Introduction

The genus Sulfitobacter was established by Sorokin (1995), and belongs to Roseobacter group which is the most widely distributed bacteria in the marine environment, and harbors enormous ecological significance during oceanic elemental cycles and interactions with marine eukaryotes (Buchan 2005; Wagner-Döbler and Biebl 2006). Sulfitobacter pontiacus isolated from the Black Sea is the type species of this genus (Sorokin 1995). At the time of writing, the genus Sulfitobacter includes 23 validated species (https://lpsn.dsmz.de/genus/ sulfitobacter) that were all isolated from marine environments (Wang et al 2021). Marine phycosphere harbors dynamic and cross-kingdom algae-bacteria interactions (ABI) which play crucial roles in aquatic ecosystems (Seymour et al. 2017). Within the phycosphere interface, the exopolysaccharides (EPS) produced by algae-associated bacterial consortia serves as one essential intermediary component during those dynamic interactions (Amin et al. 2012; Cho et al. 2004; Yang et al. 2021; Zhang et al. 2020). Previously, we performed the Phycosphere Microbiome Project (PMP) to convey microbial structures of phycosphere microbiota (PM) of marine harmful algal blooms (HAB) dinoflagellates (Duan et al. 2020; Yang et al. 2018, 2020a, b; Yang et al. 2020a, b, c; Zhang et al. 2015a, b, 2020; Zhou et al. 2021). We found Sulfitobacter spp. was one dominant genus distributed in diverse PM of six toxic HAB Alexandrium spp. (Fig. S1) (Zhang et al. 2015a, b). After isolating the cultivable PM, a new lightly yellowish and versatile bacterium designated strain AM1-D1T was isolated from Alexandrium minutum amtk4 that produces high levels of paralytic shellfish poisoning toxins (PSTs) (Chou et al. 2004; Zhang et al. 2015b; Yang et al. 2020a, b, c). We found that strain AM1-D1T produces active bioflocculanting EPS (Duan et al. 2020; Mu et al. 2019), and also demonstrates obvious microalgae growth-promoting (MGP) ability. To specify its phylogenetic position, in this study, we proposed that strain AM1-D1T represents a novel species of the genus Sulfitobacter based on polyphasic characterizations. Additionally, whole-genomic mining of strain AM1-D1T gained insight into the application of this new isolate for furthering exploration of bacterial PST biosynthesis and detailed mechanisms underlying those dynamic algae-bacteria interactions (Amin et al. 2012; Seymour et al. 2017).

Materials and methods

Bacterial strains and culture conditions

Strain AM1-D1T was isolated from the cell culture of Alexandrium minutum amtk4 according to our protocols described previously (Yang et al. 2018a; Zhang et al. 2015a, 2020; Zhou et al. 2021). The isolation medium of marine agar (MA, Difco, BD) 2216 was supplemented with algal culture extract of LZT09 at approximate 0.05 mg/L (Yang et al. 2021). The strain was routinely cultivated on MA at 30 °C, or maintained as a glycerol suspension (20%, v/v) and stored at − 80 °C for long-term preservation. Five reference type strains, including S. noctilucae NB-68T (= KCTC 32122T = JCM 18833T = DSM 100978T), S. noctilucicola NB-77T (= KCTC 32123T = JCM 18834T) and S. geojensis MM-124T (= KCTC 32124T = JCM 18835T) purchased from Korean Collection for Type Cultures (KCTC, Republic of Korea), Roseobacter denitrificans OCh 114T (= ATCC 33942T = DSM 7001T = JCM 21267T) obtained from the Japan Collection of Microorganisms (JCM, Japan), and S. pacificus SCM2-10T (= LMG 27113T = NBRC 109915T) obtained from the NITE Biological Resource Center (NBRC) were used for the comparative study of the taxonomic position of strain AM1-D1T.

16S rRNA phylogenetic analysis

PCR amplification of bacterial 16S rRNA gene was performed using an universal bacterial primers of 27F/1492R according the standard procedure (Yang et al. 2021; Zhang et al. 2021), and sequenced at MajorBio (Shanghai, China). The identifications of phylogenetic neighbor and the calculations of gene similarity values using the 16S rRNA gene sequences obtained by both PCR amplification, and extracted 16S rRNA region from the bacterial genomes were achieved using online EzTaxon server (http://www.ezbiocloud.net). Reference sequences of type strains were downloaded from the NCBI database (www.ncbi.nlm.nih.gov). Phylogenetic analysis was performed using MEGA 7.0 after multiple alignment of the data via ClustalW. Phylogenetic distances were calculated according to neighbour-joining (NJ), maximum-parsimony (MP) and maximum-likelihood (ML) methods. In each case, bootstrap values were calculated based on 1000 replications to evaluate the phylogenetic tree topology.

