Enrichment and isolation
For the initial enrichment, the mud sample was inoculated (10% w/v) into sterile anaerobic liquid medium with pyruvate as the growth substrate. After 2 days of incubation at 30 °C, microbial growth was observed. After three subsequent transfers and following serial 10-fold dilutions in the same medium, only one morphological type was observed in the highest positive dilution (10-9). Attempts to obtain separate colonies either anaerobically in agar blocks or aerobically on the surface of the medium with 1.5% of agar were unsuccessful. The purity of strain F-1T was assessed by routine microscopic examination and confirmed by results of 16S rRNA gene and complete genome sequencing.
Phenotypic and chemotaxonomic characteristics
Mid-exponential-phase cells of strain F-1T grown on sulfate and lactate were motile vibrios with a single polar flagellum, 2.0 – 4.0 µm in length and 0.5 µm in diameter (Fig. 1a). Cells stained Gram-negative in both the exponential and the stationary growth phases. The formation of endospores was not observed in the cultures grown under optimal or suboptimal conditions. Ultrathin sections of the strain F-1T revealed a Gram-stain-negative cell wall type (Fig. 1b).
The temperature range for growth of strain F-1T was 6–37 °C, with an optimum at 24 °C. No growth was detected at 4 °C or below and 42 °C or above after incubation for a month. The pH range for growth was 7.0-10.5, with an optimum at pH 9.5. No growth was observed at pH values 6.5 or bellow or 11.0 or above. Growth of strain F-1T was observed at NaCl concentrations from 0.3 to 3.0% (w/v) with an optimum at 0.5-1.0%, no growth was evident at 3.5% (w/v) NaCl or above. The doubling time on lactate/SO42- under optimal growth conditions was 1.47 h-1. Addition of yeast extract (0.1 g/l) did not stimulate growth.
Strain F-1T grew with sulfate (14 mM) as an electron acceptor and lactate (20 mM), fumarate (20 mM), D-glucose (5 mM), D-cellobiose (20 mM) or molecular hydrogen (H2/N2; 80/20; v/v in the gas phase) as an electron donor. In the presence of sulfate the end products from lactate oxidation were acetate (13 mM), propionate (0.8 mM) H2S (5 mM) and CO2 (0.5 mM). Pyruvate (10 mM) , malate (5mM), formate (20 mM), acetate (5 mM), butyrate (5 mM), ethanol (10 mM), propanol (10 mM), and arabinose (10 mM) were not used as electron donors with sulfate as an electron acceptor. In the absence of sulfate lactate and pyruvate were fermented and supported growth. The major product of lactate fermentation was acetate (1.6 mM); trace amounts of propionate (0.4 mM), CO2 (0.11 mM) and hydrogen (0.06 mM) were also produced. Fumarate, glucose and cellobiose were not fermented. Strain F-1T demonstrated a weak (5*106 cells ml-1), but sustainable (at least 5 consequent 5% (v/v) transfers) autotrophic growth with sulfate, sulfite, thiosulfate, elemental sulfur, fumarate or arsenate as an electron acceptors and molecular hydrogen (H2/N2; 80/20; v/v in the gas phase) as an electron donor. Addition of acetate (10 mM) as a carbon source did not have any effect on autotrophic growth. With lactate as an electron donor sulfate (14 mM), sulfite (2 mM), thiosulfate (20 mM), elemental sulfur (5 g/l), fumarate (20 mM) or arsenate (5 mM) were used as an electron acceptor for growth, but nitrate (20 mM), nitrite (5mM), selenate (5 mM) or ferrihydrite (poorly crystalline Fe(III) oxide, 90 mmol Fe(III) l–1) were not utilized. Strain F-1T was not able to grow by disproportionation of sulfite (5 mM), thiosulfate (10 mM) and elemental sulfur (5 g/l).
Major fatty acids were anteiso-С15:0 (13.8%), С16:0 (12.5%), С18:0 (11.8%), and iso-С17:1ω8 (12.0). Other branched saturated and monounsaturated fatty acids were detected in fewer amounts (Supplementary Table S1 and Figure S1).
Phylogeny
The 16S rRNA gene sequences of strain F-1T obtained by amplification with universal bacterial primers and retrieved from whole-genomic data were identical. A comparison of 1541 nucleotides of 16S rRNA gene sequences of strain F-1T with those available in GenBank (Benson et al. 1999) and EzBioCloud (Yoon et al. 2017) databases showed that the novel isolate belongs to the genus Pseudodesulfovibrio and had the highest sequence similarity to Pseudodesulfovibrio aespoeensis DSM 10631T (98.05 %) and Pseudodesulfovibrio indicus J2T (96.00 %). The 16S rRNA gene phylogenetic tree reconstruction revealed that the strain F-1T constituted a monophyletic branch clearly separated from the most closely related species (Fig. 2).
