Complete genome sequence of “Thiodictyon syntrophicum” sp. nov. strain Cad16T, a photolithoautotrophic purple sulfur bacterium isolated from the alpine meromictic Lake Cadagno

“Thiodictyon syntrophicum” sp. nov. strain Cad16T is a photoautotrophic purple sulfur bacterium belonging to the family of Chromatiaceae in the class of Gammaproteobacteria. The type strain Cad16T was isolated from the chemocline of the alpine meromictic Lake Cadagno in Switzerland. Strain Cad16T represents a key species within this sulfur-driven bacterial ecosystem with respect to carbon fixation. The 7.74-Mbp genome of strain Cad16T has been sequenced and annotated. It encodes 6237 predicted protein sequences and 59 RNA sequences. Phylogenetic comparison based on 16S rRNA revealed that Thiodictyon elegans strain DSM 232T the most closely related species. Genes involved in sulfur oxidation, central carbon metabolism and transmembrane transport were found. Noteworthy, clusters of genes encoding the photosynthetic machinery and pigment biosynthesis are found on the 0.48 Mb plasmid pTs485. We provide a detailed insight into the Cad16T genome and analyze it in the context of the microbial ecosystem of Lake Cadagno.


Introduction
PSB belonging to the family of Chromatiaceae are generally found at the interface of aerobic and sulfidic-anaerobic zones that are exposed to sunlight such as stagnant, hypertrophic water bodies, littoral zones and bacterial mats [1]. The genus Thiodictyon was first described by Winogradsky in 1888 [2] and comprises two type strains, Thiodictyon elegans strain DSM 232 T and Thiodictyon bacillosum strain DSM 234 T . "Thiodictyon syntrophicum" sp. nov. strain Cad16 T is the proposed type strain of the species "Thiodictyon syntrophicum" [3] within the family of Chromatiaceae of the genus Thiodictyon [4]. Cultures of strain Cad16 T were isolated from the chemocline of the alpine meromictic Lake Cadagno (Ticino, Switzerland). This lake is characterized by high influx of sulfate, magnesium and calcium in the euxinic monimolimnion which favors the formation of a steep chemocline at 10 to 14 m depth [5,6]. Within this zone a dense population (up to 10 7 cells per ml in summer) of mainly anaerobic phototrophic sulfur bacteria belonging to the PSB genera Chromatium, Lamprocystis, Thiodictyon, Thiocystis, and the GSB Chlorobium [7] is responsible for up to 40% of the total CO 2 fixation measured in Lake Cadagno [8]. Strain Cad16 T has been shown to be highly active in CO 2 fixation both in situ and in vitro [9]. Furthermore, aggregation of strain Cad16 T with SRBof the genus Desulfocapsa has been described [3]. In this publication we describe the first complete genome of strain Cad16 T providing details especially on CO 2 fixation, sulfur metabolism and on CRISPRs. The sequencing of strain Cad16 T is part of a larger sequencing project that includes the key species of the microbial community from the anoxic layers of Lake Cadagno.

Classification and features
Strain Cad16 T is Gram-negative, the cells are oval-sphere shaped and 1.4-2.4 μm in diameter, non-motile, vacuolated and contain BChl a. Isolate Cad16 T can grow as single cells, as well as in cell aggregates with up to 100 cells contained in EPS layer (Fig. 1). It was isolated from the chemocline of Lake Cadagno in a depth of 10-14 m where it grows in a non-obligate mutualistic association with sulfur-reducing bacteria of the genus Desulfocapsa [10]. Based upon morphology and partial 16S rRNA sequence analysis, the strain Cad16 T was classified as a member of the genus Thiodictyon within the family Chromatiaceae before [10]. Figure 2 shows the phylogenetic placement of strain Cad16 T (complete 16S rRNA sequence) in a 16S rRNA based maximum likelihood phylogenetic tree. The closest relatives of isolate Cad16 T are T. bacillosum DSM 234 T and T. elegans DSM 232 T with 99% sequence identity (partial 16S rRNA sequences). A comparison of the strain Cad16 T core genome with other whole genome sequenced PSB confirmed the phylogenetic placemant (Additional file 1: Figure S1).
