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Applied and Environmental Microbiology, October 2005, p. 6353-6359, Vol. 71, No. 10
0099-2240/05/$08.00+0     doi:10.1128/AEM.71.10.6353-6359.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.

Diversity and Distribution of Sulfate-Reducing Bacteria in Permanently Frozen Lake Fryxell, McMurdo Dry Valleys, Antarctica

Elizabeth A. Karr,{dagger} W. Matthew Sattley, Melissa R. Rice, Deborah O. Jung, Michael T. Madigan, and Laurie A. Achenbach*

Department of Microbiology, Southern Illinois University, Carbondale, Illinois 62901-6508

Received 30 March 2005/ Accepted 27 May 2005


   ABSTRACT
Top
 Abstract
Introduction
 Materials and Methods
 Results
 Discussion
 References
 
The permanently frozen freshwater Lake Fryxell, located in the Dry Valleys of Antarctica, exhibits an ideal geochemistry for microbial sulfate reduction. To investigate the population of sulfate-reducing bacteria in Lake Fryxell, both 16S rRNA gene and metabolic primer sets targeting the dsrA gene for the dissimilatory sulfite reductase alpha subunit were employed to analyze environmental DNA obtained from the water column and sediments of Lake Fryxell. In addition, enrichment cultures of sulfate-reducing bacteria established at 4°C from Lake Fryxell water were also screened using the dsrA primer set. The sequence information obtained showed that a diverse group of sulfate-reducing prokaryotes of the domain Bacteria inhabit Lake Fryxell. With one exception, the enrichment culture sequences were not represented within the environmental sequences. Sequence data were compared with the geochemical profile of Lake Fryxell to identify possible connections between the diversity of sulfate-reducing bacteria and limnological conditions. Several clone groups were highly localized with respect to lake depth and, therefore, experienced specific physiochemical conditions. However, all sulfate-reducing bacteria inhabiting Lake Fryxell must function under the constantly cold conditions characteristic of this extreme environment.


   INTRODUCTION
Top
 Abstract
 Introduction
Materials and Methods
 Results
 Discussion
 References
 
Lake Fryxell is a permanently ice-covered lake located in the McMurdo Dry Valleys, Antarctica. A perennial ice cover of 4 to 6 m and a small influx of glacial meltwater impose a stable chemical stratification of the water column (21). The upper portion of the water column of Lake Fryxell consists of freshwater supersaturated with dissolved oxygen, whereas the water becomes anoxic and increasingly sulfidic below 10 m, coinciding with a gradual increase in salinity that reaches a maximum of nearly 1% at the sediment-water interface (14) (see Fig. 3).



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  		FIG. 3. Profile of Lake Fryxell, summarizing key physiochemical parameters (left) and the locations with depth of different groups of dsrA phylotypes in the water column (right). The widths of the colored boxes are proportional to the number of different phylotypes residing at a particular depth according to the scale bar on the bottom of the figure. Figure 2 shows the phylotypes in each group. The 14-m group 3 enrichment clone is identical in dsrA sequence to the group F environmental clone, LFdsrC26, found at 9 m and in the sediment. DOC concentrations are expressed as millimolar carbon (adapted from data in reference 29).

 
The biota of Lake Fryxell is strictly microbial (21), and the lake geochemistry lends itself to the development of vertically stratified microbial populations. For example, our previous work on anoxygenic phototrophic bacteria in Lake Fryxell showed a distinct vertical stratification of purple phototrophic bacteria (1, 11). Interestingly, however, and despite the high levels of sulfide present, enrichment cultures for purple bacteria in Lake Fryxell yielded only purple nonsulfur bacteria (10, 11). One such organism obtained in pure culture was shown to be extremely sulfide tolerant and to contain gas vesicles (10).

Because Lake Fryxell is highly sulfidic, it has been assumed that biological sulfate reduction was responsible. Sulfate-reducing bacteria (SRB) reside within the {delta} subdivision of the Proteobacteria, the gram-positive and Thermodesulfobacterium subdivisions of Bacteria, and the archaeal domain Euryarchaeota (26). SRB reduce sulfate to sulfide by using a variety of electron donors, including H2, fatty acids, alcohols, and aromatic compounds (32). SRB are present in permanently cold environments, and several psychrophilic representatives have been isolated in recent years, all from anoxic marine environments (19). Sulfate reduction is a major process in marine sediments, an environment dominated by low temperatures, where SRB play an important role in the sulfur cycle as well as the carbon cycle (12, 22).

