doi:10.1111/j.1574.6941.2005.00021.x

competitive PCR; phylogenetic analysis; sulfate availability and salinity; estuarine sediments. The diversity of sulfate-reducing microorganisms was investigated in two con-trasting mudflats of the Seine estuary, by PCR amplification, cloning and sequencing of the genes coding for parts of the alpha and beta subunits of dissimilatory sulfite reductase (dsrAB). One site is located in the mixing-zone and shows marine characteristics, with high salinity and sulfate concentration, whereas the other site shows freshwater characteristics, with low salinity and sulfate concentration. Diversity and abundance of dsrAB genes differed between the two sites. In the mixing-zone sediments, most of the dsrAB sequences were affiliated to those of marine Gram-negative bacteria belonging to the order of Desulfobacter-ales, whereas in the freshwater sediments, a majority of dsrAB sequences was related to those of the Gram-positive bacteria belonging to the genus Desulfoto-maculum. It is speculated that this is related to the salinity and the sulfate concentration in the two mudflats