ALGICIDAL EFFECTS OF A NOVEL MARINE PSEUDOALTEROMONAS ISOLATE (CLASS PROTEOBACTERIA, GAMMA-SUBDIVISION) ON HARMFUL ALGAL BLOOM SPECIES OF THE GENERA CHATTONELLA, GYMNODINIUM, AND HETEROSIGMA

During a bacterial survey of the Huon Estuary in southern Tasmania, Au stralia, we isolated a yellow-pigmented Pseudoalteromonas strain (clas s Proteobacteria, gamma subdivision), designated strain Y, that had po tent algicidal effects on harmful algal bloom species. This organism w as identified by 16S rRNA sequencing as a strain with close affinities to Pseudoalteromonas peptidysin. This bacterium caused rapid cell lys is and death (within 3 h) of gymnodinoids (including Gymnodinium caten atum) and raphidophytes (Chattonella marina and Heterosigma akashiwo). It caused ecdysis of armored dinoflagellates (e.g., Alexandrium caten ella, Alexandrium minutum, and Prorocentrum mexicanum), but the algal cultures then recovered over the subsequent 24 h, Strain Y had no effe ct on a cryptomonad (Chroomonas sp.), a diatom (Skeletonema sp,), a cy anobacterium (Oscillatoria sp,), and two aplastidic protozoans. The al gicidal principle of strain Y was excreted into the seawater medium an d lost its efficacy after heating. Another common bacterial species, P seudoalteromonas carrageenovora, was isolated at the same time and did not have these algicidal effects. The minimum concentrations of strai n Y required to kill G. catenatum were higher than the mean concentrat ions found in nature under nonbloom conditions. However, the new bacte rium showed a chemotactic, swarming behavior that resulted in localize d high concentrations around target organisms. These observations impl y that certain bacteria could play an important role in regulating the onset and development of harmful algal blooms.