Microcostatus dexteri Stanek-Tarkowska, M. Rybak, Czyż, 2021

Jadwiga, Stanek-Tarkowska, Antonina, Czyż Ewa & Mateusz, Rybak, 2021, Description of a new diatom species-Microcostatus dexteri sp. nov. - from terrestrial habitats in southern Poland, Phytotaxa 509 (2), pp. 241-247 : 243-246

Microcostatus dexteri Stanek-Tarkowska, M. Rybak, Czyż sp. nov. ( Figs 2–24 )

Valve shape lanceolate to elliptical-lanceolate with rounded ends ( Figs 2–20 ). Valve length 2.1–3.1 µm, width 2.3–5.5 µm, and length-width ratio 2.1–3.1 (n=20). Raphe filiform and straight in a raised slightly asymmetric non-perforated sternum ( Figs 16–22 ). Axial area narrowing near the valve apices, forming two longitudinal depressions, enlarged in the middle and creating an almost lanceolate central area ( Figs 15–17, 20 ). Shape and size of the central area varies. External proximal raphe endings slightly expanded and teardrop-shaped ( Figs 15–17, 20–21 ). Distal raphe endings curved in the same direction and extending up to the valve margin ( Figs 15–17, 20 , 22 ). Small elongated helictoglossae present on the distal raphe endings ( Fig. 23 ). Raphe sternum well developed but without a conopeum ( Fig. 21 ). Striae uniseriate (20–24 in 10 μm), radiate, becoming strongly radiate towards the apices ( Figs 2–20 ). Large specimens exhibit low density or lack of striae in the central part of the valve ( Fig. 19 ). Areolae almost square ( Figs 15–20 ). Individual areolae not visible in LM. In internal view, the areolae are covered by hymen ( Fig. 23 ). Few areolae (2–4) present close to the valve apices on the valve mantle ( Figs 16 , 22, 24 ).

Etymology:— The new species is named in honour of Prof.Anthony R. Dexter (Institute of Soil Science and Plant Cultivation State Research Institute in Poland)

Type:— POLAND. Podkarpacie Province, Rzeszów, coordinates 49º 58’ 22.48” N, 22º 16’ 36.54” E, J. Stanek- Tarkowska, November 2016. holotype slide SZCZ27499 , deposited at the Szczecin Diatom Collection hosted by the University of Szczecin ( SZCZ). The holotype specimen is illustrated here in Fig. 4 .

Distribution and ecology:— The newly described species was observed in terrestrial environments (on bare soils and among terrestrial and corticophilous mosses). The sites where M. dexteri was found are insolated (from full insolation to deep shade). The chemical reaction was slightly acidic to circumneutral (5.2–6.5) ( Tab. 1). In each sample, individual specimens were present and never exceeded 1% in the assemblages.

Associated diatom flora:— The most frequent taxa in the assemblage from the soil habitat included Mayamaea atomus ( Kützing 1844: 108) Lange-Bertalot (1987: 72)—15–30 %, Humidophila contenta (Grunow in Van Heurck 1885: 109) Lowe Kociolek, J.R.Johansen, Van de Vijver, Lange-Bertalot & Kopalová (in Lowe et al. 2014: 357)—10– 15%, Pinnularia obscura Krasske (1932: 117) —20–35%, and Stauroneis thermicola ( Petersen 1928: 394) J.W.G. Lund (1946: 61) —25–30%. In the moss samples, together with the described taxa, the most frequent taxa were H. contenta 7–30%, P. obscura —ca. 10%, and Luticola acidoclinata Lange-Bertalot (in Lange-Bertalot & Metzeltin 1996: 76)— ca. 35% ( Tab. 1).

Similar species:— Microcostatus stapputana Lange-Bertalot & Wydrzycka (in Wydrzycka & Lange-Bertalot 2001: 12), M. ornicollegarum Lange-Bertalot & Wydrzycka (in Wydrzycka & Lange-Bertalot 2001: 10), and M. elisabethianus Van de Vijver & Ector (2019: 541) ( Tab. 2).

Due to their small size, identification of Microcostatus species is difficult and light microscopy is often insufficient, as many characteristic valve features can only be observed by scanning electron microscopy. In comparison with all known European Microcostatus species , the newly described M. dexterii has much lower density of striae, which allows identification thereof even under a light microscope. Morphologically, M. dexteri is most closely related to M. stapputana and M. ornicollegarum , species known only from tropical South America ( Wydrzycka & Lange-Bertalot 2001, Metzeltin & Lange-Bertalot 2007), and to M. elisabethianus described recently from the sub-Antarctic region ( Van de Vijver & Ector 2019). Some important morphological differences between the newly described species and similar M. stapputana and M. ornicollegarum can be observed only using SEM.

Despite the very similar cell dimensions, M. dexteri can be distinguished from M. stapputana by the difference in the structure of areolae which are mainly square (not circular as in M. stapputana ), and the presence of a single row of areolae on the mantle restricted to the valve apices (not on the whole mantle). Additionally, the valves of M. dexteri are more lanceolate than those of M. stapputana . The frustules of M. ornicollegarum are very similar in shape and dimensions to M. dexteri ( Wydrzycka & Lange-Bertalot 2001, Van de Vijver et al. 2004). Nevertheless, both species can be distinguished by the structure of striae: M. dexteri has striae composed of multiple areolae, while M. ornicollegarum has striae composed of single elongated areolae ( Wydrzycka & Lange-Bertalot 2001). Also, M. elisabethianus described recently from the Antarctic region has similar dimensions to those of the newly described species. However, it has a more lanceolate valve outline with more protracted apices and striae composed by single elongated areolae. Additionally, M. elisabethianus has a conopeum, which is not present in M. dexteri .

The accompanying dominant diatom species, mainly typical terrestrial and aerophytic taxa, suggest that the newly described Microcostatus species has a similar preference for humidity of the environment.

Figures

FIGURES 2–20. Light microscopy images of Microcostatus dexteri sp. nov. in size diminution series (scale bar = 10 µm). Figs 15–20 SEM images of Microcostatus dexteri sp. nov. (Fig. 15: scale bar = 3 µm, Figs. 17, 20: scale bar = 4 µm, Figs 16, 18, 19: scale bar = 5 µm)

FIGURES 21–24. Detailed SEM images of Microcostatus dexteri sp nov. (Figs 21, 24: scale bar = 1 µm, Figs 22, 23: scale bar = 2 µm).