Pairwise ANI value of the genome of the strain F-1T and the genome of the closest relative organism, P. aespoeensis (DSM 10631T) was 82.07%. The in silico DDH value predicted between strain F-1T and P. aespoeensis (DSM 10631T) by the recommended formula 2, was 24.50%. Both these values are much lower than the threshold for prokaryotic species delineation proposed to be 95–96 % (ANI) and 70% (DDH) (Meier-Kolthoff et al. 2013, Rodriguez-R and Konstantinidis 2016).
Genome analysis
The draft genome assembly of strain F-1T has a total length of 3227153 bp and N50 value of 302886 bp within 29 contigs and the genomic DNA G+C content was 61.93 mol%. The genome of F-1T was predicted to contain 3061 protein-coding sequences and 54 RNA genes. Most of the annotated genes were responsible for the synthesis of amino acids and derivatives (167), protein metabolism (155), cofactors, vitamins, prosthetic groups and pigment formation (86) (Supplementary Table S2 and Figure S2).
The genome of strain F-1T contains a full set of genes required for dissimilatory sulfate reduction (Pereira et al. 2011) including sulfate adenylyltransferase (WP_155932275), manganese-dependent inorganic pyrophosphatase (WP_155934818), APS reductase subunits AprA (WP_155932273) and AprB (WP_155932274), the subunits of dissimilatory sulfite reductase DsrABCD (WP_155934369 - WP_155934373), and electron transfer complexes DsrMKJOP (WP_155932960 - WP_155932964) and QmoABC (WP_155932272 - WP_155932270).
The genome of strain F-1T possessed all genes for glycolysis via the Embden-Meierhoff-Parnas pathway. Surprisingly, the reductive pentose phosphate pathway in the genome of strain F-1T is absent, although ribulose biphosphate carboxylase, key enzyme of rPP, was present in the proteomes of several Pseudodesulfovibrio strains (Bell et al. 2018).
Strain F-1T can grow autotrophically, but its genome does not harbor the genes encoding the key enzymes of six well-characterized microbial carbon fixation pathways, viz. ribulose 1,5-bisphosphate carboxylase (Calvin-Benson cycle), carbon monoxide dehydrogenase/acetyl-CoA synthase complex (reductive acetyl-CoA pathway), ATP-citrate lyase and citryl-CoA lyase (two variants of the reductive tricarboxylic acid cycle), 4-hydroxybutyryl-CoA dehydratase (3-hydroxypropionate/4-hydroxybutyrate and dicarboxylate/4-hydroxybutyrate cycles) or malonyl-CoA reductase (3-hydroxypropionate bi-cycle). A recently described reductive glycine pathway (Sánchez-Andrea et al. 2020, Song et al. 2020) is incomplete in the genome of strain F-1T. However, the genome of strain F-1T contains all enzymes of the TCA cycle, including citrate synthase (WP_155934624), aconitase (WP_155932278), isocitrate dehydrogenase (WP_155932020), succinyl-CoA synthetase (WP_155935188), fumarase (WP_155932566, WP_155932568), succinate dehydrogenase/fumarate reductase (WP_155932561 - WP_155932564), fumarate hydratase (WP_155932566, WP_155932568) and malate dehydrogenase (WP_155934516). Therefore, it can be hypothesized that in strain F-1T CO2 fixation can occur via “reversed oxidative TCA cycle” (Mall et al. 2018, Nunoura et al. 2018).
Strain F-1T is capable of utilizing molecular hydrogen as an energy source. The genome of strain F-1T encodes two subunits of periplasmic HynAB hydrogenase (WP_155931905, WP_155931907) which has a bifunctional activity and is required either for the uptake of molecular hydrogen or for H2 release during fermentation of organic substances. The [Ni-Fe] hydrogenase maturation system HypABCDEF in the genome of strain F-1T is encoded in WP_155931624, WP_155931626, WP_155931901, WP_155935612, WP_196772909 and WP_155931785.
The genome of strain F-1T contains two copies of gene of arsenate reductase, arsC (WP_155934701, 155934643). The presence of arsC is the common feature through the genus Pseudodesulfovibrio. However, there are` no published data on the ability of the members of Pseudodesulfovibrio to grow with arsenate as an electron acceptor.
In contrast to the canonical fumarate reductase/succinate dehydrogenase consisting of four subunits (frdABCD), the genome of strain F-1T contains genes only for three subunits (frdABC) (WP_155932561, WP_155932563, WP_155932564), as it was previously reported for Desulfovibrio vulgaris and Desulfovibrio desulfuricans (Zaunmüller et al., 2006).
The genome of strain F-1T contains genes– of the nitrogenase complex nifDHK, which is required for nitrogen fixation. Two components the iron protein (WP_155934608) and the molybdenum-iron protein (WP_155934611, WP_155934612), as well as two genes of P-II family nitrogen regulators (WP_155934609, WP_155934610) are present