Abstract
Distributions of bacterial populations in the anoxic Cariaco Basin were studied over 2 yr (five cruises) at three stations using fluorescence in situ hybridization (FISH) and terminal restriction fragment length polymorphism (T‐RFLP) of 16S ribosomal ribonucleic acid (rRNA) genes. FISH results demonstrate that the contribution of ɛ‐proteobacterial abundances to total prokaryotic inventories was relatively constant among cruises, and this group was dominant in the redoxcline, accounting for 18% of DAPI (4'6‐diamidino‐2‐phenylindole)‐stained counts on average. In contrast, β‐proteobacteria were more variable. Sulfate reducers, α‐, and γ‐proteobacteria accounted for minor components of the anaerobic microbial communities. T‐RFLP fingerprinting also showed significant temporal shifts in bacterial community composition. The greatest variations in bacterial community structure were evident among oxic, transitional, and anoxic zones, but significant horizontal variations were also evident within redoxclines of three different stations. A statistical analysis of T‐RFLP patterns revealed clear spatiotemporal patterns in bacterial community structure along the redox gradient among sites and over time. Total prokaryotic abundances corresponded well with seasonal variations in organic carbon fluxes above the redoxcline. However, no single measured environmental variable (O 2 , H 2 S, S 2 O 3 2‐ , H 4 SiO 4 , NO 3 − , PO 4 3‐ , microbial biomass, chemoautotrophic production) explained the temporal oscillations of the dominant bacterial groups. Analyses suggest that redox gradients, terrestrial inputs, and disturbance due to lateral intrusions of adjacent waters could significantly affect bacterial community composition in the Cariaco Basin.