Strain Cad16 T was anaerobically grown in Pfennigs medium [11], containing per liter: 0.25 g KH 2 PO 4 , 0.34 g NH 4 Cl, 0.5 g MgSO 4 ·7H 2 O, 0.25 g CaCl 2 ·2H 2 O, 0.34 g KCl, 1.5 g NaHCO 3 , 0.5 ml trace element solution SL 10 , and 0.02 mg vitamin B 12 with 2 mM acetate in 100 mL serum bottles with rubber stoppers. The medium was prepared in a 2 l bottle with a N 2 /CO 2 (80%/20%) gas phase. The medium was then reduced with 0.3 g l − 1 Na 2 S·9H 2 O (1.10 mM final concentration) and adjusted to a pH of 7.2. Cultures were incubated at 20-23°C under photoheterotrophic conditions with 6 h light/dark photoperiods with a 40-W tungsten bulb placed at a distance of 60 cm from the cultures (ca. 10 μE m − 2 s − 1 ).
Different electron donors and carbon substrates were tested under phototautotrophic conditions by Peduzzi et al. [3,10]. Photolithoautotrophic growth was observed under anoxic conditions with hydrogen sulfide, thiosulfate and elemental sulfur as electron donors. Thereby, elemental sulfur is stored within the periplasma as intermediate oxidation product (Fig. 1). The carbon sources acetate, butyrate, ethanol, formate, fructose, fumarate, glucose, glycerol, lactate, malate, propanol, propionate, pyruvate and succinate were added at 5 mM concentration, respectively. Strain Cad16 T was observed to assimilate only acetate, pyruvate and fructose in the presence of sulfide and bicarbonate. Strain Cad16 T was additionally tested for chemolithoautrophic growth with bicarbonate under a headspace atmosphere containing 5% O 2 , 10% CO 2 and 85% N 2 , in the dark. Growth was observed with 0.02% hydrogen sulfid and 0. 07% thiosulfate, or with 0.07% sulfide only, respectively. The pigments responsible for the purple-red color of strain Cad16 T were analysed spectrometrically in vivo by Peduzzi et al. [3]. Local absorption maxima at 833 nm, 582 nm and 374 nm gave evidence for the presence of BChl a, and at 528 nm for the carotenoid okenone, respectively [10].
A further characterization of strain Cad16 T can be found in Table 1.
A circular representation of the genome sequence and annotation according to the COG criteria is shown in Fig. 3.

Genome project history
Sampling was done in August 2001 using a Friedingertype bottle on Lake Cadagno. Subsequent isolation and cultivation of strain Cad16 T was done in Pfennig's medium I [11]. gDNA was isolated in November 2014 and sequencing was performed in January 2015. Raw data was assembled in with the SMRTview assembly platform and annotated using the NCBI Prokaryotic Genome Annotation Pipeline. Completeness of the isolate Cad16 T sequence was verified using the 31 single copy genes of the Amphora Net analysis platform [12].
The genome sequence was deposited in GenBank under BioProject PRJNA354524, with the accession numbers CP020370-CP020372. The key elements of the genome studied are listed in Table 2.

Growth conditions and genomic DNA preparation
Strain Cad16 T was anaerobically grown in Pfennigs medium [11] Cells were collected by centrifugation for 15 min at 10,600 g. DNA was extracted using phenol/ Fig. 1 Phase-contrast photomicrograph of "T. syntrophicum" sp. nov. strain Cad16 T . The elementary sulfur globule inclusions are visible as black dots within the cell chloroform/isoamylalcohol solution (25:24:1, v/v, Sigma, Buchs, Switzerland) following the protocol provided by Pacific Biosciences [13] in combination with phase lock gels (VWR International). gDNA was purified using AMPure beads (Agencourt, Beckman Coulter Life Sciences, Indianapolis, USA) following the E2612 protocol form New England Biolabs [14].
