Structure and diversity of Gram-negative sulfate-reducing bacteria on rice roots.

Specific PCR assays were used to amplify the 16S rRNA genes of the Desulfobacteriaceae and the Desulfovibrionaceae from extracted environmental DNA from rice roots. 16S rDNA-based community patterns of the Desulfobacteriaceae were generated via terminal restriction fragment length polymorphism analysis from rice roots and compared with bulk soil. The molecular fingerprints showed no significant difference between rice roots and bulk soil, but changes during the vegetation period. 16S rDNA clone libraries and sequencing showed that the predominant terminal restriction fragments represented distinct phylogenetic groups. The 16S rDNA clone sequences of the Desulfobacteriaceae fell in the phylogenetic radiation of Desulfonema and Desulfosarcina or grouped within the Desulforhabdus-Syntrophobacter assemblage. Three of the latter sequences were closely affiliated with the MPN isolate EZ-2C2 from rice roots. All Desulfovibrionaceae 16S rDNA clone sequences, with one exception, were affiliated with the MPN isolate F1-7b from rice roots. The clustering of the clone sequences and the close phylogenetic affiliation with isolates from MPN enrichments from the same habitat in two cases indicated that these sequence clusters may represent predominant Gram-negative sulfate reducers on rice roots. Quantification of the bacterial abundances was accomplished by rRNA dot blot hybridization. In total the Gram-negative sulfate reducers accounted for approximately 2-3% of the total rRNA content. The relative rRNA abundance of the Desulfobacteriaceae was, at 1.4%, higher than that of the Desulfovibrionaceae (0.5%).