SHORT COMMUNICATION Bacterial chemotaxis towards the extracellular products of the toxic phytoplankton Heterosigma akashiwo

The occurrence of harmful algal blooms (HABs), caused by toxic phytoplankton, has detrimental environmental, economic and human health effects within many aquatic habitats (Horner et al., 1997). Understanding the environmental conditions that control the development and decline of HABs has consequently become an important element of ecosystem management within heavily affected environments. Potentially significant links between HAB dynamics and heterotrophic bacteria have been reported, with various species of aquatic bacteria implicated in both the promotion and inhibition of HABs (Kim et al., 1998; Skerratt et al., 2002; Liu et al., 2008a, b). This has revealed the unseen complexity of HAB ecology, as well as arousing interest in the potential application of algicidal bacteria as biocontrols to terminate HABs (Kim et al., 2007). The raphidophyte phytoplankton Heterosigma akashiwo is a toxic species that has been implicated in several major fish kills worldwide (Honjo, 1993). The mechanism behind H. akashiwo toxicity is still unresolved, but increases in the toxic effects of this species have been linked to ecological interactions with heterotrophic bacteria (Carrasquero-Verde, 1999). Furthermore, heterotrophic bacteria can both enhance (Liu et al., 2008a, b) and inhibit the growth of H. akashiwo (Kim et al., 1998; Lovejoy et al., 1998; Yoshinaga et al., 1998; Liu et al., 2008a, b), indicating that bacterial-algal interactions play a fundamental role in the ecology and environmental impact of this toxic species. Behavioural responses, including localized bacterial clustering around H. akashiwo cells (Lovejoy et al., 1998), forming specific algal–bacterial consortia (Carrasquero-Verde, 1999), have been predicted to be an important element of these interactions. In this study, we examined whether the chemical products of H. akashiwo act as chemoattractants for three environmentally relevant strains of marine bacteria, to elucidate how behavioural