Community composition of a hypersaline endoevaporitic microbial mat

Abstract

A hypersaline, endoevaporitic microbial community in Eilat, Israel, was studied by microscopy and by PCR amplification of genes for 16S rRNA from different layers. In terms of biomass, the oxygenic layers of the community were dominated by Cyanobacteria of the Halothece, Spirulina, and Phormidium types, but cell counts (based on 4',6'-diamidino-2-phenylindole staining) and molecular surveys (clone libraries of PCR-amplified genes for 16S rRNA) showed that oxygenic phototrophs were outnumbered by the other constituents of the community, including chemotrophs and anoxygenic phototrophs. Bacterial clone libraries were dominated by phylotypes affiliated with the Bacteroidetes group and both photo- and chemotrophic groups of alpha-proteobacteria. Green filaments related to the Chloroflexi were less abundant than reported from hypersaline microbial mats growing at lower salinities and were only detected in the deepest part of the anoxygenic phototrophic zone. Also detected were nonphototrophic gamma- and delta-proteobacteria, Planctomycetes, the TM6 group, Firmicutes, and Spirochetes. Several of the phylotypes showed a distinct vertical distribution in the crust, suggesting specific adaptations to the presence or absence of oxygen and light. Archaea were less abundant than Bacteria, their diversity was lower, and the community was less stratified. Detected archaeal groups included organisms affiliated with the Methanosarcinales, the Halobacteriales, and uncultured groups of Euryarchaeota.

FIG. 1.

Phase-contrast and epifluorescence micrographs of organisms in the gypsum crust. (a) Halothece-like unicellular cyanobacterium from the upper brown layer; (b) Spirulina/Halospirulina- like cyanobacterium from the green layer; (c) Phormidium-like and Halothece-like Cyanobacteria from the green layer; (d) DAPI stain of a P/O layer sample showing the abundant thread-like organisms and various small morphotypes; (e) Halochromatium-like bacterium with internal granules (presumably sulfur).

FIG. 2.

Phylogenetic trees showing phylotypes affiliated with Bacteroidetes (A), α-proteobacteria (B), and other groups (C). The trees were constructed by using Jukes-Cantor distance calculations and neighbor joining. Bootstrap values were determined with 1,000 replicates. Nodes with bootstrap values smaller than 60% are marked with squares (40 to 60%) or circles (<40%).

FIG. 2.

Phylogenetic trees showing phylotypes affiliated with Bacteroidetes (A), α-proteobacteria (B), and other groups (C). The trees were constructed by using Jukes-Cantor distance calculations and neighbor joining. Bootstrap values were determined with 1,000 replicates. Nodes with bootstrap values smaller than 60% are marked with squares (40 to 60%) or circles (<40%).

FIG. 2.

Phylogenetic trees showing phylotypes affiliated with Bacteroidetes (A), α-proteobacteria (B), and other groups (C). The trees were constructed by using Jukes-Cantor distance calculations and neighbor joining. Bootstrap values were determined with 1,000 replicates. Nodes with bootstrap values smaller than 60% are marked with squares (40 to 60%) or circles (<40%).

FIG. 3.

Phylogenetic tree showing the affiliation of the archaeal phylotypes detected in the gypsum crust. The tree was constructed by using Jukes-Cantor distance calculations and neighbor joining. Bootstrap values were determined with 1,000 replicates. Nodes with bootstrap values smaller than 60% are marked with squares (40 to 60%) or circles (<40%). MBGD, marine benthic group D; MGIII, marine group III.

FIG. 4.

Representative DGGE gels with PCR products obtained with primers specific for Bacteria (a) and Archaea (b). The gels contained 10% acrylamide-bisacrylamide and a 40 to 70% denaturing gradient. Bands that were identified as a phylotype from one of the clone libraries are indicated in the figure.

FIG. 5.

Number and distribution of bands on DGGE gels loaded with bacterial PCR products from the white (W), green (G), purple/olive-green (P/O), and deep sulfidic (S) layers.