LM (Figs 1–36): Valves lanceolate in longer specimens to elliptic-lanceolate in shorter valves. Margins usually linear to weakly convex. Apices only weakly wedge-shaped, moderately acutely to bluntly rounded. Occasionally, strictly lanceolate valves with convex margins and acutely rounded apices observed. Valve dimensions (n=45): length 19.0– 51.5 μm, width 5.5–7.5 μm. Axial area narrow, linear, slightly widening close to the central area. Sternum weakly but still visibly developed. Central area small, rounded to transapically rectangular, bordered by usually one, rarely 2–3 irregularly shortened and more distantly spaced striae. Raphe filiform with enlarged proximal raphe endings, deflected towards the secondary side. Striae radiate becoming parallel and even weakly convergent close to the apices, 10–16 in 10 μm. Lineolae almost not discernible in LM. SEM (Figs 37–40): External raphe sternum distinctly raised, less developed towards the apices, irregularly widened near the central area (Figs 37, 39). Proximal raphe endings drop-like enlarged, clearly deflected to the secondary side (Fig. 39). Distal raphe fissures hooked, continuing onto the mantle (Fig. 37). Striae composed of slit-like lineolae. Virgae larger than the lineolae (Figs 37, 39). Lineolae ca. 35–40 in 10 μm. Internally, raphe running weakly lateral of the raphe sternum with the raphe slit opening on the side, but with proximal raphe endings in the middle of the central area (Figs 38, 40).

Type: — FRANCE. Sainte-Marguerite: Tennis Spring, sample S1_BA_151114 (45° 40’ 06.9” N, 3° 13’ 20.3” E), A. Beauger, 15 November 2014 (holotype: CLF!, slide no. 104186; isotypes: CLF! slide no. 104187, Herbiers Universitaires de Clermont-Ferrand, France, and BR! slide no. 4427).

Etymology: — The new species is named after the village where it was discovered, Sainte-Marguerite, situated in the French Massif Central.

Ecology and associated diatom taxa: — The type population of N. sanctamargaritae was found in Tennis spring where it entirely dominated the diatom flora. The new species was also found in two other springs such as Tambour Spring and Petit Saladis Spring. The latter two were co-dominated by N. sanctamargaritae and Crenotia thermalis (Rabenhorst 1864: 107) Wojtal (2013: 81). Fragilaria famelica (Kützing 1844: 64) Lange-Bertalot (1980a: 749), Nitzschia valdecostata Lange-Bertalot & Simonsen (1978: 58) and Rhopalodia acuminata Krammer in Lange-Bertalot & Krammer 1987: 75 were also identified.

The springs have a circumneutral pH, an elevated conductivity level (>8000 μS cm-1) and are enriched with by sodium-chloride and bicarbonate sodium with CO 2 present as a gas phase as observed by other authors in the same area (Negrel et al. 1997, Rihs et al. 2000, Gal et al. 2012). The water temperature ranged between 17.5 and 27.0 °C. The concentrations of the different ions were very similar between the three springs although Lithium ions were present in each spring (8.6 mg l−1) underlining deep water origin (Michard 1990). Tables 1 show the main physical and chemical values for the three springs.