Phylogenetic diversity of bacterial and archaeal communities in the anoxiczone of the Cariaco Basin

Microbial community samples were collected from the anoxic zone of the Cari aco Basin at depths of 320, 500, and 1,310 m on a November 1996 cruise and were used to construct 16S ribosomal DNA libraries. Of 60 non-chimeric sequ ences in the 320-m library, 56 belonged to the E subdivision of the Proteob acteria (epsilon -Proteobacteria) and 53 were closely related to ectosymbio nts of Rimicaris exoculata and Alvinella pompejana, which are referred to h ere as epsilon symbiont relatives (ESR). The 500-m library contained sequen ces affiliated with the fibrobacteria, the Flexibacter-Cytophaga-Bacteroide s division, the division Verrucomicrobia, the division Proteobacteria, and the OP3 candidate division. The Proteobacteria included members of the gamm a, delta, epsilon and new candidate subdivisions, and gamma -proteobacteria l sequences were dominant (25.6%) among the proteobacterial sequences. As i n the 320-m library, the majority of the epsilon -proteobacteria belonged t o the ESR group. The genus Fibrobacter and its relatives were the second la rgest group in the library (23.6%), followed by the delta -proteobacteria a nd the epsilon -proteobacteria. The 1,310-m library had the greatest divers ity; 59 nonchimeric clones in the library contained 30 unique sequences bel onging to the planctomycetes, the fibrobacteria, the Flexibacter-Cytophaga- Bacteroides division, the Proteobacteria, and the OP3 and OP8 candidate div isions. The proteobacteria included members of new candidate subdivisions a nd the beta, gamma, delta, and epsilon -subdivisions. ESR sequences were st ill present in the 1,310-m library but in a much lower proportion (8.5%). O ne archaeal sequence was present in the 500-m library (2% of all microorgan isms in the library), and eight archaeal sequences were present in the 1,31 0-m library (13.6%). All archaeal sequences fell into two groups; two clone s in the 1,310-m library belonged to the kingdom Crenarchaeota and the rema ining sequences in both libraries belonged to the kingdom Euryarchaeota. Th e latter group appears to be related to the Eel-TA1f2 sequence, which belon gs to an archaeon suggested to be able to oxidize methane anaerobically. Ba sed on phylogenetic inferences and measurements of dark CO2 fixation, we hy pothesized that (i) the ESR are autotrophic anaerobic sulfide oxidizers, (i i) sulfate reduction and fermentative metabolism may be carried out by a la rge number of bacteria in the 500 and 1,310-m libraries, and (iii) members of the Euryarchaeota found in relatively large numbers in the 1,310-m libra ry may be involved in anaerobic methane oxidation. Overall, the composition of microbial communities from the Cariaco Basin resembles the compositions of communities from several anaerobic sediments, supporting the hypothesis that the Cariaco Basin water column is similar to anaerobic sediments.