$¢PhytoKeys

PhytoKeys 259: 81-102 (2025) DOI: 10.3897/phytokeys.259.150757

Research Article

Four new Planothidium species (Achnanthidiaceae, Bacillariophyceae) from the Karst region of Guizhou in China

Yun Li'®, Lin-Xin Lu'®, Hui-Wen Zhou'™®, Pan Yu2®, John Patrick Kociolek?®, Wan-Ting Pang'™, Quan-Xi Wang™®, Qing-Min You'®

1 College of Life Sciences, Shanghai Normal University, Shanghai 200234, China

2 School of Environment and Geographical Sciences, Shanghai Normal University, Shanghai 200234, China

3 Museum of Natural History and Department of Ecology and Evolutionary Biology, University of Colorado, UCB 218, Boulder CO80309, USA Corresponding author: Qing-Min You (yougm1117@shnu.edu.cn)

OPEN Qaceess

Academic editor: Kalina Manoylov Received: 20 February 2025 Accepted: 20 May 2025

Published: 25 June 2025

Citation: Li Y, Lu L-X, Zhou H-W, Yu P Kociolek JP Pang W-T, Wang Q-X, You Q-M (2025) Four new Planothidium species (Achnanthidiaceae, Bacillariophyceae) from the Karst region of Guizhou in China. PhytoKeys 209: 81-102. https://doi.org/10.3897/ phytokeys.259.150757

Copyright: © Yun Li et al. This is an open access article distributed under terms of the Creative Commons Attribution

License (Attribution 4.0 International - CC BY 4.0).

Abstract

Four new species of monoraphid diatoms belonging to the genus Planothidium were studied and described from the karst region of Guizhou Province, China. The morpho- logical characteristics of these new species were observed and documented using light and scanning electron microscopy. Planothidium pseudoreichardtii sp. nov. exhibits a broad sinus at the central area on the interior of the rapheless valve, linear shallow de- pressions and a unique enlarged areola at the end of the striae on the exterior rapheless valve. Planothidium liboensis sp. nov. is characterized by 4-5 rows striae on both valves, a cavum at the central area on the interior rapheless valve and irregular depressions on the exterior rapheless valve. Planothidium angustirostratum sp. nov. features elliptical valves with narrowly rostrate apices, a cavum at the central area of interior rapheless valve and circular depressions on the exterior rapheless valve. Planothidium maolanen- sis sp. nov. has elliptical valves without protracted apices, coarsely-spaced striae on the one side of the central area of the both valves. These four new species were compared with other similar Planothidium taxa. In addition, ecological information was provided and the stability of some features was discussed.

Key words: Diatoms, karst, monoraphid, new taxa, Planothidium, taxonomy

Introduction

Monoraphid diatoms are particularly intriguing due to their structural asymmetry, characterized by the presence of a raphe on one valve and its absence on the oth- er (Kulikovskiy et al. 2016). The genus Planothidium Round & Bukhtiyarova was established to accommodate monoraphid diatoms previously classified within the genus Achnanthidium Kitzing (Round and Bukhtiyarova 1996). Currently, ap- proximately 129 accepted taxa of Planothidium are recorded in AlgaeBase (Gui- ry and Guiry 2024). The genus is characterized by the presence of multiseriate striae on both the raphe valve (RV) and rapheless valve (SV), as well as asym- metrical structures in the central area of the rapheless valve in many species (Morales 2006). Since its establishment, Planothidium has undergone signifi- cant taxonomic refinement, with studies describing new species and exploring

81

Yun Li et al.: Four new Planothidium species

its morphological and ecological diversity. The application of scanning electron microscopy and molecular techniques has further enhanced the differentiation of Planothidium species and deepened our understanding of their morphological features (Novis et al. 2012; Alvarez-Blanco and Blanco 2013; Van De Vijver et al. 2013; Jahn et al. 2017; Van De Vijver and Bosak 2019; Wetzel et al. 2019).