Purity of the DNA was tested using the Qbit UV/VIS absorption reader (Thermo Fisher Scientific, Rheinach, Switzerland).

Genome sequencing and assembly
The library construction and genome sequencing was done on the Pacific Biosciences RS II platform at the Functional Genomic Center Zurich, Zurich, Switzerland. A 10 kb SMRTbell library was constructed using the DNA Template Prep Kit 1.0 (Pacific Biosciences, Menlo Park, USA). SMRTbell template fragments over 10 kb length were used for creating a SMRT bell-Polymerase Complex with P6-C4 chemistry (Pacific Biosciences) according to the manufacturer instructions.
Four SMRT cells v3.0 (Pacific Biosciences) for PacBio RS II chemistry were used for sequencing. Separate sequencing quality reports for all four cells were created through the SMRT portal software.
The SMRT web portal was used for genome assembly with the RS_HGAP_Assembly.2 pipeline from the SMRT Analysis 2.3 server. The polished assembly consists of 153 scaffolds with a mean coverage of 175× and a N50 value of 6,849,178. Thereof, three scaffolds were distinctly longer (6.85, 0.50 and 0.43 Mb, respectively) and showed a coverage greater than 200×, whereas mean coverage dropped below a value of 50× for the remaining 150 scaffolds. Phylogenetic tree indicating the position of "T. syntrophicum" sp. nov. strain Cad16 T relative to other genomes sequenced within the family Chromatiaceae. The tree was inferred from 16S rRNA sequences (≥ 1300 bp) using the maximum likelihood algorithm of the IQ-TREE software [62]. 1000 bootstrap replicates were performed. Numbers at the nodes indicate the SH-aLRT support (%) and ultrafast bootstrap support (%), respectively. Asterisk mark strains with complete genome sequences available. Open circles as node shapes indicate bootstrap support values below 50% These three scaffolds showed self-similar ends in dotplot graphs and could be circularized manually.

Genome annotation
NCBI Prokaryotic Genome Annotation Pipeline (Annotation Software revision 4.1) NCBI Prokaryotic Genome Annotation Pipeline (Annotation Software revision 4.1) was used for gene calling and gene annotation. To identify CRISPR-Cas sequences the CRISPRfinder server was used [15]. The Pfam-A v29 database was used to predict Pfam domains [16]. Transmembrane domains were predicted with the webserver based TMHMM2 program [17] and signal peptides were predicted with SignalP 4.1 server [18].

Genome properties
The complete genome of strain Cad16 T comprises one circular chromosome (6,837,296 bp) and two circular plasmids pTs485 (484,824 bp) and pTs417 (416,864 bp) ( Table 3). The average GC content for the chromosome, and plasmids pTs485 and pTs417, is 66.28%, 65.59 and 65. 97%, respectively. A total of 6601 coding sequences were predicted. Thereof, 6237 were predicted to encode proteins whereas six rRNA, 49 tRNA and four ncRNA sequences were predicted. A putative function is assigned for 3471 (46.57%) protein encoding genes ( Table 4). The classification of genes into COGs functional categories is given in Table 5. The replicons pTs485 and pTs417 could be made circular, have their own origin of replication each, but do not contain any RNA or house-keeping genes. Therefore, to our understanding, both pTs485 and pTs417 fulfill the plasmid definition. Class Gammaproteobacteria TAS [3] Order Chromatiales TAS [3] Family Chromatiaceae TAS [3] Genus Thiodictyon TAS [2,60] Species "" TAS [3] Strain: Cad16 T TAS [3] Gram stain Negative TAS [3] Cell shape Coccus TAS [3] Motility Non-motile TAS [3] Sporulation a Evidence codes -IDA Inferred from Direct Assay, TAS Traceable Author Statement (i.e., a direct report exists in the literature), NAS Non-traceable Author Statement (i.e., not directly observed for the living, isolated sample, but based on a generally accepted property for the species, or anecdotal evidence). These evidence codes are from the Gene Ontology project [10] Fig. 3 Schematic representation of the complete genome sequence of "Thiodictyon syntrophicum" sp. nov. strain Cad16 T and gene classification