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1 S. N. Winogradskii Institute of Microbiology RAS, Prospect 60-let Octyabrya 7/2, 117811 Moscow, Russia
2 Department of Environmental Biotechnology, Delft University of Technology, 2628 BC Delft, The Netherlands
3 Department of Microbiology, Faculty of Biology, Moscow State University, Moscow, Russia
4 Department of Biological Sciences, Louisiana State University, Baton Rouge, LA 70803-1715, USA
Correspondence
Dimitry Yu. Sorokin
soroc{at}inmi.host.ru or
D.Y.Sorokin{at}tnw.tudelft.nl
| ABSTRACT |
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The GenBank/EMBL/DDBJ accession number for the 16S rRNA gene sequence of strain TL 2T is AY656719 and the accession numbers for partial cbbL gene sequences of strain TL 2T, Rdv. sulfidophilum DSM 1374T, Rdv. euryhalinum DSM 4868T and Rdv. adriaticum DSM 2781T are AY656720AY656723.
| INTRODUCTION |
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A near-shore sulfidic hydrothermal area in Matupi Harbour on New Britain (Papua New Guinea) is an example of an environment that can be beneficial for proliferation of facultatively autotrophic sulfur bacteria. There, a hot sulfide-containing volcanic seep mixes with cold sea water containing oxygen and organic compounds, originated from decaying shore vegetation (Ferguson & Lambert, 1972
). However, the concentration of these compounds fluctuates significantly because of the active water dynamics, which do not favour domination of obligate species. Enumeration of different types of sulfur-oxidizing bacteria demonstrated the dominance of heterotrophic forms in most of the samples oxidizing thiosulfate either to sulfate or to tetrathionate (Sorokin, 1991
). Among the tetrathionate-producing heterotrophs, a peculiar coccoid morphotype was dominant and was described as Catenococcus thiocycli (Sorokin, 1992
; Sorokin et al., 1996
). Among the ten sulfate-forming heterotrophic strains, one isolate, strain TL 2T,was able to grow autotrophically. The physiological properties and taxonomic position of this novel bacterium are described in this paper.
| METHODS |
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. The sulfur-oxidizing population from this sample was first enriched and then serially diluted on mineral medium with thiosulfate and sea water. The culture grown from the greatest dilution was streaked onto solid medium of the same composition and single colonies were isolated. When inoculated in a liquid mineral medium, these colonies always resulted in the growth of two types of bacteria. One type was represented by a Thiomicrospira-like, obligately autotrophic, sulfur-oxidizer unable to grow heterotrophically. Another type, represented by a filamentous heterotroph, was a minor component in mineral medium but dominated under purely organotrophic or mixotrophic conditions. Despite differences in metabolism, separation of the two organisms was difficult, because they grew in a stable association and even formed fused colonies. Finally, after eight to ten sequential passages on purely organic medium with acetate, the heterotrophic member was isolated in pure culture and designated strain TL 2T.
The mineral medium for primary enrichment culture was prepared on the basis of filter-sterilized sea water supplemented with 10 mM thiosulfate and the pH-indicator bromothymol blue (from water-saturated solution, 1 ml l1). For further batch cultivation, an artificial medium was used containing (g l1): NaCl 25, K2HPO4 0·5, NH4Cl 0·5, MgSO4.7H2O 0·4, CaCl2.2H2O 0·1 and trace metal solution (Pfennig & Lippert, 1966
) 1 ml l1. The last three components were added after sterilization. TL 2T required biotin and thiamine for growth; pyridoxine had a growth-stimulating effect. Therefore, normally, a filter-sterilized mixture of these vitamins was supplied after medium sterilization to a final concentration of 50 µg l1 each. Thiosulfate was added after sterilization from 2 M stock solution to a final concentration of 1020 mM. The initial pH was set at 7·5. Upon acidification, the pH of the medium was adjusted to 7·5 with a sterile 1 M NaHCO3 solution. For heterotrophic and mixotrophic growth, the mineral base was supplemented with an organic carbon source (1020 mM) and yeast extract (0·2 g l1) instead of the vitamin mixture. Autotrophic growth with hydrogen was tested on a basic mineral medium plus the vitamin mixture in a closed jar under a 1 : 1 mixture (v/v) of air/H2. Growth under denitrifying conditions was tested using acetate or thiosulfate as electron donor and nitrate, nitrite or N2O as electron acceptor. The medium was made anoxic by flushing with argon for 20 min. The methylotrophic potential of the isolate was examined with methanol (2 ml l1) or formate (20 mM) as substrates. For continuous cultivation, a 1·5 l laboratory fermenter with a 1 l working volume, fitted with pH and oxygen controls (Applicon), was used. The dissolved oxygen concentration was controlled at 50 % air saturation by the stirring speed. The temperature was controlled at 30 °C. For chemical stability, the medium was supplied from two reservoirs, containing acidic and alkaline solutions in double strength. The acidic solution contained (l1) 1 g KH2PO4, 0·8 g NH4Cl, 0·4 g MgCl2.6H2O, 0·2 g CaCl2.2H2O, 2 ml trace metal solution, 25 µl silicone antifoam and 10 ml HCl. The alkaline base included 50 g l1 NaCl and 2 g l1 of NaOH. Thiosulfate and acetate were sterilized separately as 2 M solutions and added to the alkaline base after sterilization. For autotrophic growth, the final concentration of thiosulfate was 40 mM, and for mixotrophic conditions 10 mM acetate and 1020 mM thiosulfate were used.