The enzyme dissimilatory sulfite reductase (DSR) catalyzes the final steps in sulfate reduction and is therefore an essential enzyme. As such, DSR is synthesized by all known SRB (30). The ubiquity of DSR and its high sequence conservation has made this enzyme ideal for assessing the biodiversity of SRB in anoxic environments (30). In this regard, genes encoding the {alpha} (dsrA) and ß (dsrB) subunits of DSR have been employed in several environmental studies utilizing PCR to profile communities of SRB (5, 6, 17, 18). To explore the biodiversity of SRB in Lake Fryxell, we have performed dsrA analyses of water and sediments from this extreme environment by using a combination of PCR and denaturing gradient gel electrophoresis (DGGE) and have complemented these studies with 16S rRNA gene analyses and enrichment cultures. From our results, we conclude that a diverse group of SRB inhabit Lake Fryxell, including phylotypes only distantly related to known species of this group.


   MATERIALS AND METHODS
Top
 Abstract
 Introduction
 Materials and Methods
Results
 Discussion
 References
 
Sample collection and processing.
Water and sediment samples were collected from Lake Fryxell (Taylor Valley, McMurdo Dry Valleys, Southern Victoria Land, Antarctica) during November 2001 and November 2003 at global positioning system coordinates 77°36.570S, 163°08.969E and 77°36.604S, 163°08.853E, respectively. Samples were collected using a 5-liter Niskin bottle through a hole drilled and melted into the lake ice cover. Water depth was determined from the level to which the lake water rose within the sampling hole. Water samples were collected at depths of 8, 9, 11, 14, and 17 m. Surface sediment samples were collected with an Eckman dredge. All samples were transferred to Crary Lab, McMurdo Station, in sterile containers for immediate processing; samples were kept cold but unfrozen and shielded from light during transport.

Water chemistry and growth media.
Sulfide, temperature, and oxygen measurements were performed as previously described (11). Sulfate concentrations were measured using a standard turbidimetric method in which sulfate is precipitated with barium at acidic pH to form barium sulfate (3). Following primary cultivation of SRB in sealed serum bottles containing lake water amended with 20 mM lactate and 50 mM Na2SO4 (4°C), subcultures were established in anoxic 10 mM MOPS (4-morpholinepropanesulfonic acid)-buffered medium (pH 7.2) containing lactate (20 mM) as a carbon source, Na2SO4 (70 mM) as an electron acceptor, and the following mineral salts (per liter of deionized water): NaCl, 2.0 g; MgSO4·7H2O, 0.25 g; CaCl2·2H2O, 0.1 g; NH4Cl, 1.0 g; KCl, 0.5 g; KH2PO4, 1.0 g; and trace elements (31), 1 ml. Yeast extract (0.005%, wt/vol) was added as a source of vitamins. Either sulfide (1.4 mM) or sodium ascorbate (7.6mM) was added as a reducing agent, and resazurin (1 mg/liter) was added to confirm anoxic conditions. Cultures were maintained in crimp-sealed serum vials or completely filled 17-ml tubes and were incubated at 10°C in darkness. Pyruvate (5 mM final concentration) was added later to enhance growth of all cultures.

DNA extraction.
To concentrate the biomass, 500-ml aliquots of lake water were filtered through sterile 0.2-µm Nalgene analytical test filters. The filters were aseptically cut into approximately 2-mm2 pieces and transferred to a biopulverizer tube (Qbiogene, Carlsbad, CA) for extraction of total environmental DNA. Biopulverizer tubes containing lysing matrix E, 978 µl sodium phosphate buffer, and 122 µl MT buffer (Fast DNA Spin Kit for Soil; Qbiogene) and test filters were processed in a mini-bead beater (Biospec Products, Bartlesville, OK) for 30 seconds at 4,600 rpm. The remainder of the DNA extraction procedure was performed according to the manufacturer's instructions.

