Nitzschia hantanense 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 : 163-170

Nitzschia hantanense L. Tan & B.H. Kim , sp. nov. ( Figs 26–39 , LM; Figs 40–47 , SEM)

Description: —Valve lanceolate, central part of valve sides convex, tapering acutely near the apices, weakly protracting up to slightly subcapitate apices ( Figs 27–43 ). Valve length 9.3–10.5 µm, width 2.5–2.8 µm, 38–40 striae in 10 µm, 20–22 fibulae in 10 µm, 4–6 areolae in 1 µm. Striae not discernible under LM ( Figs 27–39 ). Cells solitary. Striae are uniseriate with large, round areolae under SEM. Face of valve almost without parallel striae, most striae curved towards the poles ( Figs 40, 41 ). Canal striae double ( Fig. 44 ). Striae separated by raphe; a wider separation of the stria areolae present between the raphe canal ( Fig. 45 ). On the mantle, each stria composed of two round areolae, one areola within the raphe canal and one outside; vimines between two round areolae concave inwards ( Figs 46, 47 ). Areolae occluded by hymenes, which are visible externally and internally. Vimines slightly thinner and narrower than virgae ( Fig. 46 ). Raphe system eccentric, on opposite sides (‘nitzschioid’ symmetry) of the valve, which situated between the valve face and the mantle. Raphe slit continuous. Keeled raphe simple, shallow. Canal raphe subtended by fibulae, continuous with vimines or virgae to forming round to elliptic portulae, two lines of areolae under it ( Fig. 47 ). Distal raphe fissures strongly hooked towards the same side (either valve face or mantle face), ending with small helictoglossae internally ( Figs 42, 43 ). Marginal fibulae of almost the same widths, evenly spaced throughout the valve, each fibula spaced at every 1–2 striae ( Fig. 41 ). Valve with open bands, with a single row of small puncta ( Fig. 45 ).

Type: — Republic of Korea. Gangwon-do Province: Hantangang River, 38°14′54.5′′N, 127°17′10.3′′E, river epilithon, H.K. Kim & I.H. Cho, 4 April 2018 (Holotype HTG!, population on slide HTG 180409B7B2, illustrated here in LM as Figs –. Paratype HTG!, population on slide HTG 180409B7B2a, illustrated here in SEM as Figs –. Isotype HYU!, population on slide HYU-D018) 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 Hantangang River, where the species was collected.

Ecology: —The type population of Nitzschia hantanense was epilithic in Hantangang River, South Korea. Water conditions were: circumneutral pH (6.4), low specific conductance (170 μS/cm), low turbidity (11.7 NTU) and moderate dissolved oxygen (7.6 mg /L). Relatively common species recorded in the same location were: Achnanthidium minutissimum , Diatoma vulgaris ( Bory 1824: 461) , and Achnanthes convergens H. Kobayasi ex H. Kobayshi, Nagumo & Mayama (1986: 84) .

Molecular characterization: —The nucleotide sequences of the SSU rRNA and rbcL genes of the strain were deposited in GenBank (NCBI), under the accession numbers MK567893 and MK576040.

Remarks: — Nitzschia hantanense is diagnosed by the continuous raphe slit, distinct outline, denser striae, and denser fibulae ( Table 6). In the morphologic and phylogenetic approaches, N. hantanense was found to be closely related to N. supralitorea Lange-Bertalot (1979: 215) , from which it is distinguished by its relatively high density of striae and fibulae. The original description of N. supralitorea mentions that it has fewer striae (25–34 in 10 µm, striae visible under LM in most specimens) and fibulae (14–18 in 10 µm), and that it has a lanceolate to linear-lanceolate valve outline, with capitate apices. Because of the lack of a historical description, the analysis of the type material of N. supralitorea (specimen ID TCC 950 in the Thonon Culture Collection, France) revealed several differences, i.e., that the striae are slightly curved near the apices and in the central part of the valve are parallel in the central part of the valve. In contrast, N. hantanense has almost no parallel striae; most striae are curved towards the poles, and the central part of the valve sides are convex. There is a significant overlap in the range of valve length, width, and the density of striae between N. hantanense and N. microcephala Grunow in Cleve & Grunow (1880: 96). The valve size of N. hantanense is compatible with that of N. inconspicua Grunow (1862: 579) but it differs in the shape of valve apices; N. hantanense has subcapitate apices and N. inconspicua has rounded apices. In addition, N. inconspicua has a discontinuous raphe slit, whereas the raphe slit of N. hantanense is continuous. N. fonticola (Grunow) Grunow in Van Heurck (1881: pl. LXIX [69]) showed narrower valves than N. hantanense . None of these species are in accordance with the morphologic description of N. hantanense .