Respiratory activity of washed cells was measured in a 4 ml thermostatted cell with a Clark oxygen electrode (Yellow Spring Inc.) at 30 °C in 0·05 M potassium phosphate buffer, pH 7·5, supplemented with 25 g NaCl l1 and 0·2 g MgCl2.6H2O l1.
Cell-free extract was prepared by ultrasonic disruption of washed cell suspension (1015 mg protein ml1) either in 0·05 M phosphate or Tris/HCl buffers at pH 7·0 and 8·0, respectively. RubisCO activity in cell-free extracts was measured according to Beudecker et al. (1980)
. Activities of sulfite-cytochrome c oxidoreductase and AMP-dependent sulfite dehydrogenase were determined in the cell-free extract as described by Kelly & Wood (1994)
. Qualitative tests for the presence of cytochrome oxidase and catalase were performed with single colonies from an acetate-grown culture using 1 % TMPD (N,N,N',N'-tetramethyl-1,4-phenylenediamine dihydrochloride) and 3 % H2O2, respectively. Thiosulfate and tetrathionate were analysed by cyanolysis (Kelly et al., 1969
), sulfide and sulfite colorimetrically according to Trüper & Schlegel (1964)
and sulfate by HPLC (column ET 250/8/4 Nucleosil 10 Anion; 30 °C, carrier 0·025 M sodium salicylate, pH 4·0, flow rate 1·5 ml min1). Biomass protein was estimated by the Lowry method, after washing the pellet with a 2·5 % NaCl solution twice to remove interfering sulfur compounds. Pigments were extracted with a methanol/acetone mixture (3 : 7, v/v) at 4 °C for 12 h and the extract was analysed spectrophotometrically. For electron microscopy, cells were pre-fixed with glutaraldehyde (2·5 % v/v) and then post-fixed with 1 % OsO4+2 % NaCl (w/v), dehydrated and embedded into resin. Thin sections were stained with uranyl acetate and lead citrate.
Total DNA extraction, amplification and sequencing of 16S rRNA and of cbbL genes were performed as described previously (Sorokin et al., 2003
; Spiridonova et al., 2004
). Phylogenetic analyses based on nucleotide sequences of 16S rRNA genes and putative amino acid sequences deduced from cbbL sequences were performed using different treeing algorithms realized in the TREECON (Van de Peer & De Wachter, 1994
) and PHYLIP (Felsenstein, 1989
) software packages.
| RESULTS |
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Metabolism
TL 2T grew chemoautotrophically with thiosulfate as an electron donor and heterotrophically with acetate, succinate, malate, lactate, pyruvate, propionate, butyrate, glycolate, glyoxylate, malonate, ethanol, glycerol, mannitol, D-glucose, D-maltose, sucrose, D-cellobiose, proline, lysine, alanine, glutamate, glutamine, aspartate and asparagine. Growth was not observed with methanol, formate, methylamine, benzoate, D-ribose, D-fructose, D-galactose, D-melibiose, starch, casein, Tween 80, aromatic amino acids, leucine, arginine, threonine or serine. It was unable to ferment glucose or to grow anaerobically under denitrifying conditions either autotrophically or heterotrophically. Ammonium and nitrate, but not nitrite, urea or dinitrogen served as nitrogen sources.
In batch cultures with acetate, thiosulfate was oxidized mostly to sulfate during the exponential phase of growth (Fig. 2
). Significant amounts of sulfite were detected in the medium as an intermediate of thiosulfate oxidation. To obtain a stable autotrophic culture of TL 2T on mineral medium with thiosulfate as energy source, two or three adaptive transfers from the mixotrophic and five or six transfers from the heterotrophic media were necessary. Up to 5 mM sulfite was transiently produced in the course of thiosulfate consumption. The µmax and Ymax values in the activated autotrophic culture were 0·06 h1 and 1·61·7 g protein (mol thiosulfate)1, respectively. In continuous mixotrophic (acetate+thiosulfate) culture with 10 mM of each substrate, sulfite was not accumulated in the effluent at a dilution rate (D) up to 0·05 h1 but, when the influent thiosulfate concentration was increased up to 20 mM, sulfite started to accumulate at D>0·04 h1. The autotrophic continuous culture with 40 mM thiosulfate started from the mixotrophic culture after 2 weeks adaptation period at low D (0·03 h1) reached µ=0·1 h1, after which wash-out of the culture was observed. In contrast to batch cultures, no sulfite production was found in the autotrophic continuous culture and the molar growth yield was about 2-fold higher (Table 1
). One of the possible reasons for such differences might be more stable pH conditions in the continuous culture.