PCR and nested PCR.
The SRB-specific 16S rRNA gene primer sets were used as previously described (7). The dsr primer set (1F [5'-ACSCACTGGAAGCACG-3'] and 4R [5'-GTGTAGCAGTTACCGCA-3']) (30) was used to amplify the dsrAB genes. PCR components (Promega, Madison, WI) consisted of 32.5 µl water, 5 µl 10x buffer, 1.25 µl bovine serum albumin (0.25 mg/ml) in initial amplifications, 3 µl 25 mM MgCl2 (1.5 mM), 1 µl each deoxynucleoside triphosphate at 10 mM, 0.5 µl each primer at 125 µg/ml, 1 µl template DNA (30 to 300ng), and 2.5 µl (2.5 units) Taq polymerase (Sigma, St. Louis, MO). Cycling parameters were as follows: an initial denaturation for 15 s at 94°C followed by 30 cycles of 15 s at 94°C, 20 s at 54°C, and 54 s at 72°C, concluding with a 1-min extension at 72°C. Amplification products were analyzed on a 0.7% agarose gel to ensure correct size (1.9 kb) and eluted from the agarose by using the GENECLEAN spin kit (Qbiogene) for use in nested PCR.

To obtain a fragment of the dsrA gene of a size suitable for DGGE analysis, a nested PCR approach was undertaken. The 1.9-kb product from the initial PCR amplification was diluted 1:15 and used as template DNA in a subsequent PCR (with parameters as described above) using the dsr 1F primer (see above) with the addition of a GC clamp on the 5' end (5'-CGCCCGCCGCGCCCCGCGCCCGTCCCGCCGCCCCGCCCC-3') and the 5R primer (5'-TGCCGAGGAGAACGATGTC-3') (30). Amplification products were analyzed on a 2% agarose gel to ensure correct size (243 bp) and eluted from the agarose by using the GENECLEAN spin kit (Qbiogene) for DGGE analysis.

DGGE and sequencing.
Four gel-purified dsrA PCR mixtures were combined (for a total volume of 200 µl) and concentrated to 10 µl in a DNA SpeedVac (Savant, Farmingdale, NY) for loading onto a DGGE gel. Samples were electrophoresed on a 10% acrylamide-bisacrylamide gel prepared from 40% acrylamide-bisacrylamide (38.93 g acrylamide and 1.07 g bisacrylamide dissolved in 100 ml double-distilled water) with a 30% to 90% denaturant range (100% denaturant is 7 M urea and 40% [vol/vol] formamide) at 130 V for 8 h using a DCode universal mutation detection system (Bio-Rad, Hercules, CA). DGGE gels were stained using SYBRI-green (Sigma) and visualized on a UV transilluminator. Bands were excised from the gel and eluted in tissue culture water at 4°C overnight. The eluted DNA was then reamplified using the dsrA primer set (1F/5R) and cloned into the pCR 2.1-TOPO vector (Invitrogen, Carlsbad, CA). Sequences of the dsrA inserts were generated using vector primers (M13F and M13R; Invitrogen, Carlsbad, CA) and the ThermoSequenase cycle sequencing kit (USB, Cleveland, OH).

Sequence analysis.
Similarity to dsrA sequences from cultured SRB was determined using the BlastX function of BLAST (2). Lake Fryxell dsrA clone sequences were manually aligned to dsrA sequences from known SRB obtained from GenBank using the computer program SeqApp (8). The nucleotide alignment was used to create phylogenetic trees by using the PAUP* software package (27). Tree topology was determined using the distance optimality criterion with a neighbor-joining algorithm and the maximum likelihood option using 215 nucleotide positions. The overall tree topology was constrained to the topology of a backbone phylogenetic tree generated using the entire dsrAB gene sequences of cultured SRB. This technique ensured that branching patterns were more consistent with trees generated based on the entire dsrAB operon and that biases imposed from phylogenetic analyses of shorter sequences were less evident in the resulting tree.