Morphological studies of Planothidium have focused on key characteris- tics, such as the structure of the rapheless valve's central area, valve outline, and striae patterns. Morales (2006) proposed a classification framework for Planothidium species based on the central area morphology of the rapheless valve, dividing them into four groups: (1) species with continuous, uninterrupt- ed striae in the central area, such as P. daui (Foged) Lange-Bertalot; (2) spe- cies with a variable, clear space in the central area that does not show any indentation or convexity, such as P minutissimum (Krasske) Lange-Bertalot; (3) species with a depressed central area without striae, such as P. lanceolatum (Brébisson ex Kiitzing) Lange-Bertalot; (4) species with a depressed region on one side of the central area, which is capped to form a cave-like structure called a “cavum”, such as P. biporomum (Hohn & Hellerman) Lange-Bertalot (Morales 2006). In the study by Stancheva et al. (2020), Planothidium species were cate- gorized into three groups: those with a sinus (single horse-shoe shaped mark), such as P lanceolatum; those with a cavum (double horse-shoe shaped mark), such as P. frequentissimum (Lange-Bertalot) Lange-Bertalot; and those lacking both a sinus and a cavum, such as P. minutissimum (Krasske) Morales and P. daui (Foged) Lange-Bertalot. Moreover, beyond classic morphological mea- surements (such as width, length, length-to-width ratio, and number of striae per 10 um), several other features observed in the SEM have been suggested as useful for species identification, including (1) the pattern of surface smooth- ness on the rapheless valve; (2) the number of areolae per stria on the raph- eless valve; and (3) whether the striae on the rapheless valve interrupt at the valve margin, thus influencing the number of areolae at the valve edge (Wetzel et al. 2019). In addition to morphological studies, molecular tools have provid- ed new insights into the taxonomy and phylogeny of Planothidium. Molecular research has confirmed the presence of two distinct clades within the genus, one with species possessing a sinus or cavum in the rapheless valve and the other lacking these structures (Jahn et al. 2017).

Currently, research on the genus Planothidium in China remains limited. In marine environments, one new species has been reported and described (Li et al. 2024). However, in freshwater ecosystems, only a few species have been recorded as new records, and no new species have been reported so far (Liu et al. 2015, 2016; Wang et al. 2019). Karst ecosystems, characterized by carbon- ate-rich and nutrient-poor waters, remain underexplored. These unique habi- tats may drive morphological differentiation and adaptive evolution, offering immense potential for diatom taxonomy studies. Recently, several new diatom species have been described from karst regions, such as Sellaphora gologonica Lai, Ector & Wetzel, Germainiella legionensis Blanco, Borrego-Ramos & Olenici, Achnanthidium mediolanceolatum Yu, You & Kociolek, Amphora baotuensis Li, Nagumo & Xu, and Fallacia cinariana Somek, Hamilton, Solak, Beauger & Sevin- dik et al. The discovery of these species highlights the underexplored diatom biodiversity in karst environments (Borrego-Ramos et al. 2018; Lai et al. 2018; You et al. 2019b; Li et al. 2022; Somek et al. 2025).

PhytoKeys 259: 81-102 (2025), DOI: 10.3897/phytokeys.259.150757 99

Yun Li et al.: Four new Planothidium species

In recent years, we carried extensive surveys of diatoms species diversity of China (Kociolek et al. 2016b, 2016a, 2019; Lowe et al. 2017; You et al. 2017, 2019a; Yu et al. 2017; Zhou et al. 2024). Based on the collections developed for these studies, four new Planothidium species within karst landform of Guizhou have been discovered. This study provides detailed descriptions, as well as light and scanning electron microscopy of these new species: Planothidium pseudore- ichardtii sp. nov., Planothidium liboensis sp. nov., Planothidium angustirostratum sp. nov. and Planothidium maolanensis sp. nov., comparing them with similar species within the genus. This work contributes valuable reference material for the taxonomy and morphology of freshwater Planothidium, while emphasizing the importance of karst habitats as biodiversity hotspots for diatoms.

