Algicidal bacteria associated with blooms of a toxic dinoflagellate in a temperate Australian estuary

Abstract

Gymnodinium catenatum is an introduced toxic dinoflagellate that blooms intermit- tently and causes shellfish farm closure in the Huon Estuary, Tasmania, Australia. Seventy-five bac- teria isolated from the estuary were tested for algicidal activity against this and other toxic and non- toxic algal species. Five isolates produced algicidal extracellular exudates. These algicidal species were a Pseudoalteromonas sp. (ACEM 4), a novel Zobellia sp. (ACEM 20), a strain of Cellulophaga lytica (ACEM 21) and 2 Firmicutes: a novel Planomicrobium sp. (ACEM 22) and a strain of Bacillus cereus (ACEM 32). This study is the first time Gram-positive bacteria have been associated with algi- cidal activities. Further data are presented on an algicidal Pseudoalteromonas species previously iso- lated from the Huon Estuary (Strain y). Supernatant produced by all 5 strains caused cell lysis and death in G. catenatum vegetative cells. No change or reversible ecdysis was noted for 2 other endemic dinoflagellate species. Algicidal or inhibitory activity was not activated via homoserine lac- tones, but bacterial quorum sensing for the isolates was shown by means of the AI-2 mechanism. Algicidal activity from field isolates was also influenced by strain or environmental variation. Bacte- ria were capable of losing or switching off their algicidal ability indicating that the presence of an algicidal species in the environment may not necessarily signify that they are currently algicidal. Concentrations of algicidal compounds required for algal lysis in laboratory experiments indicate that the 5 bacterial species can be effective against G. catenatum vegetative cells if they dominate the bacterial population in the estuary, particularly when attached to particles.