Kinetics of attachment of potentially toxic bacteria to Alexandrium tamarense

Abstract

Interactions between bacteria and harmful algae are potentially important regulating factors for population dynamics of both organisms and for toxin production. These interactions are still poorly understood. To monitor the physical associations between potentially toxic bacteria and dinoflagellates in controlled conditions, we inoculated an axenic non-toxic strain of Alexandrium tamarense (Dinophyceae) together with reputed paralytic shellfish toxin (PST)-producing bacteria belonging to the genus Alteromonas (γ subdivision of the division Proteobacteria) and to the clade Roseobacter (α subdivision of the division Proteobacteria). The attachment behavior of both bacterial strains was monitored using TSA-FISH (tyramide signal amplification and fluorescent in situ hybridization) and confocal microscopy. Our results suggest that ageing dinoflagellate cultures stimulate both free bacterial growth and attachment. However, toxin production by originally non-toxic dinoflagellate cells was not induced by the physical interaction of either of the bacterial strains with the dinoflagellate cells. This does not support the hypothesis that toxic bacteria could simply control toxin production by attachment to particle surfaces such as eukaryotic organisms.KEY WORDS: Attached bacteria · Dinoflagellate · Alexandrium · Bacteria-phytoplankton interactions · In situ hybridization · Tyramide signal amplification · Confocal micoscopy Resale or republication not permitted without written consent of the publisherAquat Microb Ecol 28: [249][250][251][252][253][254][255][256] 2002 autonomous bacterial production of phycotoxins (Gallacher et al. 1997 has, in some cases, cast doubt that these toxins are indeed produced by the algae alone (see reviews by Rausch de Traubenberg & Lassus 1991. Nevertheless, several authors have reported on the production of PST by 'axenic' dinoflagellate cultures with levels of toxicity similar to those observed for non-axenic strains (Dantzer & Levin 1997, John & Flynn 1999 but the absence of bacteria (free, attached or endocellular) was not clearly demonstrated. Tosteson et al. (1989) and suggested that bacteria could modify algal toxicity via attachment. However, mechanisms by which physical interactions can influence toxin production levels for the partners of the bacteria-dinoflagellate association remain unknown. Indeed, techniques have been lacking to identify specifically the bacteria associated to Alexandrium spp. and to study their spatial relationships. Bell & Mitchell (1972) created the term 'phycosphere' to qualify a zone surrounding algal cells in which bacteria are influenced by algae and conversely algae by bacteria. Bacteria can be free in the phycosphere (Blackburn et al. 1998), attached to the surface of algal cells (Kogure et al. 1982, Vaqué et al. 1990, Worm & Sondergaard 1998 or intracellular (Silva & Franca 1985, Lewis et al. 2001. Few investigators have examined the process of attachment to algal cells (Kogure et al. 1982, Vaqué et al. 1990, Worm & Sondergaard 1998. Me...

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