Phenotypic, physiological and biochemical characterizations

Cell morphology was observed by light microscope (Olympus CX21) and transmission electron microscopy (TEM) JEM-1200; JEOL, Tokyo, Japan). The morphology of cell colonies were observed after 24 h incubation on MA plates at 30 °C. Motility was tested microscopically under the phase-contrast mode by the hanging drop technique. Growth range and optimum at different temperatures (4, 10, 15, 20, 25, 30, 37, 40, 45 and 50 °C), and pH values (4.0–11.0, at 0.5 unit interval) was investigated in marine broth (MB) incubated at 30 °C for 2 days. Growth at various NaCl concentrations (0–11.0%, w/v, interval of 0.5%) was investigated in MB at 30 °C for 2 days. The following sterile buffer system: pH 4.0–5.0, 0.1 M citric acid/0.1 M sodium citrate; pH 6.0–8.0, 0.1 M KH2PO4/0.1 M NaOH; pH 9.0–10.0, 0.1 M NaHCO3/0.1 M Na2CO3; pH 11.0, 0.05 M Na2HPO4/0.1 M NaOH, was used to adjust the desired pH of the test media. Growth under anaerobic condition was determined after incubation in an anaerobic chamber (Bactron EZ-2; Shellab) grown on MA plate and cultivated at 30 °C for 2 week. Growth was determined by monitoring OD600nm using an Agilent spectrophotometer. Oxidase activity was determined using a 1% (v/v) solution of tetramethyl-p-phenylenediamine. Catalase activity was detected by assessing production of bubbles after addition of a drop of 3% (w/v) H2O2. Hydrolysis of aesculin, casein, DNA, gelatin, starch and Tween 80, Voges–Proskauer and methyl red tests, production of indole, phenylalanine deaminase, phosphatase, nitrate and nitrite reduction and Simmon’s citrate were determined as described by Smibert and Krieg (1994). Activities of extracellular enzymes were determined by using API 20E, API 20NE and API ZYM strips (bioMérieux, Marcyl’E´toile, France) according to the manufacturer’s instructions.

Chemotaxonomic characteristics

Strain AM1-D1T was incubated in MB in order to extract the fatty acids. The exponential-phase of the cell culture was harvested, and the fatty acids were analyzed as described by Sasser (1990). Identification and quantification of cellular fatty acids were performed using the Sherlock Microbial Identification System (MIDI) (Sherlock version 6.1; MIDI database TSBA6.0). Polar lipids of were extracted using a chloroform/methanol system, separated by two-dimensional silica gel TLC, and analyzed according to the procedures described by Tindall et al. (1987). Respiratory quinones were extracted and determined by HPLC method as described previously (Hiraishi et al. 1996).

Phylogenomic calculations and functional genes analysis

The genome sequences of type strains of Sulfitobacter were available until December 2020, and downloaded from GenBank (https://www.ncbi.nlm.nih.gov/genome). The up-to-date bacterial core gene set (UBCG) was used to construct a phylogenetic tree using the genomes of strain AM1-D1T and other type strains of Sulfitobacter. DNA G+C content of strain AM1-D1T was calculated based on its whole-genome sequence. Three measures of similarities based on the average nucleotide identity (ANI), average amino acid identity (AAI) and digital DNA-DNA hybridization (dDDH) values were calculated using online OrthoANI tool and GGDC tool (http://ggdc.dsmz.de/distcalc2.php), respectively, using the default parameters. Additionally, the identification of essential functional genes and clusters related to quorum sensing (lux), dimethylsulfoniopropionate (DMSP) (dmd), photosynthetic capacity (puf, bch), carbon monoxide oxidation (cox), tau gene, sulfur oxidation (sox), Vitamin B12 (cob), carotenoids (crt), and nitrogen metabolism (napA, nasA, narB, narG, norB, nirS) in the genomes of strain AM1-D1T and its close relatives were performed using BLAST based on InterPro (https://www.ebi.ac.uk/interpro) database and IMG database (https://img.jgi.doe.gov).

