High bacterial diversity in permanently cold marine sediments

A 16S ribosomal DNA (rDNA) clone library from permanently cold marine sedim ents was established. Screening 353 clones by dot blot hybridization with g roup-specific oligonucleotide probes suggested a predominance of sequences related to bacteria of the sulfur cycle (43.4% potential sulfate reducers). Within this fraction, the major cluster (19.0%) was affiliated with Desulf otalea sp. and other closely related psychrophilic sulfate reducers isolate d from the same habitat. The cloned sequences showed between 93 and 100% si milarity to these bacteria. Two additional groups were frequently-encounter ed: 13% of the clones were related to Desulfuromonas palmitatis, and a seco nd group was affiliated with Myxobacteria spp. and Bdellovibrio spp. Many c lones (18.1%) belonged to the gamma subclass of the class Proteobacteria an d were closest to symbiotic or free-living sulfur oxidizers. Probe target g roups were further characterized by amplified rDNA restriction analysis to determine diversity within the groups and within the clone library. Rarefac tion analysis suggested that the total diversity assessed by 16S rDNA analy sis was very high in these permanently cold sediments and was only partiall y revealed by screening of 353 clones.