GenBank accession numbers of sequences used in dsr alignment and phylogenetic analyses are as follows: Archaeoglobus fulgidis, ARFDSRAB; Desulfomicrobium baculatum, AB061530; Desulfomicrobium escambiense, AB061531; Desulfobacter vibrioformis, AJ250472; Desulfovibrio termitidis, AB061542; Desulfovibrio longus, AB061540; Desulfovibrio sp. strain TBP-1, AF327307; Desulfotomaculum thermosapovorans, AF271769; Desulfosarcina variabilis, AF360643; Desulfosarcina sp. strain CME1, AF360645; Thermodesulforhabdus norvegica, TN0277293; Desulfovibrio piger, AB061534; Desulfovibrio desulfuricans, AF273034; Desulfovibrio cuneatus, AB061537; Desulfofaba gelida, Desulfomusa hansenii, AF419393; Desulfotignum balticum, AF420287; Desulfococcus multivorans, DMU58126; Desulfonema limicola, U58128; Desulfobotulus sapovorans, DSU58120; Desulfobulbus rhabdoformis, DRH250473; Desulfofustis glycolicus, AF418191; Desulforhopalus vacuolatus, AF334594; Desulfotomaculum putei, AF273032; Thermodesulfobacterium commune, AF271771; Desulfovirga adipica, AF334591; Desulforhopalus singaporensis, AF418196; Desulfobacterium autotrophicum, AF418182; Desulfitobacterium dehalogenans, AF337903; Thermodesulfovibrio yellowstonii, U58122; and Desulfotomaculum ruminis, U58118.

Nucleotide sequence accession numbers.
Sequences generated in this study were deposited in GenBank under accession numbers AY273266 to AY273292, AY642269 to AY642278, and AY703988.


   RESULTS
Top
 Abstract
 Introduction
 Materials and Methods
 Results
Discussion
 References
 
Phylogenetic evidence for sulfate-reducing bacteria in Lake Fryxell from 16S rRNA gene analyses.
In preliminary investigations of the SRB population in Lake Fryxell, genomic DNAs obtained from various lake depths (8, 9, 11, 14, and 17 m) as well as from a sediment grab sample were subjected to PCR amplification using 16S rRNA SRB group-specific primers (7). The primer set specific for the Desulfobulbus group yielded positive results in the 8-m and sediment samples (Fig. 1). By contrast, 16S primer sets specific for the Desulfotomaculum and Desulfobacterium groups yielded no positive results. The Desulfobacter-specific primer set yielded positive results in the 9-m, 11-m, 14-m, 17-m, and sediment samples, although the 14-m, 17-m, and sediment samples were only faintly positive (Fig. 1). The 16S primer sets targeting the Desulfovibrio group and the group that includes the genera Desulfococcus, Desulfonema, and Desulfosarcina yielded positive reactions in all the samples tested, although the 8-m positive reaction for the Desulfovibrio primer set was faint (Fig. 1). All positive reactions yielded the expected band size (7) for the primer set. Negative (no-DNA) controls were electrophoresed on a separate gel (not shown) and yielded no evidence of DNA contamination. From these data it was clear that several phylogenetic groups of SRB inhabit Lake Fryxell.



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  		FIG. 1. PCR using SRB genus-specific 16S rRNA gene primer sets on Lake Fryxell genomic DNA samples. Lanes: 1, 1-kb ladder; 2, 8 m, Desulfobulbus; 3, 8 m, Desulfobacterium; 4, 8 m, Desulfococcus-Desulfonema-Desulfosarcina group; 5, 8 m, Desulfotomaculum; 6, 8 m, Desulfobacter; 7, 8 m, Desulfovibrio; 8, 9 m, Desulfobulbus; 9, 9 m, Desulfobacterium; 10, 9 m, Desulfococcus-Desulfonema-Desulfosarcina group; 11, 9 m, Desulfotomaculum; 12, 9 m, Desulfobacter; 13, 9 m, Desulfovibrio; 14, 11 m, Desulfobulbus; 15, 11 m, Desulfobacterium; 16, 11 m, Desulfococcus-Desulfonema-Desulfosarcina group; 17, 11 m, Desulfotomaculum; 18, 11 m, Desulfobacter; 19, 11 m, Desulfovibrio; 20, 1-kb ladder; 21, 1-kb ladder; 22, 14 m, Desulfobulbus; 23, 14 m, Desulfobacterium; 24, 14 m, Desulfococcus-Desulfonema-Desulfosarcina group; 25, 14 m, Desulfotomaculum; 26, 14 m, Desulfobacter; 27, 14 m, Desulfovibrio; 28, 17 m, Desulfobulbus; 29, 17 m, Desulfobacterium; 30, 17 m, Desulfococcus-Desulfonema-Desulfosarcina group; 31, 17 m, Desulfotomaculum; 32, 17 m, Desulfobacter; 33, 17 m, Desulfovibrio; 34, sediment, Desulfobulbus; 35, sediment, Desulfobacterium; 36, sediment, Desulfococcus-Desulfonema-Desulfosarcina group; 37, sediment, Desulfotomaculum; 38, sediment, Desulfobacter; 39, sediment, Desulfovibrio; 40, 1-kb ladder.