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Phylogenetic analysis
The G+C content in the DNA of TL 2T was 63·1±0·3 mol%. Phylogenetic analysis based on the almost complete 16S rRNA gene sequence of TL 2T placed the novel isolate into the
-3 group of the Proteobacteria within the order Rhodobacteriales. In the phylogenetic tree constructed with the neighbour-joining algorithm, strain TL 2T formed a separate branch located between the purple non-sulfur bacteria of the genera Rhodobacter and Rhodovulum (Fig. 3
). However, a low value of bootstrap support (24 %) showed an uncertain position for its branch point. The phylogenetic trees constructed either by maximum-parsimony or maximum-likelihood algorithms had the same topologies (data not shown). The nucleotide sequence similarity was almost the same with different species of the genera Rhodobacter and Rhodovulum, ranging from 92·4 to 95·0 %. There was no close relatedness between the strain TL 2T and the recently described non-photosynthetic members of the RhodobacterRhodovulum clade, Albidovulum inexpectatum (Albuquerque et al., 2002
) and Pseudorhodobacter ferrugineus (Uchino et al., 2002
), which had 92·8 and 92·5 % sequence similarity to strain TL 2T, respectively.
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| DISCUSSION |
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Phylogenetic analysis placed the novel bacterium in the
-subclass of the Proteobacteria. This subclass is remarkable in the strong representation of lithoheterotrophic and facultatively autotrophic sulfur-oxidizing species. Moreover, some of the aerobic sulfur-oxidizing alphaproteobacteria still express non-functional bacteriochlorophyll a or contain its gene (Yurkov & Beatty, 1998
). This might indicate direct descent from a common ancestral anaerobic, non-sulfur, purple bacterium. The facultatively autotrophic strain TL 2T and the thiosulfate-oxidizing heterotroph Albidovulum inexpectatum (Albuquerque et al., 2002
) are phylogenetically even more closely related to anaerobic phototrophs than to chemotrophic representatives of the
-Proteobacteria. But, apart of the ability to oxidize thiosulfate and similarity of their habitats (marine thermal waters), strain TL 2T and Albidovulum inexpectatum differed significantly in phenotypes, such as autotrophy, temperature and salt range. One of the interesting questions remaining to be answered is why this particular subclass, in contrast, for example, to the
- or
-subclasses of the Proteobacteria, is almost completely lacking in obligate autotrophs.
Finally, regarding the unique phylogenetic position and distinct phenotype, this isolate is proposed to represent a novel genus and species with the name Thioclava pacifica gen. nov., sp. nov.
Description of Thioclava gen. nov.
Thioclava (Thi.o.cla'va. Gr. neut. n. thion sulfur; L. fem. n. clava stick, staff, cudgel, club; N.L. fem. n. Thioclava sulfur-oxidizing swollen rod).
Gram-negative rods of varying size from long filaments with swollen ends, clustered in aggregates, to single small rods, rarely motile by one polar flagellum. Obligately aerobic and facultatively autotrophic, sulfur-oxidizing bacteria. Grow chemoautotrophically with thiosulfate and heterotrophically with simple organic compounds. Unable to utilize hydrogen as electron donor and to grow methylotrophically. Inorganic carbon is assimilated via the Calvin cycle. Contain the green form of type I RubisCO. UQ-10 is the only respiratory lipoquinone present. The type species of the genus is Thioclava pacifica.
Description of Thioclava pacifica sp. nov.
Thioclava pacifica (pa.ci'fi.ca. L. fem. adj. pacifica pacific, peacemaking, and, by extension, pertaining to the Pacific Ocean, where the type strain was isolated).
The description is identical to that of the genus, with the following additions. The following substrates are utilized for heterotrophic growth: acetate, succinate, malate, lactate, pyruvate, propionate, butyrate, glycolate, glyoxylate, malonate, ethanol, glycerol, mannitol, D-glucose, D-maltose, sucrose, D-cellobiose, proline, lysine, alanine, glutamate, glutamine, aspartate and asparagine. No growth on methanol, formate, methylamine, benzoate, D-ribose, D-fructose, D-galactose, D-melibiose, starch, casein, Tween 80, aromatic amino acids, leucine, arginine, threonine or serine. Unable to ferment glucose or to grow anaerobically under denitrifying conditions either autotrophically or heterotrophically. Ammonium salts and nitrate serve as nitrogen sources. Thiamine and biotin are required for growth. Oxidase- and catalase-positive. Grows within a pH range from 6·5 to 8·5 (optimum 7·57·8), temperature range from 15 to 47 °C (optimum 35 °C) and NaCl range from 10 to 80 g l1 (optimum 3040 g l1). The G+C content of the DNA of the only strain known in the species is 63·1±0·3 mol%.
The type strain, TL 2T (=DSM 10166T=UNIQEM 229T), originated from sulfidic thermal sea water in Matupi Harbour, New Britain, Papua New Guinea.
| ACKNOWLEDGEMENTS |
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