One new species of Cymbella C.A. Agardh (Bacillariophyta) from high altitude lakes in the Hengduan Mountains of Southwest China

Introduction

Diatoms from the Yunnan plateau have been only cursorily studied in the past (Skuja 1937, Zhu and Chen 1994, Li et al. 2007a,b). Cymbella C.A. Agardh (1830: 1) is one of the largest diatom genera, representing nearly two thousand described taxa (Kociolek et al. 2020a). However, in the past 30 years, the family Cymbellaceae has been split by some researchers more finely (Kociolek and Stoermer 1987, Krammer 1997a,b, Lange-Bertalot and Genkal 1999, Krammer 2002, 2003, Jüttner et al. 2010a, Bahls 2015, Kulikovskiy et al. 2014, 2015, Kulikovskiy and Kuznetsova 2016, Kapustin et al. 2017, 2020, Kociolek et al. 2017, Glushchenko et al. 2019, Kezlya et al. 2020). As a result of this recent work about 20 different genera are recognized (Tab. 1).

Genera References
Encyonema Kützing (1833: 589) Kützing 1833
*Reimeria Kociolek et Stoermer (1987: 457) Kociolek and Stoermer 1987
Pseudencyonema Krammer (1997: 156) Krammer 1997a
Encyonopsis Krammer (1997: 156) Krammer 1997b
Cymbellopsis Krammer (1997: 157) Krammer 1997b
Cymbopleura Krammer (1999: 292) Krammer 2003
Navicymbula Krammer (2003: 123) Krammer 2003
Delicata Krammer (2003: 110; recently renamed as Delicatophycus M.J. Wynne 2019: 1) Krammer 2003
Gomphocymbellopsis Krammer (2003: 127) Krammer 2003
*Afrocymbella Krammer (2003: 129) Krammer 2003
Krsticiella Levkov in Levkov et al. (2007: 14–15) Levkov et al. 2007
Oricymba Jüttner et al. (2010: 408) Jüttner et al. 2010a
Ochigma Kulikovskiy, Lange-Bertalot et Metzeltin (2012: 214) Kulikovskiy et al. 2012
Khursevichia Kulikovskiy, Lange-Bertalot et Metzeltin (2012: 157) Kulikovskiy et al. 2012
Kurtkrammeria Bahls (2015: 6) Bahls 2015
Celebesia Kapustin, Kulikovskiy et Kociolek (2017: 153) Kapustin et al. 2017
Karthickia Kociolek, Glushchenko et Kulikovskiy (2019:606) Glushchenko et al. 2019
Tab. 1.A chronological listing of genera based on species formerly in the genus Cymbella or closely related genera (Zhang et al. 2021). * indicates the genus may be more closely related to freshwater gomphonemoid diatoms.

Since the revision of Cymbella, a number of other new species have been described from China (summarized in Kociolek et al. 2020b, see also Li et al. 2019, Liu et al. 2020, Zhang et al. 2020). In southwestern China, specifically, Li et al. (2003a,b) described new species of Cymbella respectively from northwest Tibet and Qinghai Province. Gong and Li (2011) described a new species of Cymbella from the Yunnan Plateau. Hu et al. (2013) described three new species of Cymbella from the high altitude lakes of the Hengduan Mountains region. In the present paper, we describe one new Cymbella species from the Hengduan Mountains area based on light and scanning electron microscopical investigations and compare its morphology with similar species.

Material and methods

Samples were taken using a Kajak-type gravity core (Renberg and Hansson 2008) from the surface sediments of Lake Shadecuo (29°44′35.8″ N, 101°21′39.8″ E, Tab. 2), located in the eastern Hengduan Mountains region of Southwest China. The pH and specific conductivity of the lake water were measured in the field using a YSI 650 multi-parameter display system (650 MDS, YSI Incorporated 1700/1725 Brannum Lane, Yellow Springs, OH 45387 USA) with a 600 XL probe. Water samples were taken from 50 cm under the water surface. Transparency was estimated using a Secchi disk. Total nitrogen (TN) and total phosphorus (TP) were measured by a Shimadzu UV 2450 ultraviolet-visible spectrophotometer at Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences, using the alkaline potassium persulfate digestion-UV spectrophotometric method (Nydahl 1978) and the ammonium molybdate spectrophotometry method (Ebina et al. 1983), respectively.

Lake Shadecuo
Latitude (°N) 29°44′35.8″
Longitude (°E) 101°21′39.8″
Altitude (m a.s.l.) 4428
Maximum depth (m) 8
pH 7.66
Secchi disk depth (m) 4.8
Total nitrogen (mg·L–1) 0.288
Total phosphorus (mg·L–1) 0.004
Conductivity (μs·m–1) 33
Tab. 2.Physical and chemical parameters in Lake Shadecuo.