Materials and methods

Diatom samples were collected from Xiaogikong Scenic Area and Maolan Nature Reserve (Libo County, Guizhou Province), which is characterized by a subtropical climate and is a typical karst landscape. The sampling sites were distributed in various water bodies, including streams, ponds and underground rivers. The water parameters (pH, Temperature and Conductivity) were mea- sured using a YSI Pro Plus multiparameter meter (YSI, Ohio, USA). Samples were collected using tweezers and a knife, then placed into sealed plastic bot- tles with the addition of formalin for preservation (final concentration of 4%). Information about the sampling sites is listed in Table 1.

In the laboratory, samples were processed to remove organic matter and excess impurities using a microwave-assisted reaction system (Model MARS, CEM Corporation, Charlotte, USA) with concentrated nitric acid. The specific steps for sample processing and methods of preservation are as described in You et al. (2021). Clean samples after processing were prepared for both light microscopy (LM) and scanning electron microscopy (SEM). For LM, samples were encapsulated in Naphrax and observed using an Olympus BX-53 micro- scope with DIC optics and a 1.4 numerical aperture, 100~ oil immersion objec- tive. For SEM, the same samples were dried on metal stubs and imaged using

Table 1. Locality data and habitat for samples studied.

No. of samples

GZ201510041P

GZ201510045

GZ201510051

GZ201510066

GZ201510099

GZ201510100

GZ201510108

Location Xiaoqikong Scenic Area Xiaogikong Scenic Area Xiaoqikong Scenic Area Xiaoqikong Scenic Area

Maolan Nature

Reserve

Maolan Nature

Reserve

Maolan Nature

Reserve

' ; Altitude Water Cond. Collection Coordinates Habitat (m) Temp (°C) pH (uS/em) Collector Date 25°15'41"N, | Attached to rocks in 780 18.0 ZB 226 Kociolek & | 10.2.2015 107°45'19"E the pond Wang Q.X. 25°15'36'N, | Attached tothe stones | 780 18.0 io | 226 Wang Q.X. & | 10.2.2015 107°45'16"E sin the rushing water Kociolek J.P. 25°15'36'N, | Attached to floating 780 18.0 7,5) 226 Wang Q.X. & | 10.2.2015 107°45'16"E things in the pond Kociolek J.P. 25°15'02'N, Attached to rocks 629 19.5 8:0:| -215 Wang Q.X. & | 10.2.2015 107°42'46"E beneath the waterfall Kociolek J.P. 25°17'32'N, | Floating in the slow 650 18.0 7.9 203 Wang Q.X. & | 10.4.2015 108°04'16"E subsurface stream Kociolek J.P. 25°17'32'N, | Floating in the slow 650 18.0 ZO) 203 Wang Q.X. & | 10.4.2015 108°04'16"E subsurface stream Kociolek J.P. 25°17'35'N, | Attached to rocks ina 811 18.0 7.8) 205 Wang Q.X.& | 10.4.2015 108°04'42"E rapid river. Kociolek J.P.

PhytoKeys 259: 81-102 (2025), DOI: 10.3897/phytokeys.259.150757

83

Yun Li et al.: Four new Planothidium species

a SU8010 SEM at 2 kV with a working distance (WD) of less than 6 mm (Hitachi High-Technologies Corp., Tokyo, Japan). To determine the relative abundance of the new species and record co-occurring diatom species, 400 intact and identifiable diatom valves from each sample were identified under light micros- copy (1000x). The images were compiled using Adobe Photoshop 2023. The morphological terminology follows (Morales 2006; Van De Vijver et al. 2018; Wetzel et al. 2019; Morais et al. 2020). The holotype images of each species correspond to specimens that have been circled on the permanent slides. All samples and permanent slides are stored at Lab of Algae and Environment, College of Life Sciences, Shanghai Normal University.

Results

Phylum Bacillariophyta

Class Bacillariophyceae Haeckel, 1878

Order Achnanthales Silva, 1962

Family Achnanthidiaceae Mann, 1990

Genus Planothidium Round & Bukhtiyarova, 1996

Planothidium pseudoreichardtii Q-M. You, P. Yu & J.P. Kociolek, sp. nov. Figs 1, 2

Holotype. SHTU! Slide GZ201510045, holotype illustrated in Fig. 1C, J. Diatom samples are housed in the Lab of Algae and Environment, College of Life Sci- ences, Shanghai Normal University, China.