 
Evidence for sulfate-reducing bacteria in Lake Fryxell from the metabolic gene dsrA.
To further analyze the SRB population in Lake Fryxell, genomic DNAs from the same five depths plus sediment were amplified using the dsr primer set 1F/4R (30). All samples tested yielded a positive result, with the 8-m reaction yielding only a faint positive, as also occurred with universal bacterial 16S rRNA gene amplification (data not shown). To obtain a gene fragment suitable for DGGE analysis, a 1.9-kbp dsrAB PCR product was used as a template in a PCR utilizing the dsrA primers 1F and 5R. As expected, this amplification yielded a 243-bp fragment (30). Subsequent DGGE analysis of these fragments revealed multiple dsrA phylotypes at each depth in Lake Fryxell (data not shown). All DGGE bands were extracted and analyzed.

Sequence analysis of the retrieved dsrA DGGE bands resulted in seven dsr clone groups (groups A to G) (Fig. 2). Groups A and C were the most diverse, containing between them 18 of the 25 total phylotypes retrieved (Fig. 2). Group A clones exhibited dsrA sequence similarities ranging from 84.6 to 89% to known species of the genus Desulfovibrio, with dsrA from Desulfovibrio termitidis being the closest of cultured relatives (Fig. 2). Clone group C was only distantly related to the Desulfococcus-Desulfonema-Desulfosarcina group of SRB (Fig. 2), with sequence similarities ranging from 66.1 to 73.8%. Group D clones formed a small but distinct sister group to group C clones and exhibited distant sequence similarity (75.2 to 77.1%) to dsrA from Desulfosarcina variabilis (Fig. 2).



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  		FIG. 2. Rooted neighbor-joining distance tree based on 215 nucleotide positions of the dsrA gene, using Thermodesulfovibrio yellowstonii as the outgroup. All designations beginning with LF are from Lake Fryxell. Environmental clone groups are represented in red, enrichment culture clones are represented in green, and the one group containing both enrichment and environmental clones is shown in blue. Bootstrap values of >50% are indicated.

 
Clone group E showed low sequence similarity (67.6 to 69.7%) to dsrA from Desulfobulbus rhabdoformis (Fig. 2). Although group E clustered with the Desulfobulbus group on the dsrA tree (Fig. 2), the two group E sequences were slightly more similar in sequence (69.6 to 71%) to the dsrA gene of Desulfovibrio longus. However, despite employing multiple treeing options, including removal of the backbone constraint tree and/or removal and replacement of relevant taxa, the position of this group within the dsrA tree never changed.

Clone groups F and G contained deeply branching SRB. Group F clones showed distant sequence similarity (63.1 to 63.9%) to dsrA from Thermodesulforhabdus norvegica, while group G clones showed sequence similarity to various dsrA genes (Fig. 2). For example, clone LFdsrC11 showed 61.2% similarity to dsrA from Desulfotomaculum putei, a gram-positive sulfate-reducing bacterium, whereas clone LFdsrC24 showed 67.6% similarity to dsrA from Desulfovibrio sp. strain PT-2 and 69.5% similarity to dsrA from Desulfovibrio desulfuricans, genes that are scattered on the dsr tree (Fig. 2). GroupB was represented by only a single dsrA sequence and showed distant sequence similarity to Desulfomusa hansenii (74.5%) and to Desulfofaba gelida (68.6%), a psychrophilic sulfate-reducing bacterium (12, 13) (Fig. 2).

