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Applied and Environmental Microbiology, October 1999, p. 4666-4671, Vol. 65, No. 10
Department of Civil Engineering, Northwestern
University, Evanston, Illinois 60208-31091;
Max-Planck-Institute for Marine Microbiology, D-28359 Bremen,
Germany2; The Moshe Shilo Center for
Marine Biogeochemistry, Alexander Silberman Institute of Life Sciences,
The Hebrew University of Jerusalem, Jerusalem,
Israel3; and Technische
Universität München, Lehrstuhl für Mikrobiologie,
D-80290 Munich, Germany4
Received 16 April 1999/Accepted 20 July 1999
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
The relationship of sulfate-reducing
bacteria (SRB) to oxygen has been of particular interest since the
publication of earlier reports of exceptionally high rates of sulfate
reduction in the oxygenated surface regions of some microbial mats
(4, 6, 16, 44). These observations are inconsistent with the
generally accepted paradigm that environmentally available electron
acceptors are depleted sequentially according to the thermodynamically
predicted order of preference. Although there is no apparent
restriction on the use of sulfate in the presence of a
thermodynamically preferred electron acceptor, such as oxygen and
nitrate, it is generally assumed that the use of sulfate under these
conditions places an organism at a selective disadvantage. However,
selective advantage or disadvantage can be evaluated only with a full
understanding of the environmental context, from considering such
factors as syntrophy, microhabitat, and varying physical and chemical
environments. In turn, this understanding cannot be achieved without
knowledge of population structure. In this study we use comparative
sequencing of genes encoding a key enzyme in sulfate respiration,
dissimilatory sulfite reductase (DSR), to directly identify SRB
populations within the oxic and anoxic regions of a microbial mat community.
Although pure culture remains fundamental to microbiology, the biases
associated with culture-based descriptions of community structure are
now generally acknowledged. Thus, this well-established approach is
increasingly complemented by the use of a variety of molecular tools.
In particular, comparative sequencing of the 16S rRNAs now provides the
most general framework for studies of natural microbial diversity and
abundance (21, 35, 38, 39; for reviews, see
references 2, 14, and 32). One
limitation of the rRNA-based analysis is that it does not provide a
direct link to physiology. To some extent, populations identified by rRNA sequence are expected to share metabolic features with close relatives characterized by results of pure culture, but little inference can be made for more distant relatives. Thus, novel lineages
of SRB A direct molecular identification of novel SRB must consider the
presence of enzymes required for sulfate respiration or the genes
encoding them. To this end we earlier demonstrated that a 1.9-kb DNA
fragment encoding most of the alpha and beta subunits of the DSR could
be amplified by PCR from all recognized lineages of SRB with a single
primer set (46). DSR catalyzes the six-electron reduction of
sulfite to sulfide and hence is required by all SRB. Development of a
general PCR primer set was possible because of the remarkable
conservation of the DSR sequence. This conservation was first suggested
by the combined studies led by Voordouw and Trüper showing that
the bacterial (Desulfovibrio) and archaeal (Archaeoglobus) genes have approximately 60% nucleotide
similarity (11, 20, 25, 45).
Our previous studies demonstrated that this PCR primer set would
amplify the appropriate DNA fragment only from sulfate-respiring microorganisms and that the phylogenies of DSRs of hitherto analyzed reference strains are consistent with that inferred from the 16S rRNA
(46). The homologous enzyme from the sulfide-oxidizing organism Chromatium vinosum is phylogenetically well
separated from those of sulfate respirers (20). Thus,
environmental studies based on DSR sequence analyses should provide a
more direct measure of SRB diversity and distribution. We here describe
the use of this approach to directly evaluate the diversity of SRB
populations in a hypersaline cyanobacterial mat from Solar Lake (Sinai,
Egypt). In particular, we addressed the issue of the relationship of
the presence of SRB to the availability of oxygen by determining the distribution of DSR sequence types within previously described oxic and
anoxic depth intervals within this mat (7, 12, 31, 34).
Mat maintenance and analysis.
Mats maintained in aquaria were
characterized after 5 h of exposure to light as described in the
accompanying paper (31) and as previously established
(16, 24, 34).
Nucleic acid analyses.
Total DNAs were extracted from mat
depths of 0.05, 0.3, 0.4, 0.6, 1.6, 2.0, 4.0, and 9.0 mm, and a
longitudinal sample was taken from a complete mat core by a
modification of the method described by Tsai and Olson (42).
Briefly, mat sections were washed with 1 ml of TE (10 mM Tris-HCl, 1 mM
EDTA [pH 8.0]) and incubated in 200 µl of lysozyme solution (0.15 M
NaCl, 0.3 M Na2-EDTA, 15 mg of lysozyme per ml) for 3 h at 37°C. Then, 200 µl of lysis buffer (0.1 M NaCl, 0.5 M Tris-HCl
[pH 8.0], 10% sodium dodecyl sulfate) was added and the suspension
was subjected to three cycles of freezing (ethanol and dry ice for 5 min) and thawing (65°C water bath for 10 min). Finally, proteinase K
was added to a final concentration of 50 µg/ml and the tubes were
incubated at 30°C for 30 min. DNA was purified by phenol extraction,
precipitated with isopropanol, washed with 80% (vol/vol) ethanol, and
resuspended in water.
