In this study, we describe a new diatom, Navicula congqiancuoensis sp. nov., which was found in epilithon samples collected from a small mountain lake, at Lake Congqiancuo, Sichuan Province, China. A detailed morphological description of the new species is provided based on light and scanning electron micrographs. Navicula congqiancuoensis sp. nov. valve is linear to linear-lanceolate, with a slight elevation in the center and broadly rounded at both apices. The striae are radiate and convergent at the apices. The axial area is narrow and linear, and the central area is small and elliptical in the middle. This new species is compared with similar species, such as Navicula angusta, N. leptostriata, N. piercei, N. heimansioides which differ in size, valve shape, striae density and ultrastructure.

China, Lake Congqiancuo, Navicula, new species, taxonomy

The genus Navicula Bory (1822), initially described by Bory de Saint-Vincent in 1822, has undergone several revisions over time, including those by Patrick (1959), Cox (1979), and Round et al. (1990). Since its original description, numerous genera have been separated from Navicula, such as Diadesmis Kützing (1844), Luticola D.G.Mann in Round et al. (1990), Gandhia Kulikovskiy et al. (2023), Trialacinia Kociolek, Liu & Fan (2025). Consequently, the current definition of Navicula sensu stricto is based on the type of Navicula tripunctata (Müller 1786) Bory (1822), and is restricted to the members of the section Lineolatae previously described by Cleve (1895) as Cox (1979). Species within the genus Navicula typically exhibit a narrow to broad lanceolate, naviculoid valve outline with apices that display a variety of shapes. The central sternum is asymmetrically thickened to varying degrees. The raphe branches are straight and filiform, with the proximal ends slightly deflected unilaterally and the distal ends strongly recurved. The striae are uniseriate and composed of apically elongated areolae called lineolae, characteristics that facilitate the identification of Navicula among other diatom genera (Lange-Bertalot 2001).

To date, according to Diatom base, > 1000 species of the genus Navicula have been accepted taxonomically (Kociolek et al. 2024). Despite this, many Navicula species in inland waters remain undescribed, with numerous new species discovered globally over the past decades. These discoveries span Africa, Europe, America, Asia, and even Antarctica (Taylor et al. 2016; Van de Vijver and Lange-Bertalot 2009; Witkowski et al. 2010; Edlund and Soninkhishig 2009; Zidarova et al. 2016).

In China, the genus Navicula is still insufficiently studied. Li and Qi (2016) listed 133 species and 32 varieties in the Diatom Flora of China, yet it is evident that an in-depth study of this genus within the country’s borders is still lacking. Recent findings by Gong et al. (2015) of new species from the Yunnan Plateau’s lakes include Navicula craticuloides Y. Li & Metzeltin, Navicula gongii Metzeltin & Y. Li and Navicula yunnanensis Y. Li & Metzeltin. Adding to this diversity, Chudaev and Georgiev (2016) added two new Navicula in a high-altitude Tibetan lake: Navicula gololobovae Chudaev in Chudaev and Georgiev (2016) and Navicula cryptofallax var. tibetica Chudaev in Chudaev and Georgiev (2016). Meanwhile, in a stark contrast of habitat, a new benthic brackish diatom species, Navicula amoyensis, was identified in the estuarine environment of the Jiulong River (Chen et al. 2017) in Southern China. Furthermore, expanding the known diatom flora of China, Fu (2018) documented six previously unrecorded Navicula species from the Changbai Mountain regions. Wang et al. (2020) reported a new species, Navicula daochengensis, from Lake Congqiancuo in Sichuan, China, highlighting the ongoing exploration and documentation of Navicula diversity in the country. Furthermore, Navicula australasiatica, Navicula perangustissima and Navicula turriformis in Li et al. (2020); Navicula fuxianturriformis in Zhang et al. (2022) and Navicula sinicomeniscus in Xiao et al. (2023).

The aim of this paper is to describe a new Navicula species, Navicula congqiancuoensis sp. nov., from a small mountain lake — Lake Congqiancuo, Haizishan Nature Reserve, Daocheng County (Sichuan Province, China) — based on its unique morphological features, revealed by detailed light and scanning electron microscopy (SEM) observations and its comparison with similar taxa.

Class Bacillariophyceae

Order Naviculales

Family Naviculaceae

Genus Navicula

 Navicula congqiancuoensis W.Zhang & S.Blanco, sp. nov.

LM: Figs 1–7, SEM: Figs 8–13, 14–18

Description.

Light microscopy (LM) (Figs 1–7). The valve is linear to linear-lanceolate, with a slight widening in the center and broadly rounded at the apices, with a length of 42.7–51.2 μm and a width of 4.9–6.1 μm, giving an average L/W ratio 8.2 (40 valves were measured to obtain these size ranges). The raphe is weakly lateral, and the external distal raphe ends are deflected on to the mantle. The striae are radiate and convergent at the ends, and there are two to three short striae in the central area, 14–16 in 10 μm. The axial area is narrow and linear, and the central area is small and elliptical in the central nodule.

Figures 1–7. 

Navicula congqiancuoensis sp. nov. Size diminution series. Scale bar: 10 μm.

