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

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 populations provided with H-3-leucine substrate. FISH and T-RFLP patterns clearly illustrated shifts in bacterial community composition as waters transitioned from oxic to anoxic conditions at depth. Predator-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 alpha-, beta-, gamma-, and epsilon-proteobacteria to predator exclusion. The alpha- and beta-proteobacteria became more prevalent in controls than in 'predator-free' samples (< 1.6 mu m filtrate) over time. In contrast, gamma- and epsilon-proteobacteria gained prevalence in 'predator-free' samples. Interestingly, abundances of beta-proteobacteria dramatically declined over time in both controls and predator-free treatments, and none actively assimilated H-3-leucine (MICRO-FISH). Among H-3-leucine-assimilating cells in control samples at 270 m, alpha-, gamma-, and epsilon-proteobacteria accounted for 12, 38, and 14 % of H-3-leucine active cells, respectively. Taxonomic distributions of H-3-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.