Core-genome alignment and phylogenetic analysis

All predicted protein-coding genes and amino acids annotated from available 16 genome of type strains of the genus Sulfitobacter were selected and compared using an all-versus-all BLAST search. This analysis identified shared reciprocal best matches (defined as > 70% nucleotide identity or > 40% amino acid identity) in all pair-wise genome comparisons (core orthologues, CO) of 17 type strains of Sulfitobacter including strain AM1-D1T. The core orthologous genes were individually aligned using MUSCLE (Tatusov et al. 2000). The resulting nucleotide alignments were concatenated to create a core-genome alignment using IQ-TREE v1.6.12 (http://www.iqtree.org). The phylogenetic tree was reconstructed by neighbour-joining method with MEGA 7 (https://www.megasoftware.net/home). Additionally, the core orthologous and unique proteins were used to construct a Venn diagram using the VennPainter tool (Lin et al. 2016).

Bacterial EPS bioflocculanting and MGP bioactivity assays

Extraction of bacterial EPS produced by strain AM1-D1T and bioflocculating activity evaluation were performed according to our procedures reported previously (Duan et al. 2020; Mu et al 2019; Zhang et al 2021). The prepared EPS was dissolved in distilled water for furthering bioflocculation activity (BFC) assay (Mu et al. 2019). Briefly, the measures using kaolin clay suspension flocculation (KCSF) assay calculated as flocculation rate (FR) were used and performed at a 96-well microplate mode with at least triplicates. Microalgae growth-promoting (MGP) activity of strain AM1-D1T toward A. minutum amtk4 in a co-culture system was performed as reported previously (Gonzalez and Bashan 2000).

Results and discussion

Morphological, physiological and biochemical characteristics

Strain AM1-D1T was observed to form lightly yellowish colonies when grown on MA at 30 °C for 2 days. Cells of strain AM1-D1T were Gram-negative, rod-shaped with polyhydroxyalkanoate (PHA) granules inside (Fig. 1). It’s motile, aerobic and weak positive for anaerobic growth. TEM observation showed that the cells of strain AM1-D1T were approximately 0.3–0.4 μm wide and 0.7–2.1 μm long (Fig. S1), which were smaller than the related type strains. Growth occurred at 15–37 °C (optimum, 30 °C) at pH 5–10 (optimum, pH 7.0) in the presence of 1–4% (w/v) NaCl (optimum, 3%). Oxidase and catalase activities were both positive. Other phenotypic features distinguishing strain AM1-D1T from its close relatives are summarized in the species description (Table 1). Compared with its five close relatives, strain AM1-D1T exhibited a wider temperature grow range, but a narrower NaCl grow range. In API 20NE tests, only two strains, S. pacificus SCM2-10T and S. geojensis MM-124T were positive for nitrate reduction, but strain AM1-D1T and other three type strains were all negative for nitrate reduction. In API ZYM tests, strain AM1-D1T and S. pacificus SCM2-10T were positive for acid phosphatase, naphthol-AS-BI-phosphohydrolase, but positive for lipase (C14).

Fig. 1
figure 1

Morphological images of strain AM1-D1T cells grown in MB medium at 30 °C for 2 days, observed by optical microscopy after Gram-staining (a), and Sudan Black staining showing the PHA granules (b), and transmission electron microscopy with black arrow indicating the PHA granules inside (c)

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Table 1 Differential characteristics of strain AM1-D1T and its phylogenetically close relatives
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It’s attractively noted that for the bacterial isolation sources, total 8 type strains of genus Sulfitobacter, including AM1-D1T, S. delicatus and S. dubius (Ivanova et al. 2004), S. faviae (Kumari et al. 2016), S. pacificus (Fukui et al. 2015), S. porphyrae (Fukui et al. 2014), S. pseudonitzschiae (Hong et al. 2015), S. undariae (Park et al. 2015) and S. algicola (Wang et al 2021) were isolated from diverse marine dinoflagellates and planktons. This interesting finding may indicate those species might involve in potential cross-kingdom associations with their hosts (Amin et al. 2012).

16S rRNA phylogenetic analysis

The extracted 16S rRNA gene sequences of strains AM1-D1T and its close relatives were subjected to phylogenetic analysis. The similarity identification using EzBioCloud alignment indicated that strain AM1-D1T shared 16S rRNA gene similarities of 97.9% with S. noctilucae NB-68T, and less than 97.8% with all other Sulfitobacter species, and below 97.5% with all type strains of the genus Roseobacter (Shiba 1991). Phylogenetic tree constructed based on NJ method demonstrated that AM1-D1T form a monophyletic group compared with other species of the genus Sulfitobacter, which was outside the group formed by the type strains of S. noctilucae, S. noctilucicola, S. pacificus, S. geojensis and S. sabulilitoris (Fig. 2). The phylogenetic relationship was also supported by the ML (Fig. S2) and MP trees (Fig. S3).