Distribution of dsrA phylotypes with depth in Lake Fryxell.
Lake Fryxell is highly stratified with respect to oxygen, sulfide, sulfate, and several other parameters (Fig. 3). Since genomic DNA was obtained from water taken at different depths, the distribution of dsrA phylotypes with respect to these physiochemical variables could be tracked. Some dsrA phylotypes, such as those in group A, were distributed throughout the water column, whereas others were found to be distinctly stratified (Fig. 3). Group A was the most widely distributed phylotype, being present at all depths tested, including 8 m, where no other phylotypes were detected. However, the highest diversity of group A phylotypes was at 14 m (Fig. 3). Groups D and F, although not highly diverse, were present both in oxic waters (9 m) and in the sediments; one group D sequence was obtained at 14 m as well, the peak of group A diversity (Fig. 3). Group G dsrA clones were present only in anoxic waters, but in both low-sulfide and high-sulfide regions of the water column (Fig. 3). The remaining clone groups (B, C, and E) were detected only in oxic to barely anoxic waters. Clone group B was the most extreme in this regard, appearing only in the 9-m sample. At this depth oxygen is readily detectable, Eh is still very positive, and sulfide is undetectable (Fig. 3).

Enrichment culture evidence for sulfate-reducing bacteria in Lake Fryxell.
Water column samples collected from various depths of Lake Fryxell were sealed in completely filled 125-ml crimp-top serum bottles immediately upon retrieval and kept cold but unfrozen for transport from the field to Crary Lab at McMurdo. The bottles were then amended with 20 mM lactate and 50 mM Na2SO4 and incubated at 4°C. Visible turbidity was achieved with the 11-, 14-, and 17-m samples within 3 to 4 weeks. Growth was very weak in a 9-m enrichment established in the same way.

Subcultures from the initial enrichments were transferred into a 10 mM MOPS-buffered defined SRB medium (pH 7.2) containing lactate and sulfate. Although growth occurred initially, the cultures could not be repeatedly transferred in this medium. However, supplementing the medium with 5 mM pyruvate significantly improved growth and allowed the cultures to be transferred repeatedly. Microscopic observations of the enrichments showed that the 11- and 14-m cultures contained at least two morphologically distinct organisms, while the 17-m culture appeared more homogeneous (Fig. 4). Vibrio-shaped organisms dominated all cultures. The 11- and 14-m cultures contained both long and short vibrios (Fig. 4A and B). In the 11-m enrichment, nearly equal numbers of a long, thin vibrio and a shorter, thicker vibrio were present (Fig. 4A). In the 14-m cultures, the latter organism was present in large numbers while the thinner organism was present in only low numbers (Fig. 4B). In contrast, the 17 m cultures appeared to be nearly axenic, as the shorter, thicker vibrio dominated the enrichments (Fig. 4C).



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  		FIG. 4. Phase-contrast photomicrographs showing cells of SRB from enrichment cultures established with (A) 11-m, (B) 14-m, and (C) 17-m Lake Fryxell water. Note the gradual loss of the longer, thinner rod (arrow in panel A) with depth, possibly because of a higher sensitivity of this organism to sulfide. Figure 2 shows the phylogenetic positions of dsrA clones obtained from each enrichment, as follows: 11 m, group 2; 14 m, LF14mE1 (group F); 17 m, group 1.

 
DNAs obtained from all enrichments except for that at 9 m yielded dsrA amplification products, and the products clustered into three groups. Two of the groups contained dsrA sequences that did not match any of the seven groups defined by community sampling (Fig. 2 and 3). Group 1 enrichment sequences were shotgun clones of dsrA PCR amplification products obtained from the 17-m enrichment. This group exhibited 91 to 93% similarity to the dsrA gene of Desulfovibrio piger (recent reclassification of Desulfomonas pigra) (16) (Fig. 2 and 3). Group 2 enrichment sequences from the 11-m sample yielded two clone groups showing ~75% similarity to the dsrA gene of the morphologically distinct Desulforhopalus singaporensis, a taurine-fermenting SRB that produces cell surface spinae (15). Finally, the single 14-m clone group matched an environmental sequence, being identical to LFdsrC26, a group F clone (Fig. 2 and 3). Although group F phylotypes were not specifically detected at 14 m by community sampling, they were detected in samples taken both above and below 14 m (Fig. 3). Thus, it is likely that this group is distributed throughout the water column of Lake Fryxell.


