Cymbella biseriata Videska, Zaova & Levkov 2022, sp. nov.

Cymbella biseriata Videska, Zaova & Levkov sp. nov. (LM Figs 2–22, SEM Figs 23–34)

Description:—LM (Figs 2–22): Valves slightly dorsiventral, semilanceolate and asymmetrical. Valve ends slightly subcapitate. Dorsal margin slightly arched, ventral side almost straight. Valve dimensions: length 29–38 μm, width 7.0–9.0 μm (n=48). Axial area very narrow, following raphe, central area small, slightly expressed on dorsal side, absent on ventral side. 4–7 stigmata present in central area on ventral side. Raphe ventrally displaced, distinctly lateral becoming reverse-lateral near central area; filiform at terminal ends. Dorsal striae slightly radiate, 12–15 in 10 μm. APF absent. Areolae not visible in LM.

SEM (Figs 23–34): Externally, raphe branches curved (Figs 23–25), proximally reverse lateral with large dropshaped central pores, slightly dorsally bent (Fig. 26). Distally, raphe ends abruptly dorsally bent (Figs 27, 28). APF absent at both valve apices (Figs 24, 26). Central area narrow, more expressed on dorsal side, semi-lanceolate. Stigmata externally with small round openings (Fig. 26). Striae entirely biseriate composed of narrow slit-like to tilde-like areolae usually in oblique orientation, ca. 40 in 10 μm (Figs 23, 24). Internally, raphe curved (Figs 29, 30) distally terminates with large knob-like helictoglossae (Figs 31, 33), proximally covered by overgrowth of silica (Figs 32, 34) hiding proximal raphe ends (lacking “intermissio”). Striae entirely biseriate, areolae internally with small round openings (Figs 32–34). Striae separated with strongly thickened vimines with same width or wider than striae (Figs 31, 32). Stigmata with small elongated internal openings, weakly or not separated from striae (Figs 32, 34).

Type:— NORTH MACEDONIA. Mountain Kozuf: Adzibarica, moss in alpine pond, 41.1841904 N, 22.2035631 E. 1690 m asl, A Videska, 4 th June 2020 (holotype MKNDC 012839 A! isotype BM 81943).

Etymology:—The epithet ‘ biseriata’ refers to entirely biseriate striae.

Ecology:— Cymbella biseriata has been observed only at the type locality, an oligo-dystrophic alpine pond on Adzibarica, mountain Kozuf. Data on the physical and chemical characteristics of the type locality have not been obtained, but floristical composition (ass. Caricetum macedonicae with Carex macedonica Bornmüller, C. paniculata Linnaeus, C. stellulata Goodenough, Eriophorum latifolium Hoppe, Geum coccineum Sibthorp John & Smith James Edward indicates oligo-dystrophic conditions of the habitat. Diatom species present in the sample are: Encyonema venticosum (Kützing) Grunow (in Schmidt et al. 1874 –1959, pl. 10, fig. 59), Odontidium mesodon (Ehrenberg) Kützing (1844, p. 44), Gomphonema elegantissimum E.Reichardt & Lange-Bertalot, (in Hofmann et al. 2011: 302), G. confusum Levkov, Mitic-Kopanja & E.Reichardt, (2016: 41) Achnanthidium microcephalum Kützing (1844: 75), Cymbella alpestris Krammer (2002: 163), C. excisiformis Krammer (2002: 160), Delicatophycus alpestris Wynne (2019: 1), Nitzshia perminuta (Grunow) M. Peragallo (1903: 672).