Fig. 2
figure 2

Neighbor-joining (NJ) phylogenetic tree of strain AM1-D1T and type strains of the genus Sulfitobacter and related taxa within the family Rhoae based on 16S rRNA gene sequences. Bootstrap values (> 50%) are expressed as percentages of 1000 replicates. Stappia stellulata DSM 5886T was used as an outgroup. Bar, 0.01 substitutions per nucleotide position

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Phylogenomic comparison of Sulfitobacter species genomes

The phylogenomic comparisons based on ANI, AAI and dDDH values between strain AM1-D1T and other type strains of the genus Sulfitobacter with available genomes are showed in Fig. S4. The ANI, AAI and dDDH values between strain AM1-D1T and S. noctilucae NB-68T were 77.2, 74.7 and 18.6%, respectively. For S. sabulilitoris HSMS-29T, the ANI, AAI and dDDH values were 78.0, 71.2 and 19.3%, respectively. For Roseobacter denitrificans OCh 114T, those three values were 71.8, 68.4 and 14.2%, respectively. All values are far lower than the thresholds (95–96% for ANI, 97% for AAI and 70% for dDDH) generally accepted for new species delineation (Meier-Kolthoff et al. 2013; Richter and Rosselló-Móra 2009). Additionally, based on the constructed phylogenomic tree (Fig. 3), strain AM1-D1T formed a separate branch clustered with S. sabulilitoris HSMS-29T which was isolated from a marine sand sample collected from the Yellow Sea, South Korea (Park et al. 2019). In addition, the comparison of genomic characteristics including the genomic sizes, G+C contents and gene numbers also can clearly distinguish strain AM1-D1T from its closely related species within the genus Sulfitobacter, and also those phylogenetic neighbors within other genus. These results strongly support the species delineation of strain AM1-D1T from other type strains of the genus Sulfitobacter, and hence strain AM1-D1T is proposed as a novel species of Sulfitobacter.

Fig. 3
figure 3

Phylogenetic tree of strain AM1-D1T and other type strains of the genus Sulfitobacter constructed by up-to-date bacterial core gene (UBCG) set, and comparison of the genomic characteristics including genomic size, DNA G+C content (%), numbers of predicted protein-coding genes, rRNA, tRNA and pseudogenes. Stappia stellulata DSM 5886T is selected as an outgroup. Gene support index (the number of individual gene trees presented the same node in total genes used) (left) and bootstrap values (right) are given at nodes. Bar, 0.1 nt substitution rate (Knuc) units

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Chemotaxonomic analysis

As shown in Table 2, the chemotaxonomic analysis of the cellular fatty acids showed that the predominant fatty acid profiles (> 10%) of strain AM1-D1T were C18:1 ω7c (42.1%), C19:0 cyclo ω8c (23.5%) and C16:0 (15.2%). Additionally, two other fatty acid compositions with lower portions including C18:1 ω7c 11-methyl (7.3%) and C18:0 (3.7%) were also found in strain AM1-D1T. The result was in agreement with those of the other members of Sulfitobacter. However, strain AM1-D1T also possessed high amount of C19:0 cyclo ω8c (23.5%) and minor amount of C12:1 3-OH (2.7%). These unique components can be used to clearly distinguish strain AM1-D1T from its closely related species. The isoprenoid quinone of strain AM1-D1T was ubiquinone-10 (Q-10). The main polar lipids of strain AM1-D1T were determined as diphosphatidylglycerol (DPG), phosphatidylethanolamine (PE), phosphatidylglycerol (PG), phosphatidylcholine (PC), one unidentified phospholipid (PL) and one unidentified lipid (L). Unlike its closet relative, Sulfitobacter noctilucae NB-68T had DPG, PG, PE and minor amounts of three unidentified lipids (L1-3) as the major polar lipids profile (Fig. S5). Both characterizations were consistent with the common chemotaxonomic profile of the other members of Sulfitobacter (Sorokin 1995; Fukui et al. 2014, 2015; Park et al. 2015, 2019; Wang et al 2021).