according to the COG criteria. The genome comprises one circular chromosome (a) and two circular plasmids, pTs485 (b) and pTs417 (c). Beginning with the outermost circle, tracks are shown in the order of: (1) predicted PGC on pTs485 (green), (2) CRISPR arrays on the chromosome and pTs485 (alternating in blue and red) (3) protein coding genes on forward strand colored according to COG categories, (4) CDS (blue), tRNA (orange) and rRNA (violet) on forward strand, (5) CDS (blue), tRNA (orange) and rRNA (violet) on reverse strand, (6) protein coding genes on reverse strand colored according to COG categories, (7) GC content (black), (8) positive and negative GC skew (green and purple, respectively) and (9) genome region by kbp. GCView [63] was used to create this genome map Extended insights from the genome sequence Phototrophy PSB typically transform light energy into chemical energy with the membrane bound type 2 photochemical reaction center. The chromosome of strain Cad16 T encodes the core antenna proteins LH1, subunits PufA and PufB (THSYN_31145 and THSYN_31140), and the regulatory protein PufQ (THSYN_31110) upstream to the reaction center genes composed of reaction RC subunits PufL, PufM, and PufC (THSYN_31125-31,135). Additional two copies of subunits LH2 alpha and beta (THSYN_31115 and THSYN_31120), respectively, are encoded further downstream, as well as pairwise in two other clusters (THSYN_30995/31005/31030/31040 and THSYN_31000/3100531010/31035/31045), similar as described for the PSB Allochromatium vinosum DSM 180 T [19]. The photosynthetic reaction center H subunit PuhA (THSYN_31405) and PucC (THSYN_31410) are clustered upstream with genes encoding RC-LH1 auxiliary proteins (THSYN_31390-31,400). Furthermore, a homologousHi-PIP (THSYN_25970) is found in strain Cad16 T . It may function as the main electron donor to the photosynthetic reaction center similar as in A. vinosum [20].
Remarkably, most of the proteins involved in photosynthesis are encoded on plasmid pTs485, forming a PGC (Fig. 3) [23]. The highly modular character of the pufLM and pufC genes of α, β and γ-proteobacteria has been demonstrated previously [24,25]. To our knowledge, this is the first description of a PGC being localized on a plasmid in a PSB species. Interestingly, the gene cluster is similarly organized as in the γ-proteobacterium Congregibacter litoralis strain KT71 T and as in members from the α-proteobacteria families Rhodobacteraceae and Rhodospirillaceae, respectively.

Sulfur metabolism
For the photoautotrophic process of CO 2 assimilation in PSB, electrons derived from the oxidation of reduced sulfur compounds, are transferred to electron carriers NAD(P) + and ferredoxin through light energy. During photolithoautotrophic growth under anaerobic conditions, strain Cad16 T uses electrons from the oxidation of sulfide, thiosulfate and elemental sulfur as reducing equivalents [3]. Strain Cad16 T can use thiosulfate as an electron source during phototrophic growth [3]. No homologous genes for the thiosulfate oxidizing multi-enzyme complex SoxAX, could be found in the strain Cad16 T genome. However, soxB (THSYN_26690) and clustered genes  [26], no homologous sequence for the tetrathionate-forming thiosulfate dehydrogenase TsdA was found. However, a c4 cytochrome type TsdB homolog (THSYN_17090) was identified. Due to this unusual combination of genes involved in thiosulfate oxidation, further studies are needed to elucidate the thiosulfate oxidation pathways in strain Cad16 T . Initial sulfide and thiosulfate oxidation is immediately followed SGB formation in strain Cad16 T (Fig. 1). In strain Cad16 T the SGB structure is mediated through envelope SGP homologues to SgpA and SgpB (THSYN_20250 and THSYN_05960) from "Thioflavicoccus mobilis" and Thiocystis violascens, respectively. The sequence of SgpC (THSYN_11025) shows homology to Marichromatium species SgpC/CV3. Predicted signal peptides suggest export of for all three SGP proteins into the periplasm in Cad16 T , as proposed for A. vinosum DSM 180 T [27].