Diatom samples were kept under 4 °C in the refrigerator before laboratory treatments. In the laboratory, the diatoms were treated with HCl and H2O2 (Battarbee 1986). Permanent slides were made from cleaned materials and mounted in Naphrax® for observation with light microscopy (Olympus, BX-51, DIC). Relative abundances of diatoms in the samples were determined with a count of 300 valves.

Cleaned materials were investigated with a Leo 1530 scanning electron microscope (SEM). Samples and slides are preserved in the Herbarium of Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences, China.

Results

Taxonomy

  1. Class Bacillariophyceae Haeckel 1878
  2. Subclass Bacillariophycidae D.G. Mann in Round et al. 1990
  3. Order Cymbellales D.G. Mann in Round et al. 1990
  4. Family Cymbellaceae Greville 1833
  5. Genus Cymbella C.A.Agardh 1830
  6. Cymbella luoyulanae Y.Li sp. nov. Fig. 1 A-E; Fig. 1C is the holotype

LM (Fig. 1): Valves moderately dorsiventral. Dorsal margin strongly convex. Ventral margin concave, slightly tumid in middle. Valve ends slightly truncated and broadly rounded. Length 59-82 μm, width 16.0-18.5 μm, maximum length/width ratio ca. 4.5. Axial area moderately broad. Central area elliptical, about 1/4-1/3 of width of valve. Raphe distinctly lateral, abruptly reverse-lateral near proximal ends. Striae slightly radiate, becoming more radiate towards valve ends. 3-5 stigmata, some larger than others, occur ventrally from central nodule, distant from middle ventral striae. Striae 7-8/10 μm at centre becoming 10-12/10 μm towards apices, with 20-22 areolae in 10 μm.

In SEM external valve view (Fig. 2): Striae uniseriate, partly biseriate with elliptical and transapically oriented areolae. Areolae slit-like with apically-orientated openings in transapical striae (Fig. 2 A-F). Stigmata rounded, separated from the areolae at central area on ventral side. Small round depressions may occur on the dorsal side of the central area (Fig. 2 C, D). Raphe centrally located on the valve face in a narrow hyaline axial area with straight to slightly dorsally arched proximal ends (Fig. 2 A, B). External distal raphe ends deflected dorsally (Fig. 2 E, F); external proximal raphe ends dilated (Fig. 2 C, D). The apical pore fields present and comprised by a group of porelli, almost entirely on the valve mantle (Fig. 2 E, F).

Fig. 2.External valve view of Cymbella luoyulanae sp. nov. by scanning electron microscope. A, B – external view of an entire valve. C, D – external view of valve center, the proximal raphe endings and the very large central area with 4-5 stigmata. E, F – valve apices, striae with slit-like areolae, some of them biseriate, and pore field separated by the distal raphe fissures.

In SEM internal valve view (Fig. 3): Striae with internal areolae openings lack any occlusions (Fig. 3 A, B). The central area distinct, projecting internally, and lacking an intermissio. Stigma opening round, with a slight expanded depression evident on the central nodule. Stigmata lack any ingrowths or occlusion (Fig. 3 C, D). Distal raphe endings bent slightly towards the dorsal margin, terminating in helictoglossae. (Fig. 3 E, F).

Fig. 3.Internal valve view of Cymbella luoyulanae sp. nov. by scanning electron microscope.. A, B – internal view of entire valve. C, D – showing the valve centre and four big and one small stigmata as elongated furrows connected to the striae. E, F – internal view of valve apex with prominent helictoglossa deflecting to the dorsal side.

Type: – CHINA. Sichuan Province: Kangding City, Lake Shadecuo, 29°44′35.8″ N, 101°21′39.8″ E, elevation 4428 m a.s.l., samples collected by Dr. Yulan, Luo, 14th October 2017. Holotype Shadecuo 1-1 in Coll. Li Yanling, Yunnan University., Kunming, China. Fig. 1 is of the holotype.

Etymology: –The specific epithet ‘luoyulanae’ refers to the collector of the sample on which these observations are based.

Ecology

Cymbella luoyulanae has been observed in the surface sediment sample from Lake Shadecuo. In Lake Shadecuo, this species was associated with Cyclotella ocellata Pantocsek (1902: 134) (20%), Achnanthidium minutissimum (Kützing) Czarnecki (Czarnecki 1994: 157) (15%), C. comensis Grunow in Van Heurck (1882, pl. 93, Fig. 3 E, F) (14%), Pseudostaurosira pseudoconstruens (Marciniak) Williames et Round (1987: 275) (15%), and Staurosirella pinnata (Ehrenberg) D.M.Williames et Round (1987: 274) (5%).