PCR amplification, cloning, and sequencing.
PCR amplification
was carried out in a 1650 Air Thermo-Cycler (Idaho Technology, Idaho
Falls, Idaho) under the reaction conditions and with the DSR1F and
DSR4R primers previously described (46). PCR products (ca.
1.9 kb) were ligated either directly with the TA Cloning System into
pCRII plasmids and transformed into ONE SHOT competent
Escherichia coli cells according to the directions of the
manufacturer (Invitrogen, San Diego, Calif.) or following recovery from
an agarose gel with an agarose gel DNA extraction kit (Boehringer
Mannheim GmbH, Mannheim, Germany). Partial sequences were obtained from
the 3' and 5' ends of each insert with a LI-COR 4000L automated
sequencer and infrared dye-labeled M13 forward and reverse primers
(LI-COR Corp., Lincoln, Nebr.). Clones and GenBank accession numbers
are listed in Table 1.
0099-2240/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.
Diversity of Sulfate-Reducing Bacteria in Oxic and
Anoxic Regions of a Microbial Mat Characterized by Comparative Analysis
of Dissimilatory Sulfite Reductase Genes


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ABSTRACT
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Abstract
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-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.
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TEXT
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Abstract
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which may contribute to sulfate reduction in oxic habitats
cannot be identified by rRNA sequence alone.
TABLE 1.
Distribution of characterized DSR clones by depth and
phylogenetic affiliation
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subgroup of the class
Proteobacteria. The F lineage is well resolved from the others and can be classified only as a member of the bacterial domain.
The more deeply diverging sequences (E and F) were recovered either
from the deep, permanently anoxic, regions of the mat or from
longitudinal slices consisting primarily of the permanently anoxic
region (2.5 to 9 mm). The depth distribution of sequence types in
relation to oxygen is shown in Fig. 1.
Clade A.
Clade A includes the greatest number of cloned
sequences; 8 of 11 clones originated from the oxic zone (0 to 1.5 mm),
and 2 of 3 clones originated from the chemocline. All 12 sequences affiliated with clade A are unique and closely related to described Desulfonema and Desulfococcus species within the
-proteobacteria.
Clade B.
Clade B is comprised of five unique sequences,
including those of the three remaining clones from the oxic zone of the
mat. This clade is affiliated with the
Desulfococcus-Desulfonema lineage within the
-proteobacteria.
Lineage C. Lineage C contains two cloned sequences, distantly related to Desulfobacter spp. These sequences were recovered from a depth of 1.6 mm (chemocline during the day and anoxic zone at night) and from a longitudinal section. However, the phylogenetic position is not fully resolved; neighbor-joining and FITCH analyses indicated a relatively close relationship with clade B sequences, which was not supported by maximum-likelihood and parsimony methods.
Lineage D. Lineage D is defined by one clone recovered from a depth of 4 mm, a region that is permanently anoxic. Although not related to any described SRB, it shows high sequence similarity to a DSR sequence recovered from an aromatic-hydrocarbon-degrading, sulfate-reducing enrichment (17a).
Clade E. Clade E is represented in the phylogenetic tree by one clone recovered from a longitudinal section of the mat and another clone isolated from the 9-mm depth. It is not related to any available pure culture sequence and only peripherally related to Desulfobulbus spp.
Lineage F. Lineage F is deeply diverging and defined by two highly similar DSR sequences not closely related to any available pure culture sequence. It demonstrates a weak specific association with Desulfotomaculum ruminis by KITCH analysis but forms an independent lineage by all other applied treeing methods.