   DISCUSSION
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
References
 
Biological sulfate reduction in Lake Fryxell has previously been suspected (9, 25), but these studies focused on the biogeochemistry of the process rather than the diversity of the organisms involved. In one isolation study, a strain of Desulfovibrio, designated strain FESu, was isolated from Lake Fryxell sediment and described (20). However, despite nearly freezing in situ temperatures, strain FESu was enriched at 25°C, grew optimally at the same temperature, and had an upper limit for growth of just above 40°C; the organism is therefore a mesophile (20).

In a more extensive exploration of the diversity of SRB in the water column and sediment of Lake Fryxell, we have employed sequence analysis of the dsrA gene from community DNA along with enrichment studies using in situ enrichment temperatures. In our study, initial evidence for the presence of SRB in Lake Fryxell came from community sampling of 16S rRNA genes. Phylogenetic analysis indicated that at least four of the six major phylogenetic groups of SRB (Desulfovibrio, Desulfosarcina group, Desulfotomaculum, Desulfobulbus, Desulfobacter, and Desulfobacterium) were present in Lake Fryxell. The only groups that could not be detected by 16S rRNA gene analysis were the Desulfobacterium and gram-positive Desulfotomaculum groups. This result was confirmed by dsrA analyses, except for a single sequence obtained that was distantly related to the gram-positive Desulfotomaculum group. However, none of the enrichment cultures showed Desulfotomaculum-like organisms.

Amplification of dsrA revealed the presence of SRB throughout the water column and in the surface sediments of Lake Fryxell. However, the dsrA sequences obtained exhibited only distant similarity to dsrA sequences from cultured SRB of the delta Proteobacteria. Moreover, some Lake Fryxell dsrA sequences showed sequence similarities to a deeply branching clade of SRB containing no cultured representatives. Collectively, the dsrA results thus suggest that novel lineages of SRB inhabit Lake Fryxell. However, in a caveat to our dsr work, we acknowledge the potential for lateral transfer of the dsrAB operon (26, 30); this constrains our use of dsrA to characterize natural communities of SRB in a robust phylogenetic framework. Despite this, in many cases our dsrA results showed such distant sequence similarity to cultured SRB that it would be indeed surprising if the sources of these dsrA genes were not themselves phylogenetically distinct. Also, considering the environmental conditions in Lake Fryxell, it is likely that at least some of these lineages will turn out to be cold-active species of SRB. Stable enrichment cultures for SRB established at 4°C support this conclusion.

Although dsrA sequences could be retrieved from all lake depths tested in this study, several clone groups clustered specifically at particular depths. Lake Fryxell displays sharp gradients of oxygen, Eh, sulfide, sulfate, and dissolved organic carbon (DOC) (Fig. 3). Oxygen levels are supersaturated in the upper depths because oxygen produced by phytoplankton and cyanobacteria becomes trapped under the ice. Conditions become anoxic at about 10 m, where detectable sulfide first appears, increasing to nearly 1.5 mM sulfide near the sediments (Fig. 3). DOC also increases with depth, while the temperature remains near 0°C throughout the water column (Fig. 3). The distribution of dsr phylotypes observed in our study may therefore be a function of the different physiochemical conditions present at different depths in Lake Fryxell, especially in regard to oxygen and sulfide. Group A phylotypes detected in the oxic zone included Desulfovibrio species, many of which show a higher tolerance for oxygen than do other SRB (26). In fact, at least one species of Desulfovibrio, D. oxyclinae, can actually respire O2 (23). Therefore, it is distinctly possible that species of SRB present at 9-m depth or above in Lake Fryxell physiologically resemble D. oxyclinae. Group B organisms in particular should fit this pattern, since they were undetectable in the anoxic zones of the water column.

