Introduction

As the third most abundant bacterial group in the seawater system, phylum Bacteroidetes plays a vital role in diverse oceanic biogeochemical processes [1]. It has been reported that phylum Bacteroidetes could mediate the degradation of HMW compounds especially in the respect of algal organic matter [2, 3]. Many heterotrophic microorganisms such as the SAR11 clade and marine Gammaproteobacteria grow partly due to phylum Bacteroidetes -derived organic products [4, 5]. Thus, phylum Bacteroidetes groups may play crucial roles in the nutrient utilization and cycling in the seawater ecosystem.

The family Cytophagaceae , currently comprising 31 genera, is one of the largest groups in the phylum Bacteroidetes [6]. The species in the family Cytophagaceae have been isolated from various habitats including freshwater river [7], seawater [8], permafrost soil [9] and even polar glacial till [10]. The genus Arcticibacterium , belonging to the family Cytophagaceae , accommodates only one recognized species: A. luteifluviistationis SM1504T (=KCTC 42716T=CCTCC AB 2015348T) [11]. Strain SM1504T was isolated from surface seawater of King’s Fjord, Arctic. However, to date, no genomes of the genus Arcticibacterium have been reported, their genomic compositions and metabolic pathways are still lacking. In the study, we reported the first genome sequence of the genus Arcticibacterium to better understand its survival strategy and ecological niche in the Arctic seawater.

Organism information

Classification and features

As the type strain of A. luteifluviistationis in the family Cytophagaceae , strain SM1504T is a Gram-negative, aerobic, non-motile and rod bacterium (Fig. 1). The yellow-pigmented colony was found after incubation at 20 °C for 2 days on a TYS agar plate. The strain could utilize glycerol, D-xylose, D-glucose, D-fructose, dulcitol, inositol D-mannitol, D-sorbitol, N-acetylglucosamine, arbutin, aesculin, cellobiose, maltose, sucrose, trehalose, starch, turanose and potassium gluconate for energy and growth, which were summarized in Table 1. Then it hydrolyzed aesculin, gelatin, tyrosine, Tween 20, 40 and 60 but did not hydrolyze DNA, agar, casein, elastin, lecithin, starch, Tween 80. In addition, various enzymes such as alkaline phosphatase, esterase (C4), esterase lipase (C8), leucine arylamidase, valine arylamidase, cystine arylamidase, trypsin and glucosidase were produced for degrading organic matter [11]. The phylogenetic placement of strain SM1504T (based on complete 16S rRNA gene sequence) through neighbor-joining phylogenetic tree was identified (Fig. 2). It formed a distinct phylogenetic branch within the family Cytophagaceae and closely relatives were species of the genera Lacihabitans , Emticicia , Fluviimonas and Leadbetterella with low sequence similarities between 88.9 and 91.6%.

Fig. 1
figure 1

Transmission electron micrographs of Arcticibacterium luteifluviistationis SM1504T cultured on TYS broth medium. Scale bar, 0.5 μm

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Table 1 Classification and general features of Arcticibacterium luteifluviistationis SM1504T [12]
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Fig. 2
figure 2

Neighbor-joining phylogenetic tree based on 16S rRNA gene sequences, showing the relationships of Arcticibacterium luteifluviistationis SM1504T and its taxonomic neighbors. Rhodothermus marinus DSM 4252T was used as as the outgroup. Bootstrap values (> 70%) based on 1000 replicates are shown at nodes. Bar, 0.02 substitutions per nucleotide position

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Genome sequencing information

Genome project history

Isolated from an extreme Arctic environment, A. luteifluviistationis SM1504T was selected for genome sequencing to elucidate the special abilities of adapting to diverse extreme stresses. We have accomplished the genome sequencing of strain SM1504T as reported in this paper. The complete genome data has been deposited in the GenBank database under the accession number CP029480.1. The project information and its association with MIGS are provided in Table 2 [12].

Table 2 Project information
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Growth conditions and genomic DNA preparation

A. luteifluviistationis SM1504T was cultivated in TYS broth at 20 °C. After cultivation for two days, genomic DNA for sequencing was extracted by using a commercial bacterial DNA isolation kit (OMEGA).

Genome sequencing and assembly

Genome sequencing was performed on both the Illumina Hiseq and the PacBio RS sequencing platforms. 400-bp Illumina paired-end libraries and 20-kb PacBio libraries were constructed and sequenced yielding 315 × and 45 × average coverages, respectively (Table 2). About 1.69 Gb and 243 Mb data from the Illumina and PacBio sequencing were assembled using SOAPdenovo [13, 14] and HGAP [15]. The final assembly resulted in one scaffold.

Genome annotation

Coding gene sequences were predicted and annotated through Prodigal v2.6.3 [16] and RAST v2.0 [17]. Functional categorization and carbohydrate-active enzymes CAZy of the predicted genes were annotated against EggNOG and CAZy databases, respectively. Then rRNAs and tRNAs were predicted by RNAmmer v1.2 [18] and tRNAscan-SE v1.3.1 [19]. In addition, the CARD analyses were performed to find resistance genes. Genomic islands and secondary metabolite biosynthesis were predicted through IslandViewer 4 [20] and antiSMASH [21].