Discussion

The features found in Cymbella luoyulanae are compared to all other known species of the genus in Tab. 3. The taxon most similar to C. luoyulanae is C. heihainensis. The valves of this species are wider, have a larger central area, drop-like proximal raphe ends, and a higher number of areolae in 10 µm, distinguishing it from C. luoyulanae. Cymbella modicepunctata has larger valves with a more distinct central area, and fewer striae in 10 µm, distinguishing it from C. luoyulanae. Cymbella luoyulanae has stigmata that may vary in size along the ventral margin, while C. modicepunctata has uniformly-sized stigmata on the ventral side. The same stigmata feature of C. luoyulanae is also found in C. schimanskii Krammer and its variety var. excelsa (Meister) Krammer. In SEM view, C. schimanskii has Y-shaped areolae openings in the middle of the valve, which is different from the biseriate of C. luoyulanae, and they differ from C. luoyulanae by having larger valves, a smaller central area, and in the shape of the central pores (rounded vs expanded as in C. luoyulanae). Cymbella luoyulanae can be distinguished from C. asiatica by valve size and striae density. Cymbella asiatica varies between 50 and 105 µm in length and 14-18 µm in breadth, has no stigmata-like depressions on the dorsal side, and only 6-7 striae in 10 µm. Cymbella luoyulanae is also similar to C. arctissima Metzeltin, which can be distinguished from our new species which has dorsally deflected proximal raphe endings, is shorter (98-105 µm vs. 59-94 µm), and has a broader central area (2/3 width vs. 1/2 width).

Species /Feature C. luoyulanae C. heihainensisLi et Gong C. modicepunctataKrammer C. asiatica Metzeltin, Lange-Bertalot et Li, Y. C. schimanskiiKrammer C. arctissimaMetzeltin
Reference this study Hu et al. (2013) Krammer(2002) Metzeltin et al. (2009) Krammer(2002) Krammer(2002)
Valve length, µm 59-94 82-100 102-140 50-105 100-200 98-105
Valve width, µm 16.0-19.5 19.0-22.5 21-22 14-18 29-36 17-18
Length/ width ratio 4.8 max 4.7 max 6.4 max 5.8 max 5 max 5.8
Striae in 10 µm 7-10 7-10 5.5-6 6-7 7-12 7-10
Areolae in10 mm 20-22 14-16 14-16 18-22 10-16 18-21
Areolae Slit-like, biseriate Slit-like,Y-, X- shaped Lineolate not known Slit-like,Y-shaped Slit-like
Stigmata ventral side:3-5 big andsome smaller,dorsal side: 0-3 ventral side:7-10 big andsome smaller,dorsal side: 0–5 ventral side:7-10 ventral side:4-6 ventral side:6-10 big andsome smaller,dorsal side:sometimes some ventral side:6-8
Central area 1/2 width 1/2 width 2/3-3/4 width 1/2 width 1/3 width 2/3 width
Central pores of raphe expandeddrop-like,slightly ventrallydeflected expandeddrop-like,slightly ventrallydeflected distinct, veryslightlyreverse-lateral distinct, abruptlyreverse-lateral small, slightlyreverse-lateral small, dorsallydeflected
Tab. 3.Morphological characteristics of Cymbella luoyulanae sp. nov. and Cymbella species sharing similar morphological features.

Another member of the genus Cymbella, C. distalebiseriata Bing Liu et D.M. Williams in Liu et al. (2018: 41), was described from Hunan Province, China. Cymbella distalbiseriata shares the feature of having striae with uniseriate and partially biseriate striae. The only other species of Cymbella exhibiting (a few) biseriate striae include C. yakii Jüttner et Van de Vijver in Jüttner et al. (2010b), C. duplopunctata Krammer 2002 and C. buechleri Krammer 2002 (both described from a fossil deposit in western North America). Three of the five species exhibiting biseriate striae are found in Asia, and two have been described from China. The other two taxa with biseriate striae are known from a fossil deposit in western North America (apparently of Miocene age, see Krammer 2002, p. 109). These are distinct from one another in size, shape and other morphological features (Krammer 2002, Jüttner et al. 2010b, Liu et al. 2018). It is unclear if the possession of biseriate striae is a homologous feature for this group or not; formal phylogenetic analysis is required to understand character evolution in this regard. However, if the taxa with biseriate striae are indeed closely related among themselves and their distribution remains between Asia and western North America, it supports the notion, noted by Ehrenberg over 180 years ago (Ehrenberg 1849) and supported by additional diatom examples (Kociolek and Stoermer 1989, Kociolek et al. 2013, 2015, Genkal and Kulikovskiy 2016, Kociolek 2019) as well as some genera of higher plants (Xiang and Soltis 2001, Nie et al. 2006, Kadereit and Baldwin 2012), of there being a close relationship between the floras of these two areas.

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