Scanning electron microscopy (SEM) (Figs 8–18). External view: Valves flat. Raphe fissure straight (Figs 8, 9). Proximal raphe endings slightly unilaterally hooked towards secondary valve side. External distal raphe endings curved to the secondary side of valve, with ends positioned on valve mantle (Figs 10, 12). Axial area very narrow and often thickened. Lineolae density ca. 50 in 10 µm (Fig. 11). Transversal striae radial, convergent at the ends. The lineolae and the virgae separating them are aligned longitudinally so that there appear to be both longitudinal and transverse striae (Figs 8–13). Internal view: the raphe is straight and slightly inclined towards the secondary valve side (Fig. 14). On the side of the raphe sternum opposite to central nodule, there is a longitudinal, semilanceolate thickening. Distal raphe endings are well-developed helictoglossae (Figs 15, 16). The accessory rib is well-developed on the primary valve side and unilaterally widened in the center, forming an asymmetrical central nodule that is visible under light microscopy (Figs 17, 18). The Voigt discordance is not visible in the valves we observed.

Figures 8–13. 

Navicula congqiancuoensis sp. nov. SEM external views 8, 9 whole valve showing the outline and external raphe structure 10, 12 distal raphe ends and areolae 11 details of external central area with slightly expanded proximal raphe ends 13 details of external view of striae. Scale bars: 10 μm (8, 9); 2 μm (10, 11, 12, 13).

Figures 14–18. 

Navicula congqiancuoensis sp. nov. SEM internal views 14 whole valve showing the outline and internal raphe structure 15, 16 distal raphe endings with helictoglossae 17 central area 18 details of areolae. Scale bars: 10 μm (14); 2 μm (15, 16, 17, 18).

Type.

China. • Sichuan Province: Tibetan Autonomous Prefecture of Garzi, Daocheng County, Haizishan Nature Reserve, Lake Congqiancuo, 29°21'58"N, 100°04'51"E, elevation 4389 m a.s.l., samples collected by Q.C. ZHOU, 20 Sep. 2018. Holotype CQC1#, SHOU! Herbarium of Hydrobiological Department, College of Fisheries and Life Science, Shanghai Ocean University (SHOU), Shanghai, China = Fig. 3.

Etymology.

The specific epithet, congqiancuoensis, refers to the locality from which the new species was described.

Ecology.

Navicula congqiancuoensis sp. nov. was found in epilithon samples collected in a high-altitude freshwater lake, with a pH of 8.9–9.4 and very low conductivity of 17.5–17.6 µS/cm. The water temperature at the time of collection was 13.5 °C, total nitrogen (TN) 0.032–0.043 mg/L, total phosphorus (TP) 0.014–0.016 mg/L.

Navicula congqiancuoensis sp. nov. shares some features with other taxa in this genus, such as Navicula angusta Grunow (1860), N. leptostriata Jørgensen (1948) and N. piercei Bahls (2012). They all are more or less similar in valve outline (linear or linear-lanceolate), raphe (almost lateral) and axial area (narrow and linear). A comparison of characteristics of these taxa in Navicula is provided in Table 1.

Navicula congqiancuoensis and N. piercei (Bahls 2012) exhibit similarities in their valve shapes, yet there are subtle distinctions. Both possess linear valves; however, in N. piercei they are distinctly tumid in the middle and with broadly rounded, somewhat wedge-shaped apices, a feature not present in N. congqiancuoensis, which is also shorter compared to N. piercei (42.7–51.2 μm vs. 64–76 μm). Additionally, the stria density is higher in N. congqiancuoensis than in N. piercei (14–16 in 10 µm vs. 11–13 in 10 µm). Furthermore, the areola density of N. congqiancuoensis is higher than that of N. piercei (50 areolae in 10 μm vs. 30 areolae in 10 µm).Navicula congqiancuoensis and N. angusta (Grunow 1860) exhibit certain differences in their valves: in N. congqiancuoensis is linear to linear-lanceolate, with a slight widening in the center and broadly rounded ends, while in N. angusta valves are linear with slightly protracted ends. Navicula congqiancuoensis is also narrower than N. angusta (width: 4.9–6.0 μm vs. 6–7 μm). The stria density of N. congqiancuoensis is higher than that of N. angusta (15–18 in 10 µm vs. 12–13 in 10 µm). Furthermore, the areola density of N. congqiancuoensis is higher than that of N. angusta (50 areolae in 10 μm vs. 32 areolae in 10 µm). Navicula congqiancuoensis and N. leptostriata (Jørgensen 1948) show also certain differences in their valves: N. congqiancuoensis has linear to linear-lanceolate valves, with a slight widening in the center and broadly rounded ends, while in N. leptostriata valves are narrowly lanceolate with acutely rounded, subtly protracted apices. Navicula congqiancuoensis is also longer N. leptostriata in length (42.7–51.2 μm vs. 33–39 μm), and the stria density is lower in N. congqiancuoensis compared to N. leptostriata (15–18 in 10 µm vs. 19–20 in 10 µm). N. congqiancuoensis is longer than N. heimansioides (length: 42.7–51.2 μm vs. 18.7–39.3 μm) (Lange–Bertalot 1993). And N. heimansioides valves are lanceolate to linear-lanceolate with slightly drawn-out ends, N. congqiancuoensis exhibit linear to linear-lanceolate valve shapes with broadly rounded ends. The stria density of N. congqiancuoensis and N. heimansioides are the same (14–16 in 10 µm), and both striae are radiate and bent at the valve center, convergent at apices.

To sum up, the discovery of N. congqiancuoensis in Lake Congqiancuo underscores the importance of continued exploration of diatom diversity in remote and understudied regions. As we expand our knowledge of diatom taxonomy in China, findings such as N. congqiancuoensis contribute to a more comprehensive understanding of biodiversity patterns and ecological dynamics in high-altitude aquatic environments. Further research into the ecology and distribution of this species may provide valuable insights into the biogeography and environmental preferences of diatoms in mountainous freshwater systems.