Fragilaria thornata L. Tan & B.H. Kim, 2020

Tan, Ling, Wang, Pengbin, Cho, In-Hwan, Hwang, Eun-A, Lee, Hyuk & Kim, Baik-Ho, 2020, Morphology and phylogenetic position of three new raphid diatoms (Bacillariophyceae) from Hangang River, South Korea, Phytotaxa 442 (3), pp. 153-182 : 157-163

Fragilaria thornata L. Tan & B.H. Kim , sp. nov. ( Figs 2–14 , LM; Figs 15–23 , SEM)

Description: —Valve narrowly lanceolate with subcapitate apices ( Figs 3–18 ). Frustules rectangular in girdle view ( Fig. 2 ). Valve length 34.3–38 µm, width 3.6–4.2 µm, 11–12 striae in 10 µm, 4–5 areolae in 1 µm. Striae uniseriate, alternate, parallel in the central area to slightly radiate towards the apices ( Figs 15, 17 ); striae composed of 1–8 areolae on valve face and 1–4 areolae on mantle ( Figs 17, 21, 22 ). Outer areolae round, closed with hemispherical silica velum (HSV) ( Fig. 23 ); inner areolae round to elongated ( Figs 16, 18 ). Axial area narrow but gradually widened near the centre, linear ( Figs 15–18 ). Well-defined hyaline area present at the centre of the valve with a slight central swelling ( Figs 15, 16, 19, 20 ). Spines present, located at junction between valve face and mantle, present along entirety of valve mantle, 1–3 spines on each apex; spines wide at base and tapered towards tips, mainly located on or near the striae, spine shape becomes irregular near the central area of the valve( Figs 15, 17, 19, 21–23 ). Connection between spine and valve with fold-like lines ( Fig. 21 ). Single rimoportula with slit-like openings present at one of the valve ends, aligned with the first stria at the valve face apex ( Figs 15, 17 ). Each valve with two apical pore fields (AFP) composed of simple pores arranged in 5–6 rows parallel to the apical axis ( Fig. 17 ). Siliceous plaques (SP) on the valve mantle edge ( Figs 21, 22 ). Valve with open band and a single row of small puncta (often obscured) ( Fig. 22 ). Colonies mostly loosely aggregated.

Type: — Republic of Korea. Gyeonggi-do Province: Heuk Stream, 37°30′12.1′′N, 127°38′6.7′′E, river epilithon, L. Tan & B.H. Kim, 4 April 2018 (Holotype HC!, population on slide HC180407B7B3, illustrated here in LM as Figs –. Paratype HC!, population on slide HC180407B7B3a, illustrated here in SEM as Figs –. Isotype HYU!, population on slide HYU-D017) The strain was deposited at the Freshwater Bioresources Culture Research Bureau, Nakdonggang National Institute of Biological Resources with deposition number is ‘ FBCC 210021D’.

Etymology: —The specific epithet refers to the spatulate spines that taper towards the tips and are thorn-shaped.

Ecology: —The type population of Fragilaria thornata was epilithic in the Heukcheon Stream, South Korea. Water conditions were: circumneutral pH (6.6), low specific conductance (156 μS/cm), low turbidity (7.5 NTU) and moderate dissolved oxygen (7.6 mg /L). Co-occurring species include Achnanthidium minutissimum (Kützing) Czarnecki (1994: 157) , Encyonema minutum (Hilse) D.G. Mann in Round, R.M. Crawford & D.G. Mann (1990: 667), and Nitzschia amphibia Grunow (1862: 574) .

Molecular characterization: —The nucleotide sequences of the SSU rRNA and rbcL genes of the strain were deposited in GenBank (NCBI), under the accession numbers MK567892 and MK576039.

Remarks: —We used LM and SEM to study the morphology of F. thornata and confirm that it is a new diatom species belonging to the genus Fragilaria . It can be seen, F. thornata differs from other closely related taxa ( Table 3). This species has been confused with F. vaucheriae (Kützing) J.B. Petersen (1938: 167) . However, the valve is shorter and narrower in F. thornata than in F. vaucheriae . Moreover, the central area of F. vaucheriae is unilateral, whereas F. thornata has a clearly bilateral central area. Finally, the numbers of striae and areolae in F. thornata are lower than in F. vaucheriae . Another taxon that was similar to F. thornata in the LM analysis was F. capucina Desmazières (1830: 453) , which has a similar valve. Krammer & Lange-Bertalot (1991) documented three different valve morphologies in the type material of F. capucina . The first valve morphology of F. capucina lacks marginal spines and differs from F. thornata , which has spines. The second valve morphology of F. capucina is linear to lanceolate with linking spines and is similar to F. thornata . However, both valve morphologies have unilateral central areas, whereas F. thornata has a bilaterally gibbous central area. The third valve morphology ( Tuji & Williams 2006a,b) is in accordance with the original description included linear valves with a rectangular to rhombic central areas, and it differs from F. thornata , which has two rimoportula per valve, one at each pole. We herein consider that F. thornata and F. capucina are separate species. The numbers of striae and areolae in F. thornata are lower than in F. vaucheriae and F. capucina . The spines of F. thornata differ from those of F. vaucheriae and F. capucina , because of F. thornata ’ s spines that taper towards the tips. Apart from these two species, it is quite different from the others. Colony formation is also used for the identification of species such as the colony type of F. thornata and F. tenera (W. Smith) Lange-Bertalot (1980: 746) are mostly loosely aggregated. However, F. vaucheriae , F. crotonensis Kitton (1869: 110) and F. rumpens (Kützing) G.W.F. Carlson (1913: 29) form ribbon-shaped colonies, more specifically F. capucina connected by entire valve face. Therefore F. thornata can be clearly distinguished from these closely related species by its areolae structure, central area, spines shape, and colony formation.

