Community composition of a hypersaline endoevaporitic microbial mat.
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Journal: 2005/December - Applied and Environmental Microbiology
ISSN: 0099-2240
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.
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Appl Environ Microbiol 71(11): 7352-7365
PMID: 16269778

Community Composition of a Hypersaline Endoevaporitic Microbial Mat

Department of Geology and Geophysics Post 701, 1680 East-West Rd., University of Hawaii, Honolulu, Hawaii 96822, Danish Center for Earth System Science, Institute of Biology, University of Southern Denmark, University of Odense, Odense, Denmark, Marine Sciences Department, 12-7 Venable Hall CB 3300, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599, The Institute of Life Sciences and the Moshe Shilo Minerva Center for Marine Biogeochemistry, The Hebrew University of Jerusalem, 91904 Jerusalem, Israel4
Corresponding author. Mailing address: Department of Geology and Geophysics Post 701, 1680 East-West Rd., University of Hawaii, Honolulu, HI 96822. Phone: (808) 956-0720. Fax: (808) 956-5512. E-mail: ude.iiawah@litek.
Received 2005 Apr 27; Accepted 2005 May 30.
Copyright © 2005, American Society for Microbiology

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 α-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 γ- and δ-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.

Abstract

Photosynthetic communities containing distinct horizontal layers of oxygenic and anoxygenic phototrophs often cover the bottom of hypersaline ponds used for the production of sea salt. The physical appearance of these microbial communities varies according to salinity (55). At salinities below ca. 15%, the photosynthetic communities tend to form compact and highly active microbial mats on the surface of the pond floor, whereas in ponds containing between 15 and 25% salt, the mats are less compact since the photosynthetic layers tend to be embedded within the crystalline salt crust on the pond bottom. Such endoevaporitic gypsum- or halite-associated microbial systems have been reported from a number of solar salterns around the world, including those from the coasts of the Gulf of Mexico (55, 66), the Mediterranean Sea (12), the Baja Peninsula (60), and the Red Sea (53). In endoevaporitic microbial ecosystems associated with gypsum deposits, several layers of oxygenic and anoxygenic phototrophs are often visible, indicating some level of stratification in the populations. In photosynthetic communities growing within halite deposits, a vertical banding of the photosynthetic populations is still observed, but the complexity of the communities seems diminished compared to populations growing within gypsum (60).

The microbial ecology of hypersaline, endoevaporitic communities is interesting for a number of reasons. Some of the earliest evidence for life in the geologic record is preserved in the isotopic composition of organic carbon and sulfur species in 3.5 billion-year-old barite, originally deposited as gypsum, at North Pole, Australia (62). The endoevaporitic communities found today in solar salterns are likely modern homologues to the microbial communities once housed in these ancient sulfate deposits. The solar salterns may provide information about the prerequisites necessary for life to develop in evaporitic, sulfate-rich environments as once existed on the Martian surface (67, 68). Finally, endoevaporitic communities grow in a salinity range where numerous physiological groups are excluded, potentially altering the biogeochemical cycling in the system (48).

The endoevaporitic microbial communities in the salterns of Eilat have previously been studied in terms of light penetration, oxygen and sulfur biogeochemistry, and the salt tolerance of phototrophs, sulfate reducers, and methanogens (8, 53, 65). The upper oxygenic zone, which is typically 1 to 2 cm deep, consists of two layers of Cyanobacteria, sometimes separated by a white layer devoid of phototrophs, overlying a layer containing various anoxygenic phototrophs. The cell volume-specific photosynthesis rates are comparable to microbial mat communities from other environments, but the lower cell density results in lower overall oxygen production rates (8). This, and the deeper distribution of the phototrophs, results in a much decreased gas exchange with the overlying water such that the major part of oxygen produced in the crust is also consumed within the crust, mainly by heterotrophic organisms. The salinity tolerance of Cyanobacteria, anoxygenic phototrophs, sulfate reducers, and methanogens were in agreement with what has been observed in pure cultures (65). Thus, sulfate reducers and anoxygenic photoautotrophic organisms in the crust are inhibited at salinities greater than 12 and 15%, respectively, whereas methanogens and Cyanobacteria are not inhibited at the in situ salinity.

The purpose of the present study is to supplement these previous investigations with a broader phylogenetic overview of the organisms present in the crust. This should allow us to identify the major components of the heterotrophic community, which are not easily identified by microscopy, and to compare the phylogenetic composition of this community with other benthic phototrophic microbial systems.

Acknowledgments

We thank Israel Salt Industries, Ltd., for allowing access to the saltern ponds and the Interuniversity Institute for Marine Sciences in Eilat and the Moshe Shilo Minerva Center for Marine Biogeochemistry for logistic support.

This study was supported by the Danish Basic Research Foundation (Grundforskningsfonden), the Danish Research Agency (Statens Naturvidenskabelige Forskningsråd), the Israel Science Foundation (founded by the Israel Academy of Sciences and Humanities), and by the NASA Astrobiology Institute “Subsurface Biospheres” and “Environmental Genomics.”

Acknowledgments

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