Moreover, the genome of strain Cad16 T encodes the membrane-bound sulfide: quinone oxidoreductases SqrD (THSYN_04215) and SqrF (THSYN_09305). These are possibly involved in the oxidation of sulfide in the periplasm.
The mode of sulfur transport across the inner membrane is not known for PSBs [28]. Organic persulfides such as glutathione or glutathione amide persulfide are proposed as possible candidates. In a next step, the rhodanese-like protein Rhd transfers the sulfur from the persulfide-carrier to the TusA protein in the cytoplasm. The further oxidation steps from sulfur to sulfite are typically mediated through the reverse acting dsr genes in PSB [29]. The strain Cad16 T genes in the dsr cluster (THSYN_22480, THSYN_22490-22,545) are arranged in a highly conserved organization similar to A. vinosum DSM 180 T , only missing dsrS that is non-essential for sulfur oxidation [30]. The DsrEFH complex mediates persulfate transfer from TusA onto DsrC. The persulfurated form of DsrC is then substrate for the cytoplasmic reverse-acting dissimilatory sulfite reductase DsrAB that catalyzes the formation of sulfite. Finally, DsrMKJOP complex reduces DsrC [30].
The genome harbors three additional sulfur relay proteins similar to DsrC (THSYN_09485, THSYN_18820 and THSYN_22565) that could function as TusA homologues. In A. vinosum DSM 180 T DsrC is able to bind DNA upstream the dsr cluster [31].
In strain Cad16 T , soeABC (THSYN_16370-16,380) encode the sulfur-iron molybdoprotein complex that further oxidizes sulfite to sulfate on the cytoplasmic site of the membrane [32]. Alternatively, strain Cad16 T oxidizes sulfite via APS by APS-reductase AprBA (THSYN_ 16395 and THSYN_16400) and ATP sulfurylase Sat (THSYN_16390), as in other PSB [33,34]. Thereby, the membrane-bound QmoABHdrCB-complex [35] (THSYN_16425-6440) possibly functions as an electron acceptor for the AprAB reductase complex since no aprM homolog was found in the strain Cad16 T sequence. For the extra-cytoplasmic export of the final oxidation product sulfate, a SulP sulfate permease (THSYN_14085) homolog to A. vinosum DSM 180 T is encoded in the strain Cad16 T sequence.
Hydrogen uptake and consumption has been shown to be linked to sulfur metabolism in Thiocapsa roseopersicina BBS [36,37]. Thereby, electrons from hydrogen oxidation in the periplasm by the hyn-type hydrogenase HydSL could be transferred via the Isp membrane complex to the disulfide bound to DsrC. In A. vinosum DSM 180 T , transcription of isp1 and isp2 encoding the Isp hydrogenase subunits is upregulated during growth on sulfide [38]. The Isp complex is composed of two subunits, Isp1 and Isp2, that contain similar catalytic domains as DsrM and DsrK, respectively. Similarly, homologous Isp1 and Isp2 proteins (THSYN_28105 and THSYN_28100) may link sulfur to hydrogen metabolisms in strain Cad16 T . In accordance, an increase in the sulfide concentration was observed while SGB were consumed by strain Cad16 T during incubation in the dark (unpublished results, F. Danza).
Additionally, other [NiFe]-hydrogenases of the Hox and Hup type (THSYN_22655, THSYN_22660 and THSYN_28115) are found in the sequence that could mediate light-dependent H 2 evolution as proposed for T. roseopersicina [39,40].
The Cad16 T genome also harbors cys genes (THSYN_ 05020-05035) that are probably involved in sulfate assimilation under sulfur-limiting conditions. Furthermore, the genome also encompasses genes encoding the CydDC (THSYN_18930 and THSYN_18935) ATPdriven cysteine transport proteins [41].

