Sinking of Heterosigma akashiwo results in increased toxicity of this harmful algal bloom species Lucas Powers a , Irena F. Creed a , Charles G. Trick a,b, * a Department of Biology, University of Western Ontario, London, Ontario N6A 5B7, Canada b Department of Microbiology and Immunology, Schulich School of Medicine and Dentistry, University of Western Ontario, London, Ontario N6A 5B7, Canada 1. Introduction The global prevalence and severity of harmful algal blooms (HABs) appears to be increasing in many marine ecosystems (Smayda, 1989; Horner et al., 1997; Anderson et al., 2002; Heisler et al., 2008). HABs are characterized by the dominance of a single phytoplankton species, eventually forming dense concentrations of algal biomass that threaten the health of the ecosystem by a number of mechanisms. It remains unclear, however, precisely what environmental parameters mediate bloom formation and what toxicological mechanisms HAB species employ that result in these harmful effects (Anderson et al., 2002; Hallegraeff and Hara, 2003), with considerable variation among the relatively small percentage of algal genera capable of forming HABs (Morris, 1999; Landsberg, 2002). Generally, potentially harmful species can affect a community in one of the following ways (Smayda, 1997a,b; Hallegraeff and Hara, 2003): (i) causing significant water discoloration due to extremely dense cellular aggregations, which creates a physical barrier to light penetration that subsequently results in anoxic conditions as blooms dissipate; (ii) producing potent toxins that bioaccumulate and are known to be directly harmful to humans; and (iii) demonstrating allelopathic and anti-biological strategies that can induce high mortality in a wide range of aquatic organisms, particularly fish, with devastating effects on aquacul- ture (Honjo, 1993). Considerable debate surrounds species in this third category, as no single process or mechanism has been conclusively identified to explain how these species achieve ichthyotoxicity. A phytoplankton species of particular concern within this third category is the red-tide forming alga, Heterosigma akashiwo (Hada) (Hara and Chihara, 1987). Coastal blooms of this fish-killing raphidophyte have been observed in both the Atlantic and Pacific Oceans and have been implicated in fish-kills in aquaculture operations in Canada, Chile, Japan and New Zealand (Hallegraeff and Hara, 2003). Additionally, severe economic losses associated with fin-fish mortality attributed to H. akashiwo have been reported in Japan (Honjo, 1993), British Colombia, Canada (Taylor, 1991; Taylor and Haigh, 1993), Washington State, USA (Connell and Cattolico, 1996), and New Zealand (Chang et al., 1990). More recently, blooms of H. akashiwo have been reported in San Francisco Bay, USA (Herndon and Cochlan, 2006) and the inner waters of British Colombia and Washington State (Horner et al., 1997). Harmful Algae 13 (2012) 95–104 A R T I C L E I N F O Article history: Received 7 June 2010 Received in revised form 21 September 2011 Accepted 12 October 2011 Available online 22 October 2011 Keywords: Heterosigma akashiwo Sinking Bioassays Toxicity Bloom propagation A B S T R A C T Notable physiological responses such as toxicity and sinking rates of the red tide forming raphidophyte Heterosigma akashiwo are correlated with high levels of macronutrient stress. Individual cells of this species are also capable of forming benthic vegetative cysts that overwinter in marine sediment and contribute to bloom propagation in subsequent seasons. It was hypothesized that there is variability in the rates of sinking within cell cultures and that sinking cells are more toxic than the neutrally buoyant or floating cells. Using laboratory-based settling columns, various isolates of H. akashiwo were allowed to separate, and the toxicities of sinking and floating populations were analyzed. Sinking and floating rates were significantly higher during the late stationary growth phase for all isolates. For two H. akashiwo isolates, sinking populations were significantly more toxic than those that were positively buoyant. A similar trend was observed in a third strain, however the relationship was not significant. Differences in adaptive ecophysiology among the different strain likely caused the variation. It is suggested that the most toxic cells within a bloom are those found at the lower depths, potentially interacting with the benthic community or ensuring that subsequent bloom propagation contains cells with the potential for toxicity. ß 2011 Elsevier B.V. All rights reserved. * Corresponding author at: Department of Biology, University of Western Ontario, London, Ontario N6A 5B7, Canada. Tel.: +1 519 661 3899; fax: +1 519 850 2343. E-mail addresses: lpowers@uwo.ca (L. Powers), icreed@uwo.ca (I.F. Creed), trick@uwo.ca (C.G. Trick). Contents lists available at SciVerse ScienceDirect Harmful Algae jo u rn al h om epag e: ww w.els evier.c o m/lo cat e/hal 1568-9883/$ – see front matter ß 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.hal.2011.10.007