Sulfate reaches a maximum concentration of 1.72 mM just below the oxycline (13 m) of Lake Fryxell and falls off slowly with depth. However, sulfate decreases sharply from 1.17 mM at 17 m to just 0.34 mM at 17.5 m (Fig. 3). This suggests that maximum rates of sulfate reduction are occurring near the sediments. If this is true, representatives of groups A, D, F, and G are likely the most abundant and physiologically active of the SRB in Lake Fryxell. The high levels of DOC and low Eh in the deeper waters of Lake Fryxell likely stimulate sulfate reduction in this narrow zone near the sediments.

In contrast to group A phylotypes, which are present mainly in regions of the Lake Fryxell water column where sulfide levels are high, phylotypes in groups B, C, and E are present where sulfide is undetectable. This suggests that species in these groups may be quantitatively and/or physiologically insignificant in terms of overall sulfate reduction in Lake Fryxell. However, this conclusion must be tempered by the known presence of other sulfur-cycling organisms in the oxycline of Lake Fryxell. In particular, sulfur-oxidizing chemolithotrophic bacteria and purple phototrophic bacteria are present in the upper regions of the Lake Fryxell water column (10, 11; W. M. Sattley, E. A. Karr, L. A. Achenbach, and M. T. Madigan, Abstr. 103rd Gen. Meet. Am. Soc. Microbiol., abstr. I-129, 2003). Thus, it is possible that sulfide produced by organisms in groups B, C, and E is rapidly oxidized by sulfide-oxidizing prokaryotes coexisting with them in a sulfuretum. In situ activity measurements of sulfate reduction are needed to resolve this question and identify the most active zone(s) of sulfidogenesis in Lake Fryxell.

Enrichment cultures from several lake depths exhibited clear signs of sulfate reduction. These included (i) the ability to grow and produce sulfide in a lactate-pyruvate mineral medium containing only vitamin levels of yeast extract and (ii) the amplification of dsrA. Cloning and sequencing of the dsrA amplification products from the enrichments yielded two groups of dsrA clones that were not detected in environmental clones, a phenomenon widely observed in microbial community profiles (4). However, of all the dsrA sequences generated in this study, group 1 enrichment clones yielded the highest similarity to the dsrA gene of a cultured SRB (>90% to Desulfovibrio piger [Fig. 2]). By contrast, group 2 clones obtained from water at 11 m were not represented by any of the environmental clones and showed only weak similarity (~75%) to cultured SRB. This suggests that the 11-m enrichments contain SRB that are not close relatives of known SRB. Finally, the sequence match to a group F environmental dsrA clone (Fig. 2) that emerged from the 14-m enrichment culture indicates that putatively deeply branching SRB can be cultured from Lake Fryxell.

In conclusion, it is clear from our combined molecular and cultural work that a phylogenetically and possibly physiologically diverse group of SRB inhabit Lake Fryxell. An understanding of the diversity and distribution of SRB in Lake Fryxell is an important prerequisite to understanding sulfur cycling in constantly cold freshwater lakes. Further investigations into the bacterial community of Lake Fryxell are also important for understanding ecosystems that are entirely microbial. Lake Fryxell is characterized by a permanent ice cover, an absence of wind mixing and higher organisms, and seasonal decoupling, with 5 months of darkness per year (24, 28). The microflora of Lake Fryxell has undoubtedly been shaped by these unusual limnological conditions, and the biodiversity of SRB that we report here exemplifies this.


   ACKNOWLEDGMENTS
 
This work was supported by NSF grants OPP0085481 and MCB0237576.

We thank Raytheon Polar Services, Petroleum Helicopters Inc., and John Priscu and the McMurdo LTER limno team for logistical support in the Antarctic. We also thank Sara Belchik for her technical contributions to the project.


   FOOTNOTES
 
* Corresponding author. Mailing address: Department of Microbiology, Southern Illinois University, Carbondale, IL 62901-6508. Phone: (618) 453-7984. Fax: (618) 453-8036. E-mail: laurie{at}micro.siu.edu. Back

{dagger} Present address: Department of Microbiology, The Ohio State University, Columbus, OH 43210. Back


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 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
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Applied and Environmental Microbiology, October 2005, p. 6353-6359, Vol. 71, No. 10
0099-2240/05/$08.00+0     doi:10.1128/AEM.71.10.6353-6359.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.



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