Detection of abundant sulphate-reducing bacteria in marine oxic sediment layers by a combined cultivation and molecular approach

The depth distribution and diversity of sulphate-reducing bacteria (SRB) wa s analysed in the upper intertidal zone of a sandy marine sediment of the D utch island Schiermonnikoog. The upper centimetre of the sediment included the oxic-anoxic interface and was cut into five slices. With each slice, mo st probable number (MPN) dilution series were set up in microtitre plates u sing five different substrates. In the deeper sediment layers, up to 1 x 10 (8) cm(-3) lactate-utilizing SRB were counted, corresponding to 23% of the total bacterial count. From the highest positive dilutions of the MPN serie s, 27 strains of SRB were isolated in pure culture. Sequencing of a 580 bp fragment of the 16S rDNA revealed that 21 isolates had identical sequences, also identical with that of the previously described species Desulfomicrob ium apsheronum. However, the diversity of the isolates was higher with resp ect to their physiological properties: a total of 11 different phenotypes c ould be distinguished. Genomic fingerprinting by enterobacterial repetitive intergenic consensus (ERIC) polymerase chain reaction (PCR) revealed an ev en higher diversity of 22 different genotypes. A culture-independent analys is by PCR and denaturing-gradient gel electrophoresis (DGGE) revealed that the partial 16S rDNA sequence of the isolated D. apsheronum strains constit uted a significant fraction of the Desulfovibrionaceae. The high subspecies diversity suggests that this abundant aggregate-forming species may have e volved adaptations to different ecological niches in the oxic sediment laye rs.