There is a general need in microbial ecology to more directly relate community structure to community functions. Within the analytical framework of comparative gene sequencing, the most direct linkages are provided by genes encoding key physiological attributes. Several genes have been used in this way, including those for nitrogenase (3, 52-54), [NiFe] hydrogenase (48), ribulose bisphosphate carboxylase/oxygenase (33), methane monooxygenase (30), and ammonia monooxygenase (36, 37). However, with the possible exception of ammonia monooxygenase (restricted to two well-defined lineages within the proteobacteria), none of these genes provide fully comprehensive or consistent coverage. The DSR gene appears to be the first example of a gene encoding a widely distributed metabolic trait of sufficiently high sequence conservation to be recoverable from all recognized archaeal and bacterial lineages with a single PCR primer set and to also display phylogenetic relationships generally consistent with the 16S rRNA. Of particular interest was the relationship of SRB to oxygen. During the diurnal cycle, these mats are exposed to changing chemical gradients, most notably of O2, H2S, and pH. With reference to O2 exposure, this and previous studies have defined three general depth intervals (8, 9, 18, 23, 24, 34): (i) permanently oxic (0 to 0.5 mm), (ii) fluctuating oxic and anoxic (0.5 to 2.5 mm), and (iii) permanently anoxic (deeper than 2.5 mm). The most striking observation was that all DSR sequences derived from the permanently oxic zone are affiliated with clades A and B (Fig. 1 and Table 1), whose members are closely related to the Desulfonema-Desulfococcus group of metabolically versatile, completely oxidizing SRB (17, 50). At this depth dissolved oxygen concentrations varied from approximately 160% saturation during the day to below 10% saturation during the night. It has been recognized for some time that many SRB are oxygen tolerant and that some may have a limited capacity to respire oxygen. Many Desulfovibrio spp. have these characteristics (1, 10, 13, 15, 22, 29, 43). Several Desulfovibrio species isolated from oxic regions of microbial mats (e.g., Desulfovibrio oxyclinae and Desulfovibrio halophytica) have been shown to have a high affinity for oxygen but a limited capacity to respire it for growth (26, 27). However, no DSR sequences related to Desulfovibrio were recovered in this study. In addition, our previous studies of a similar mat in Guerrero Negro (Baja California Sur, Mexico) with group-specific rRNA probes revealed a minor presence of Desulfovibrio species in the near surface (upper 2 mm). A common feature of both mat systems is the near-surface abundance of Desulfonema-like populations, as revealed by the DSR sequences recovered in this study and with group-specific probes to characterize the population structure of the Guerrero Negro mat (35). Since DNAs recovered from environmental samples may be derived in part from dead or inactive cells, the DSR sequence alone does not provide direct evidence for an active sulfate-respiring population. However, rRNA-based analyses provide additional support for the presence of an active SRB microbiota in the oxic regions of this mat community. The general patterns of DSR distribution are also consistent with the results of the high-resolution study of rRNA abundance in this mat community presented in our accompanying paper (31). Although pure culture isolates of Desulfonema have not been examined for their relationship to oxygen, other members of this family have been demonstrated to either reduce oxygen or be oxygen tolerant, including Desulfobacterium autotrophicum strains (13, 15, 29), several Desulfobacter species (10, 13), and a Desulfococcus multivorans strain (13, 15). More recently, Desulfonema spp. were identified by in situ hybridization in sea sediments (28), and high numbers of Desulfonema spp. have been identified by most-probable-number counts in the upper 2 mm of a field sample of the Solar Lake cyanobacterial mat and identified by sequencing of 16S rRNA gene fragments amplified from DNAs isolated near the chemocline (41). Thus, molecular and microbiological data derived from two independent field sites and from the aquarium systems consistently show the Desulfonema-like SRB to be dominant in the permanently oxic region of hypersaline cyanobacterial mat communities. However, no identical DSR sequences were recovered in our initial analysis and we anticipate that continued studies will reveal much greater sequence diversity, pointing to a very complex ecology of SRB in the near surface of the mat. We anticipate that comparative DSR sequence analyses will provide a useful complement to the microbiological and molecular tools now used to study the ecology of sulfate-respiring microorganisms. For example, DSR sequence type could be used to assist in monitoring successful enrichment of previously unknown SRB. Or, if a close phylogenetic relationship implies phenotypic similarity, this information could assist in the design of specific enrichment strategies. Since this sequence is an explicit indicator of the capacity for sulfate respiration, DSR-based analyses should also foster a better understanding of the more general environmental roles of these organisms (9a). The metabolic diversity of this group has been revised repeatedly in recent years, suggesting a more general participation in the flow of carbon and electrons in anoxic habitats than earlier thought. For example, their carbon sources, long thought to be limited to simple organic acids and alcohols, now include a wide variety of aliphatic and aromatic compounds (5, 49, 51; for a review, see reference 19). Also, although sulfate respiration unites them, SRB are not restricted to this mode of existence. As a group, they have the capacity to use a broad variety of electron acceptors, including sulfite, thiosulfate, sulfur, nitrite, and nitrate (10, 15, 19, 22, 29, 43). Some members derive energy from disproportionation of sulfur, thiosulfate, and sulfide and incomplete sulfate reduction to thiosulfate and sulfur (for a review, see reference 50). It is conceivable that within certain habitats, sulfate respiration may be a minor metabolic mode for some members of this functionally defined assemblage.Nucleotide sequence accession numbers. See Table 1 for nucleotide sequence accession numbers.
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ACKNOWLEDGMENTS |
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This research was supported in part by research grants from the Office of Naval Research (ONR N00014-95-1-00887) and the National Science Foundation and by a postdoctoral grant from the Deutsche Forschungsgemeinschaft (Wa 1027/1-1) to M.W. This research was partially supported by a grant from the German-Israeli Foundation for Scientific Research and Development and a grant of the BMBF to Y.C.
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FOOTNOTES |
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* Corresponding author. Mailing address: Department of Civil Engineering, Northwestern University, Evanston, IL 60208-3109. Phone: (847) 491-4997. Fax: (847) 491-4011. E-mail: d-stahl{at}nwu.edu.
Present address: Volcani Research Center, Soil Microbiology,
Bet-Dagan, 50-250 Israel.
Present address: Netherlands Institute for Sea Research, Den Burg,
The Netherlands.
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