Table 2 Cellular fatty acid compositions of strain AM1-D1T and five reference strains of the genus Sulfitobacter
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Genome comparison of strain AM1-D1 T

Circular representation of the genome of strain AM1-D1T including one circular chromosome and five circular plasmids is shown in Fig. 4. The size of the whole genome of strain AM1-D1T is 4.69 Mb with one chromosome of 3.84 Mb with a DNA G+C content of 64.9%. And the lengths of five plasmids were 185, 95, 15, 205 and 348 Kb with their corresponding G+C contents of 65.6, 60.3, 58.7, 63.7 and 61.6%, respectively (Table 1). Based on the complete genome sequence, the whole genomic DNA G+C content of strain AM1-D1T was calculated as 64.6%, which was higher than its five closely related type strains. Transmission electron microscopy showed that the cells of strain AM1-D1T were approximately 0.3–0.4 μm wide and 0.7–2.1 μm long (Fig. S1), which were smaller than the related strains.

Fig. 4
figure 4figure 4

Circular representation of the genome of strain AM1-D1T including one circular chromosome and five circular plasmids. Typical bacterial metabolic gene clusters (cob, crt, cys, dmd, sox and tau) found in the genome were also shown. The scale of the genome size was shown in the outer line. From the outer to inner circle: the two outer circles show the predicted protein-coding sequences (CDs) on the plus and minus strand. Different colors in these two circles show genes with different COG categories; the third circle shows rRNA and tRNA (red); the fourth and inner fifth circles show G+C content and G+C skew, respectively

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According to NCBI database records, total 17 genomes of type strains of Sulfitobacter were chosen to conduct the core- and pan-genome analysis. The constructed two functional curves showed that the total gene families and core gene families reached to maximal and minimal values of 16,295 and 1530, respectively (Fig. 5). The gene accumulation curve exhibited an increasing tendency with the increasing number of the pan-genome genes. But the core-genome curve showed a descending trend (Fig. 5). Therefore, the pan-genome was considered in an open state since the pan-genome trend curve did not reach a plateau with the increasing numbers of bacterial strains. In contrast, the core genome curve was thought to be conserved. As shown in Fig. 5, the gene occurrence plot showed that 1530 core genes were shared by 17 type Sulfitobacter genomes, accounting for 33.2% to 45.8% of the genome repertoire, respectively. Additionally, about 7.7% to 25.5% of unique genes were only found in S. dubius DSM 16472T and S. pseudonitzschiae DSM 26824T, respectively, indicating a species-specific profile within Sulfitobacter members.

Fig. 5
figure 5

Venn diagram of core- and pan-genome analysis of members of Sulfitobacter and the constructed function curves

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Comparative analysis of function genes

The distribution of functional categories of core and unique genes was compared and the result is shown in Fig. 6. It indicated that the most abundant functions in core genes of Sulfitobacter spp. were associated with amino acid transport and metabolism, general function prediction only, translation, ribosomal structure and biogenesis, and energy production and conversion, which were closely related to necessary nutrients obtaining from varied environments and maintaining a basic lifestyle. Function unknown accounted for a large proportion, which indicated current lacking of in-depth revealing of Sulfitobacter genomes. For KEGG assignment, the five most abundant core genes were distributed into KEGG categories about amino acid metabolism, carbohydrate metabolism, energy metabolism, nucleotide metabolism and overview, suggesting adaptive evolution exist within member of the genus Sulfitobacter. However, genes related to cellular, development, digestive system, immune diseases, immune system and substance dependence were only assigned in core genes. Functional analysis revealed that the abundances and categories of core and unique genes assigned into KEGG categories were diverse in member of the genus Sulfitobacter. Additionally, series of essential functional gene clusters responsible for vitamin B12 (cob), photosynthesis pigments carotenoids (crt), cysteine biosynthesis (cys), sulfur oxidation (sox), and utilization of two dissolved organic sulfur (DOS) exometabolites from marine dinoflagellates, taurine (tau) and DMSP (dmd) (Landa et al. 2019; Landry et al. 2018), were found in the genome of strain AM1-D1T (Fig. 3), which are pivotal components governing the dynamic algae-bacteria interactions (Amin et al. 2012).

