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During a May 2005 cruise, depth-dependent distributions of bacterial populations and their responses to predator exclusion were investigated in the anoxic Cariaco Basin, Venezuela. Community structure was assessed using fluorescence in situ hybridization (FISH) and terminal restriction fragment length polymorphism (T-RFLP) of 16S rRNA genes. FISH combined with microautoradiography was used to determine single-cell activity of heterotrophic bacterial popula- tions provided with 3 H-leucine substrate. FISH and T-RFLP patterns clearly illustrated shifts in bac- terial community composition as waters transitioned from oxic to anoxic conditions at depth. Preda- tor-exclusion experiments were conducted at 3 depths across the redoxcline. Although richness of bacterial operational taxonomic units (OTUs) was initially reduced at all 3 depths (250, 270, 400 m) as a likely consequence of confinement, distinct responses of individual OTUs to predator exclusion were detected in T-RFLP patterns. FISH analyses also detected very consistent responses of α-, β-, γ-, and e-proteobacteria to predator exclusion. The α- and β-proteobacteria became more prevalent in controls than in 'predator-free' samples (<1.6 µm filtrate) over time. In contrast, γ- and e-proteobacte- ria gained prevalence in 'predator-free' samples. Interestingly, abundances of β-proteobacteria dra- matically declined over time in both controls and predator-free treatments, and none actively assim- ilated 3 H-leucine (MICRO-FISH). Among 3 H-leucine-assimilating cells in control samples at 270 m, α-, γ-, and e-proteobacteria accounted for 12, 38, and 14% of 3 H-leucine active cells, respectively. Taxonomic distributions of 3 H-leucine-assimilating cells in predator-exclusion treatments did not detectably differ from controls. Overall, results demonstrated that redox gradients structured microbial communities and that predator exclusion significantly altered bacterial abundances and community composition.

Compositional responses of bacterial communities to redox gradients and grazing in the anoxic Cariaco Basin