Autotrophic growth
In PSB, CO 2 fixation is essentially achieved through the reductive pentose phosphate also known as the CBB cycle. In accordance, the strain Cad16 T genome harbors the complete CBB enzymatic pathway. On the chromosome, the dimeric RuBis-CO form II (THSYN_ 13250) clusters with RuBis-CO activation protein subunits CbbR, CbbQ and CbbO, (THSYN_13245, THSYN_13255 and THSYN_13285). Interestingly, small and large RuBis-CO subunits form I (THSYN_29475 and THSYN_29480) cluster together with carboxysome shell and auxiliary proteins on plasmid pTs417 (THSYN_29485-29,520 and THSYN_29530-29,535). The carboxysome may allow efficient photoassimilation across varying CO 2 concentrations as proposed for A. vinosum DSM 180 T [42]. Previous studies showed different expression regulation for RuBis-CO type I and type II genes from Cad16 T suggesting that only the type II is involved in the process of CO 2 fixation [8]. Interestingly, the plasmid pTs485 also harbors a RuBis-CO -like protein form III gene (THSYN_31160) upstream the PGC.
The missing sedoheptulose-1,7-bisphosphatase SBP is possibly bypassed by via the fructose-1,6-bisphosphatase (THSYN_25630). The genes gltA citrate synthase (THSYN_12620), fumA fumarate hydratase (THSYN_ 24360) and sucCD succinyl-CoA ligase (THSYN_00880 and THSYN_00885) that are essential for the TCA cycle, and isocitrate lyase (THSYN_16275) and malate synthase (THSYN_15655) that are essential for the glyoxylate cycle, respectively, are identified in the strain Cad16 T sequence. Recently a proteomic study about the capacity of Cad16 T to fix CO 2 in the dark suggested the presence of a particular archael DC/HB cycle [42]. However, nofurther genes coding for this DC/HB cycle were found. Also a complete set of genes coding for polyhydroxyalkanoic acid synthase PhaC (THSYN_ 06910) and poly-(3-hydroxybutyrate) depolymerase PhaE (THSYN_06905) are found in the strain Cad16 T genome. Strain Cad16 T additionally encodes genes necessary for glycogen polymerisation. The glucose 1-phosphate adenylyltransferase GlgC (THSYN_00810), the glycogen synthase GlgA (THSYN_11615) and the 1,4-alpha-glucan branching enzyme GlgB (THSYN_00805) allow the synthesis of glycogen.
Interestingely, strain Cad16 T also has the potential to produce the storage compound cyanophycin normally found in caynobacteria [43], since the two subunits of the enzyme cyanophycin synthetase (THSYN_26990 and THSYN_26995) are found.
Togheter, these finding provide genetic evidence for the high carbon fixation potential of strain Cad16 T in the dark [8,44].
Anaerobic Fe(II)-oxidation was described for other Thiodictyon strains [45,46] and evidence of cryptic in situ iron cycling has been demonstrated recently [47]. In accordance with these findings, we found cbb3 type terminal cytochrome C oxidases (THSYN_06760-08775) possibly involved in Fe(II) driven carbon fixation in strain Cad16 T genome.
Strain Cad16 T grows chemoautotrophically under microaerobic conditions (5% O 2 ) with sulfide, thiosulfate, or sulfide only [3], as also observed in other PSB in vitro studies with Lamprocystis purpurea [10,48], Thiocystis violacea and A. vinosum [49]. In situ, strain Cad16 T is possibly exposed to low concentration of oxygen produced by oxygenic microbiota at the mixolimnionchemocline interface [8]. Accordingly, we observe genes encoding sod-type superoxide dismutases (THSYN_20405 and THSYN_22720), as well as fnr and fur-type transcriptional regulators involved in peroxide stress response. In situ, strain Cad16 T is possibly exposed to oxygen produced by oxygenic microbiota at the mixolimnion-chemocline interface [8].

