of C. polykrikoides led us to an investigation on its allelopathic effects on other phytoplankton. Our results demonstrated highly inhibitory and toxic effects exhibited in clonal cultures of C. polykrikoides isolated from the east and west coasts of North American on a variety of common phytoplankton species, including Rhodomonas salina, Isochrysis galbana, Aureococcus anophagefferens, Thalassiosira weissflogii, Chattonella marina, Heterocapsa rotundata, Scrippsiella cf. trochoidea, Akashiwo sanguinea, Gymnodinium aureolum and Gymnodinium instriatum, with the latter five commonly co-occurring with C. polykrikoides during bloom seasons. The allelopathic impacts on all species of microalgae were noticeable in minutes to hours, causing loss of motility, morphological change, cell lysis, and death. The alleopathic effects depended on both the absolute and relative cell densities of C. polykrikoides and target species and exposure time (i.e. longer co-culturing caused higher mortality of cells in the competing species). Treatments of freezing-and-thawing, heating, sonication, and filtration for the culture of C. polykrikoides led to complete loss of its allelopathic effect on A. sanguinea, the most sensitive algal species, suggesting that the responsible allelochemicals are short-lived and associated with live cells. Separation of C. polykrikoides and other algae by a 5μm-mesh nylon membrane resulted in growth inhibition of the competitors which was significantly greater than control cultures, but significantly lower than direct contact indicating that allelopathic agents could be released and dissolved in culture medium and that direct contact between the C. polykrikoides and target cells maximizes the allelopathic effect. Additions of the free radical-scavenging enzymes, peroxidase and catalase, into C. polykrikoides culture eliminated its allelopathic effect, implying a reactive chemical(s) are responsible for the algae-killing effect of C. polykrikoides. All the above results are consistent with our results obtained from fish and shellfish larvae bioassays, suggesting that the toxicity of C. polykrikoides to fish, shellfish, and shellfish larvae and the allelopathic effects on other phytoplankton were caused by similar mechanisms. The dramatic allelopathic effects exhibited by C. polykrikoides imply that this process may play an important role in the imitation and maintenance of mono-specific blooms formed by this species.