Unexpected population distribution in a microbial mat community: sulfate-reducing bacteria localized to the highly oxic chemocline in contrast to a eukaryotic preference for anoxia

Abstract

The distribution and abundance of sulfate-reducing bacteria (SRB) and eukaryotes within the upper 4 mm of a hypersaline cyanobacterial mat community were characterized at high resolution with group-specific hybridization probes to quantify 16S rRNA extracted from 100-microm depth intervals. This revealed a preferential localization of SRB within the region defined by the oxygen chemocline. Among the different groups of SRB quantified, including members of the provisional families "Desulfovibrionaceae" and "Desulfobacteriaceae," Desulfonema-like populations dominated and accounted for up to 30% of total rRNA extracted from certain depth intervals of the chemocline. These data suggest that recognized genera of SRB are not necessarily restricted by high levels of oxygen in this mat community and the possibility of significant sulfur cycling within the chemocline. In marked contrast, eukaryotic populations in this community demonstrated a preference for regions of anoxia.

FIG. 1

Normalized dissociation curves were obtained with the probe for Desulfonema spp. (S-G-Dsnm-0657-a-A-16) that hybridized to a target rRNA (Desulfonema limicola) and a single-mismatch nontarget rRNA (Synergistes jonesii). Curves were constructed with average values from triplicate experiments. Error bars define the standard deviation for each temperature point. The estimated T_d (midpoint of probe dissociation) for each probe-rRNA duplex is indicated by an arrow on the temperature axis.

FIG. 2

Distribution of total and Desulfonema-like rRNAs at 100-μm depth intervals in relation to oxygen. The abundance of Desulfonema is represented by the unshaded region in the total-abundance plot and individually in the inset plot. The medians and ranges for the one-half maximum-oxygen (open triangle) and minimum-oxygen (filled triangle) concentrations were calculated from six independent microelectrode measurements as discussed in the text.

FIG. 3

Distribution of Desulfovibrio-like rRNA at 100-μm depth intervals.

FIG. 4

Distribution of populations encompassed by the general probe for the provisional family “Desulfobacteriaceae” and for the desulfococcus subgroup within this family (Table 1). The abundance of the desulfococcus group is represented by the unshaded region in the abundance plot for the family and individually in the inset plot.

FIG. 5

Distribution of eukaryotic rRNAs at 100-μm depth intervals.