Bacterial and algal interactions in a Tasmanian estuary

The microbial communities and the physical and chemical environment of the Huon Estuary, Tasmania, Australia, were sampled in an 18 month program. Analysis of field samples, and laboratory experiments were used to examine the characteristics of estuarine and marine bacteria and algae, relationships between bacteria and algae, and their ecological roles. A multidisciplinary approach was used, including morphological, molecular, and chemical techniques. Five new algicidal bacteria strains were isolated from the estuary. The species were identified, based on molecular and phenotypic analyses, as a novel Pseudoalteromonas sp., a novel Planococcus sp., a novel Cellulophaga sp., and two isolates that were closely related to the species Cellulophaga lytica and Bacillus cereus. The bacteria exhibited a powerful lytic effect on the vegetative lifestage of the toxic dinoflagellate Gymnodinium catenatum, an introduced species that blooms intermittently causing shellfish farm closures in the estuary. Excystment or encystment of the cyst lifestage of G. catenatum was unaffected by algicidal activity. The bacteria had no observable effect on rotifer, cyanobacteria and diatom species tested. Algicidal ability of the bacteria varied both in culture and in the environment. These variations depend on environmental conditions or may be due to differences in species-specific modes of action. Antimicrobial brominated compounds, unrelated to the algicidal activity of the bacteria, were also identified in one Pseudoalteromonas species. A novel Shewanella species was isolated that contains the highest proportion of the essential fatty acids 20:5(n-3) reported for a temperate member of this genus. These levels are close to those produced by psychrophilic Shewanella species. Fluorescence in situ hybridisation (FISH), signature lipid profiling, and morphological data obtained during two Gymnodiniuni catenatum blooms and one diatom bloom, demonstrate that algicidal bacteria can form a major part of the bacterial community. Fatty acid analysis differentiated between different bacterial taxa isolated in this study and between field samples from the estuary. Variation of the branched chain fatty acids (BCFA) reflected the strong association of the Cytophaga-Flavobacterium-Bacteriodes (CFB) cluster with microbial degradation of algal blooms in the estuary. Results from fatty acid analyses indicated that the CFB cluster are more common in the photic zone and during the chlorophyll maximum, while low levels of BCFA indicated that ˜í‚â• proteobacteria may be more abundant in the deeper, marine influenced waters. FISH analysis also showed that the CFB cluster was common in the estuarine community during blooms of the dinoflagellate G. catenatum. Member of the genera Pseudoalteromonas-Colwellia were a significant component during diatom blooms. Alpha (˜í¬±) and gamma (˜í‚â•) proteobacteria were common in the estuary, however, beta (˜í‚â§) proteobacteria were not. Throughout the year in the Huon Estuary, dominance of bacterial genera varied demonstrating distinct and systematic progressions related to the progression of algal blooms.