The evolutionary history of F. thornata was inferred based on results of the Maximum Likelihood (ML) analyses. The highest log likelihood ratios of the SSU rRNA ( Fig. 24 ) and rbcL ( Fig. 25 ) trees were -4160.84 and -1797.77, respectively. We obtained 22 SSU rRNA and 18 rbcL aligned sequences ( Table 13) from NCBI to perform the Maximum Likelihood analyses. The molecular phylogenetic analyses of both the SSU rRNA and rbcL genes indicated that F. thornata is nested within Fragilaria clade. In the SSU rRNA phylogenetic tree, the relationship of Fragilaria and Synedra (subgenus Ulnaria ) is supported as sister clade. Also in Medlin (2008)’ study, they were distinct clades molecularly. Fragilaria vaucheriae formed a clade with Fragilaria sp. , and they were homologous to F. thornata (ML bootstrap = 97%). The highest similarity score (0.985) was between F. thornata and F. vaucheriae , which had a genetic distance of 0.014 ( Table 4). In the rbcL phylogenetic tree, F. thornata formed a clade with Fragilaria sp .. The similarity score between F. thornata and Fragilaria sp. was highest (0.993) and the genetic distance value is lowest (0.008) ( Table 5). Fragilaria vaucheriae forms a clade with Fragilaria sp. , and these species are homologous with F. thornata (ML bootstrap = 97%) in the SSU rRNA phylogenetic tree. However, in the rbcL phylogenetic tree, which lacks information on F. vaucheriae , F. thornata forms a clade with Fragilaria sp. (ML bootstrap = 72%). Although the similarity (0.985) between F. thornata and F. vaucheriae is higher, the genetic distance value (0.011) between F. thornata and Fragilaria sp. is lower, probably because the query cover between them is only 98%. As we did not find the morphological information on Fragilaria sp. , this taxon was not included in the comparison. Nevertheless, F. thornata is different from F. vaucheriae and Fragilaria sp. because of the higher support of the F. vaucheriae : Fragilaria sp. in the SSU rRNA tree (ML bootstrap = 100%) and the ML bootstrap between F. thornata and Fragilaria sp. is 72% in the rbcL tree. Above all, the phylogenetic analyses strongly support our view, building on morphological characteristics, that F. thornata is a new species belonging to genus Fragilaria .

Phylum Bacillariophyta

Class Bacillariophyceae

Subclass Bacillariophycidae

Order Bacillariales

Family Bacillariaceae

Genus Nitzschia Hassall (1845: 435)

Figures

FIGURE 24. Molecular phylogenetic analysis by maximum likelihood (ML) method based on the SSU rRNA showing the phylogenetic position of Fragilaria thornata sp. nov. and other species. The tree with the highest log likelihood (-4160.84) is shown. The analysis involved 22 nucleotide sequences.All positions with less than 95% site coverage were eliminated. There were a total of 1570 positions in the final dataset. Only bootstrap values over 50% are shown on tree. Cyclotella meneghiniana was chosen as outgroup.

FIGURE 25. Molecular phylogenetic analysis by maximum likelihood (ML) method based on the rbcL gene showing the phylogenetic position of Fragilaria thornata sp. nov. and other species. The tree with the highest log likelihood (-1797.77) is shown. The analysis involved 18 nucleotide sequences.All positions with less than 95% site coverage were eliminated. There were a total of 508 positions in the final dataset. Only bootstrap values over 50% are shown on tree. Cyclotella meneghiniana was chosen as outgroup.

FIGURES 2–14. Light microscope images of Fragilaria thornata sp. nov.. 2. Girdle view. 3–14. Valve views. Scale bar = 10 μm.

FIGURES 15–23. Scanning electron microscopy images of Fragilaria thornata sp. nov.. 15, 16. External and internal view of valve. 17, 18. One side of valve apices in external and internal view, showing the apical pore field (APF) and the rimoportula (R). 19, 20. External and internal view of the central area. 21. External view of valve mantle in detail, showing the fold-like lines (FL). 22. Oblique view of valve in detail, showing the clearly visible siliceous plaques (SP). 23. View of areolae and spines on valve face/mantle in detail, showing the hemispherical silica velum (HSV). Scale bars: 15, 16 = 10 μm; 17–20, 22 = 1 μm; 21, 23 = 0.5 μm.