We obtained 21 aligned sequences of SSU rRNA and 15 aligned sequences of rbcL sequences ( Table 13) from GenBank to perform the phylogenetic analyses. The relationships between N. hantanense and other species are shown in Figure 48 and Figure 49 , N. hantanense was closely related to the clade that included other Nitzschia species in the SSU rRNA and rbcL phylogenetic trees. It can be seen that Nitzschia genus is not monophyletic, this is also confirmed in Suriyanti (2017) ’s research. The SSU rRNA tree revealed that N. hantanense is a sister to the group of N. supralitorea with strong support (ML bootstrap = 91%). The highest similarity score is 0.978 between N. hantanense and N. supralitorea and the distance value is 0.007 ( Table 7). In the rbcL tree, N. hantanense and N. supralitorea formed a clade (ML bootstrap = 98%); the similarity and genetic distance these species were 0.979 and 0.021 ( Table 8), respectively. These species were homologous with N. amphibian and N. inconspicua in the SSU rRNA (ML bootstrap = 93%). We propose N. hantanense as a new species based on molecular and morphological data, as it did not have a very high similarity score (0.978 and 0.979 in the SSU rRNA and rbcL trees, respectively). The separate SSU rRNA and rbcL phylogenetic analyses agreed with the morphological comparisons, showing several differences in the relationships between N. hantanense and other related species. Therefore, we herein present N. hantanense as a new species for this genus.

Phylum Bacillariophyta

Class Bacillariophyceae

Subclass Bacillariophycidae

Order Cymbellales

Family Gomphonemataceae

Genus Encyonema Kützing (1834: 589)

Figures

FIGURE 48. Molecular phylogenetic analysis by maximum likelihood (ML) method based on the SSU rRNA showing the phylogenetic position of Nitzschia hantanense sp. nov. and other species. The tree with the highest log likelihood (-4924.42) is shown. The analysis involved 21 nucleotide sequences.All positions with less than 95% site coverage were eliminated. There were a total of 1387 positions in the final dataset. Cyclotella meneghiniana was chosen as outgroup.

FIGURE 49. Molecular phylogenetic analysis by maximum likelihood (ML) method based on the rbcL gene showing the phylogenetic position of Nitzschia hantanense sp. nov. and other species. The tree with the highest log likelihood (-2581.97) is shown. The analysis involved 15 nucleotide sequences.All positions with less than 95% site coverage were eliminated. There were a total of 627 positions in the final dataset. Only bootstrap values over 50% are shown on tree. Cyclotella meneghiniana was chosen as outgroup.

FIGURES 26–39. Light microscope images of Nitzschia hantanense sp. nov.. 26. Girdle view. 27–39. Valve view. Scale bar = 5 μm.

FIGURES 40–47. Scanning electron microscope images of Nitzschia hantanense sp. nov.. 40, 41. External and internal view of valve. 42, 43. One side of the valve apices in external and internal view. 44, 45. Oblique view of valve, showing the continuous raphe slit. 46. Internal view of valve, showing the vimines (M) and virgae (R). 47. Internal view of raphe canal, showing the portulae (P). Scale bars: 40, 41, 44, 45 = 2 μm; 42, 43, 46 = 0.5 μm; 47 = 0.2 μm.