Diversity of sulfate-reducing bacteria in oxic and anoxic regions of a microbial mat characterized by comparative analysis of dissimilatory sulfite reductase genes

Abstract

Sequence analysis of genes encoding dissimilatory sulfite reductase (DSR) was used to identify sulfate-reducing bacteria in a hypersaline microbial mat and to evaluate their distribution in relation to levels of oxygen. The most highly diverse DSR sequences, most related to those of the Desulfonema-like organisms within the delta-proteobacteria, were recovered from oxic regions of the mat. This observation extends those of previous studies by us and others associating Desulfonema-like organisms with oxic habitats.

FIG. 1

Distribution of DSR sequence type by sample depth in the mat in relation to oxygen concentration. The number of clones recovered from each sample depth affiliated with each sequence type (A to F) is represented by the size of the corresponding circle (one to three clones). The medians and ranges for the one-half maximum-oxygen (open triangle) and minimum-oxygen (filled triangle) values were calculated from six independent microelectrode measurements (31).

FIG. 2

Phylogenetic tree reflecting the relationships of the analyzed DSR clones retrieved from the mat with the DSRs from characterized sulfate-reducing prokaryotes (46). The DSR sequences of Desulfobulbus propionicus and Desulfonema ishimotoi (47) were added to the data set so that we could more accurately define the phylogenetic depth of the δ-subclass SRB in the DSR tree and provide additional reference. Tree topology was obtained from FITCH distance matrix analysis of the DSR alpha- and beta-subunit amino acid data set. Bootstrap values were determined from parsimony analysis with an identical data set. Branches for which no bootstrap value is indicated were not recovered in the majority of bootstrap replicates by the parsimony method. The scale bar indicates the number of expected amino acid substitutions per site per unit of branch length.