Genome properties

The total size of the genome of A. luteifluviistationis SM1504T is 5,379,839 bp with an average GC content of 37.20% (Fig. 3). Total 4595 protein-coding genes (CDSs) were identified, which occupied 89.73% of the genome. Therein, 3045 CDSs were annotated with putative functions and 1550 CDSs matched hypothetical proteins (Table 3). Then 4 rRNAs and 36 tRNAs were found in the genome. CRISPR repeat, transmembrane helice, signal peptide and Pfam protein family predictions were done. In addition, distribution of genes into COG functional categories was shown in Table 4.

Fig. 3
figure 3

Circular map of the Arcticibacterium luteifluviistationis SM1504T genome. From the outside to the center: CDSs on forward strand (colored by COG categories), CDSs on reverse strand (colored by COG categories), RNA genes (tRNAs and rRNAs), G + C content and GC skew

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Table 3 Genome statistics
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Table 4 Number of genes associated with general COG functional categories
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Insights from the genome sequence

Adaption to diverse stresses

Strain SM1504T genome owned two putative gene clusters for secondary metabolite biosynthesis. The cluster 1 belonged to terpene type - the largest group of natural products [22], matching the carotenoid biosynthesis. The cluster 2, affiliated to arylpolyene type, was predicted to produce flexirubin. Furthermore, we found that the yellow-pigmented strain SM1504T harbors a complete set of genes required for zeaxanthin biosynthesis (e.g., isopentenyl-diphosphate delta-isomerase, phytoene synthase, phytoene dehydrogenase, lycopene cyclase and beta-carotene hydroxylase), which was commonly detected in other species of the phylum Bacteroidetes [23, 24]. The pigment maybe help the strain to obtain energy and for cold adaption and ultraviolet light protection in the Arctic environments [25].

A total of 150 resistance genes were found to encode 24 kinds of antibiotics (such as gentamicin, kanamycin, tetracycline and streptomycin), which was consistent with the experimental antibiotic susceptibility results [11]. The genes encoding heat shock proteins dnaK and cold shock protein cspA were detected in the genome. In line with this, SM1504T had a wider growth temperature ranges (4–30 °C) [11]. Besides, the genome harbored several genes coding for catalase and superoxide dismutase to assist the strain at cellular and molecular levels in dealing harsh radiation in the Arctic. Dozens of genes related to osmotic stress (such as choline and betaine uptake and betaine biosynthesis) and carbon starvation responses were discovered in the A. luteifluviistationis genome, which would endow cells with tolerance to hyperhaline and oligotrophic environments.

As another feature, a 245-kb genomic island coding for 208 genes was predicted. Therein, 9 genes encoded proteins related to glucide biosynthesis, such aslipopolysaccharide core biosynthesis glycosyltransferase (lpsD), UDP-glucose dehydrogenase and capsular polysaccharide synthesis enzyme (Cap8C). In addition, the presence of transposases, integrases and mobile element proteins indicated that gene transfer has occurred in the A. luteifluviistationis SM1504T genome [26]. Also, phage tail fiber proteins were predicted, which was in line with the analysis by PHAST [27] that a 15-kb incomplete prophage region could encode phage tail fiber proteins in the genome.

Degradation and utilization of carbohydrates

Totally, 3319 (71.61%) genes could be assigned a COG function, of which the wall/membrane/envelope biogenesis (5.89%), carbohydrate transport and metabolism (4.94%) and inorganic ion transport and metabolism (4.83%) were enriched (Table 4). The high percentage of proteins related to carbohydrate transport and metabolism suggested that the strain SM1504T could use various carbohydrates. On the other hand, the analyses from dbCAN showed that the strain SM1504T possessed 341 genes which encoded carbohydrate metabolism enzymes, including 69 carbohydrate esterases (11 families), 125 glycoside hydrolases (46 families), 62 glycosyltransferases (22 families), 17 polysaccharide lyases (6 families), 12 auxiliary activities (3 families) and 56 carbohydrate-binding modules (15 families). Therein, a variety of enzymes are related to the degradation of macromolecular polysaccharides (e.g., xylanase, chitinase, mannanase, alpha amylase, endoglucanase, glucoamylase and alginate lyase) derived from marine macroalgae and phytoplankton. Those polysaccharases could hydrolyze a variety of macromolecular polysaccharides into small molecules that can be absorbed and metabolized by strain SM1504T and other microorganisms in the seawater [4, 5].

Conclusions

The genomic analyses showed that the strain SM1504T could adapt to extreme Arctic seawater environments, such as high solar radiation, cold temperature and high salinity. Besides, it may act as a vital macromolecular polysaccharide decomposer and would play an important role in organic carbon cycling in the Arctic seawater ecosystem.