Transmembrane transport proteins
Several membrane transport genes were found in the strain Cad16 T genome, including protein secretion system Type II, genes encoding the TAT pathway and several TRAP transporter genes, as well as genes encoding Ton-Tol type and ABC-type transporter complexes. Additionally, a complete TSS4 pilus machinery is encoded in six clusters dispersed on the strain Cad16 T chromosome. Notably, also structural components of TSS6 secretion system are found in two clusters on the chromosome (THSYN_11395-11,410) and on pTs485 (THSYN_32540-THSYN_32580). Two effector proteins of the VrgG family were identified. THSYN_15360 belongs to the vgr_GE type Rhs family proteins similar sequences found in β-proteobacterial family of the Burkholderiaceae whereas THSYN_32425 is conserved in γ-proteobacteria and contains a type IV Rhs element. Togheter, the secretion machinery allows strain Cad16 T to interact within the highly populated chemocline with up to 10 7 bacterial cells per milliliter. The secretion and uptake mechanism may also play a key role in the cellto-cell contact with Desulfocapsa thiozymogenes.
The diurnal and sesonal behavior of vacuolated Chromatiaceae has been described for different lakes [50,51].
Five CRISPR repeat regions (CRR1-CRR5) were identified in the genome of strain Cad16 T , four being located on the chromosome and one on the plasmid pTs485 (Fig. 3). The number of DRs ranges from 19 (CRR4) to 146 (CRR2) as seen in Table 6.

Conclusions
We report on the first complete genome sequence of "Thiodictyon syntrophicum" sp. nov. strain Cad16 T and the metabolic versatility of this environmentally relevant organism. The observed carbon fixation potential can be explained by the highly developed photosynthesis machinery that is coupled to the sulfur and carbon metabolism. Within the changing conditions in the chemocline, strain Cad16 T is able to optimally use light, different organic and inorganic carbon compounds, reduced sulfur, nitrogen and oxygen. The two 0.4 Mb plasmids found in Cad16 T are unique for known PSB species and we report structural similarity to sequences from αand γ-proteobacterial phototrophs. The availability of the complete genome sequence of strain Cad16 T will facilitate further studies that elucidate its role as key species of the chemocline and the tight association with the Desulfocapsa sp. and the interaction with different PSB and GSB species present in the anoxic part of Lake Cadagno. Due to the limited molecular data on other Thiodictyon strains and no reference strains available, no (digital) DNA-DNA hybridization experiments could be performed. However, the result from phylogenetic analyses on 16S rRNA sequence level, comparative genomic analyses as well a morphological and physiological differences (see above) indicate a novel species within the genus Thiodictyon.
The decribed isolate is therefore proposed as "Thiodictyon syntrophicum" sp. nov. strain Cad16 T , a novel species within the genus Thiodictyon.
Gram-negative, cells are oval-round shaped and 1.4-2. 4 μm in diameter, non-motile, vacuolated and contain BChl a and okeneone. Growth as single cells, as well as in aggregates with up to 100 cells in a EPS layer. Chemolitoautotrophic growth was observed with 5% Oxygen and 0.02% hydrogen sulfid and 0.07% thiosulfate, or with 0.07% sulfide only, respectively. The type strain Cad16 T (=JCM 15483 T =KCTC5955 T ) was isolated from a sulfidic chemocline in the alpine Lake Cadagno in Switzerland. The genome size of the type strain is 6.84 Mb (chromosome), contains two plasmids, pTs485 (0.49 Mb) and pTs417 (0.42 Mb) and the G + C content of the genome is 66.22%. The 16S RNA gene sequence of strain Cad16 T is deposited under the GenBank/EMBL/ DDBJ accession number AJ511274. The complete genome sequence of the type strain Cad16 T is deposited under the GenBank ID CP020370, CP020371 and CP020372. The type strain has been deposited both at the Japan Collection of Microorganisms (JCM 15483 T ) and at the Korean Collection for Type Cultures (KCTC 5955 T ).