Fig. 6
figure 6

Comparison of the core genes, unique genes, KEGG and COG characteristics of the genomes between strain AM1-D1T and the other 16 type strains of Sulfitobacter with available genomes

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Bacterial EPS bioflocculanting and MGP potential

Based on our bacterial bioflocculanting assay, the extracted EPS produced by strain AM1-D1T were subjected to BFC bioactivity screening assay (Mu et al. 2019). The comparison of BFC of series of EPS concentrations on KCSF was performed. Pane A in Fig. 7 shows the concentration-dependent manner of BBF bioactivity of EPS produced by strain AM1-D1T. It can be seen that the BFC reached the maximum of 92.5 ± 6.2% at 0.40 g L−1 of bacterial EPS produced by strain AM1-D1T, which exhibiting higher bioflocculanting deficiency compared with the strains we previously reported (Duan et al. 2020; Yang et al. 2020a, b, c; Yang et al. 2021; Zhang et al 2021). For MGP assay, strain AM1-D1T demonstrated obvious microalgae growth-promoting activity when co-cultured with host A. minutum amtk4 (pane B in Fig. 7). Those results clearly indicate that strain AM1-D1T is a new MGPB isolated from marine phycosphere, and produces novel bioactive bioflocculating EPS with potential environmental and biotechnological implications (Mu et al 2019; Duan et al. 2020; Yang et al 2021; Zhang et al 2021).

Fig. 7
figure 7

The bioactivity evaluations by the bioflocculanting assay of bacterial exopolysaccharides (EPS) produced by strain AM1-D1T (a), and microalgae growth-promoting potential assay (b)

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Taxonomic conclusion

On the basis of phenotypic, phylogenetic and genomic analyses, strain AM1-D1T represents a novel species of the genus Sulfitobacter. Therefore, we formally propose Sulfitobacter alexandrii sp. nov., and AM1-D1T as the type strain. Strain AM1-D1T appears to be a versatile bacterium with potential for environmental and biotechnological applications including microalgae growth-promoting potential and the production of natural EPS biosurfactans.

Description of Sulfitobacter alexandrii sp. nov.

Sulfitobacter alexandrii (a.le.xan'dri.i. N.L. gen. n. alexandrii of the dinoflagellate Alexandrium minutum, the isolation source of the type strain).

Cells are Gram-stain-negative, aerobic, non-sporulating, non-motile, rod-shaped with approximately 0.3–0.4 μm wide and 0.7–2.1 μm long with PHA granules inside. Colonies are circular, convex, smooth, lightly yellowish grown on MA for 2 days at 30 °C. Growth occurs at 15–37 °C, optimum at 30 °C. Grows at 1.0–4.0% (w/v) NaCl with an optimum at 3.0% (w/v) NaCl. Tolerates a range of pH between 5.0 and 10.0 with the optimum at 7.0. Aesculin and Tween 20 are hydrolysed. Gelatin, Tween 40 and 80 are not hydrolysed. Nitrate is reduced to nitrite. Indole is not produced from tryptophan and glucose fermentation does not occur. Oxidase and catalase are both positive. Positive for urease activities, alkaline phosphatase, esterase (C4), esterase lipase (C8), leucine arylamidase, valine arylamidase, acid phosphatase and naphthol-AS-BI-phosphohydrolase, and negative for lipase (C14), cystine arylamidase, trypsin, α-galactosidase, β-galactosidase, β-glucuronidase, α-glucosidase, β-glucosidase, N-acetyl-β-glucosaminidase, α-mannosidase and α-fucosidase. The predominant cellular fatty acids are C18:1 ω7c, C19:0 cyclo ω8c and C16:0. The predominant isoprenoid quinone is Q-10. The main polar lipids are phosphatidylglycerol, phosphatidylethanolamine and diphosphatidylglycerol. The genome has 3.84-Mb circular chromosome and five circular plasmids of the lengths of 185, 95, 15, 205 and 348 Kb, respectively, with the genomic DNA G+C content of 64.6 mol%.

The type strain is AM1-D1T (= CCTCC AB 201696T = KCTC 52626T), which was isolated from phycosphere microbiota of highly-toxic and HAB dinoflagellate Alexandrium minutum amtk4 which was collected from a Milkfish culture pond at TungKang Taiwan during an algal bloom occurred in July, 1994, and then routinely cultivated in the laboratory. The DDBJ/EMBL/GenBank accession numbers for 16S rRNA gene sequences of the strain AM1-D1T is MH197128. The complete genome sequences of strain AM1-D1T have been deposited at DDBJ/EMBL/GenBank under the accession numbers CP018076, CP018077, CP018078, CP018079, CP018080 and CP018081 for its circular chromosome and five circular plasmids, respectively.