AME 47:57-72 (2007)  -  doi:10.3354/ame047057

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

Xueju Lin1,4,*, Mary I. Scranton1, Ramon Varela2, Andrei Chistoserdov3, Gordon T. Taylor1

1Marine Sciences Research Center, Stony Brook University, Stony Brook, New York 11794-5000, USA
2Estacion de Investigaciones Marinas de Margarita, Fundacion La Salle de Ciencias Naturales, Apartado 144, Punta de Piedras, Edo, Nueva Esparta, Venezuela
3Department of Biology, University of Louisiana at Lafayette, Lafayette, Louisiana 70504, USA
4Present address: Stroud Water Research Center, Avondale, Pennsylvania 19311, USA

ABSTRACT: 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 3H-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 α-, β-, γ-,and ε-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 ε-proteobacteria gained prevalence in ‘predator-free’ samples. Interestingly, abundances of β-proteobacteria dramatically declined over time in both controls and predator-free treatments, and none actively assimilated 3H-leucine (MICRO-FISH). Among 3H-leucine-assimilating cells in control samples at 270 m, α-, γ-, and ε-proteobacteria accounted for 12, 38, and 14% of 3H-leucine active cells, respectively. Taxonomic distributions of 3H-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.

KEY WORDS: Redox gradients · Grazing · Bacteria · Cariaco Basin · FISH · T-RFLP

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