Isotype. COLO! Material 11111, Slides are housed in the Kociolek Collection, Uni- versity of Colorado, Museum of Natural History Diatom Herbarium, Boulder, U.S.A.

Type locality. CHINA. Xiaogikong Scenic Area, Libo County, Guizhou Province, 25°15'36'N, 107°45'16'E, altitude: 780 m, collected by Kociolek J.P. & Wang Q.X., on October 2, 2015.

Description. Light microscopy (LM) (Fig. 1A—P). Valves elliptical with slight- ly constricted ends and rostrate extensions. Valve dimensions (n = 60): Length 11.5-19.0 um, width 5.5—7.5 um. Rapheless Valve (Fig. 1A—H): Axial area narrow, straight, linear. Central area features a U-shaped hyaline region widening towards the valve margin, on the opposite side, striae extend to the axial area. Striae radi- ate along the valve outline, 14-17 in 10 um (measured opposite hyaline area). Ra- phe Valve (Fig. 1I-P): Axial area narrow, straight, linear. Central area rectangular to nearly circular, bordered by 3-4 shortened, asymmetrical striae on each side. Raphe branches straight. Striae radiate along the valve outline, 14-18 in 10 um.

Scanning electron microscopy (SEM) (Figs 1Q, R, 2A, B). Rapheless Valve (Fig. 1Q, R): Striae composed of 3-4 rows of circular areolae, the outer two rows larger than those near the center. Striae narrow to 2 rows towards the axial area and may expand to 3-4 rows near the valve margin, a larger areo- la marks the end of striae near the valve margin (Fig. 1Q, white arrow). The central and axial areas feature irregular, shallow, linear depression. Internally, areolae are covered. A clearly visible sinus is present on one side of the central area, forming a shallow circular depression (Fig. 1R, white arrow). Raphe Valve (Fig. 2A, B): Striae composed of 3-4 rows of circular areolae. Striae narrow towards one end near the axial area. The central area typically displays 2 to 3

PhytoKeys 259: 81-102 (2025), DOI: 10.3897/phytokeys.259.150757 34

Yun Li et al.: Four new Planothidium species

|

- ~ _ - , * ~ 7 -—

—)

*

”

ad

*:

Figure 1. Planothidium pseudoreichardtii sp. nov. LM; A-H. Rapheless valves; I-P. Raphe valves. “=” indicates the dif-

ferent valves of the same frustule. Q. SEM external view of an entire rapheless valve. The white arrow points to a larger areola. R. SEM internal view of an entire rapheless valve, showing a distinct sinus in the central area (white arrow). Scale

bar as shown. Scale bars: 10 um (A-P).

shortened striae. Striae almost extend onto the valve margin. Raphe branches are Straight, with proximal raphe endings expanding into pores, surrounded by shallow, drop-like depressions. Distal raphe ends are curved in the same direc- tion, briefly extending onto the mantle. Internally, proximal raphe endings are slightly deflected to the opposite side, distal raphe endings terminate in a small helictoglossa. Central nodule is raised, with striae wider raised virgae and sunk-

en between them, and areolae are individually covered.

PhytoKeys 259: 81-102 (2025), DOI: 10.3897/phytokeys.259.150757 95

Yun Li et al.: Four new Planothidium species

; A _ va : , Bak i!

Figure 2. Planothidium pseudoreichardtii sp. nov. SEM; A. External view of an entire raphe valve. B. Internal view of an entire raphe valve, scale bar as shown. Scale bar as shown.

Etymology. The species was named for its outline being similar to P reichardtii.

PhycoBank registration. http://phycobank.org/105527

Distribution and ecology. This species has currently only been found at its type locality, where it was collected from rocks in a rapid stream (water temperature 18.0 °C, elevation 780 m). The species was observed in sample GZ201510041P (2.0%), GZ201510045 (2.5%). In the type material (GZ201510045), P pseudore- ichardtii was rare. The associated diatom flora included Navicula lundii Reichardt (19.5%), Achnanthidium delmontii Pérés, le Cohu & Barthés (12.0%), Planothid- ium rostratum (@strup) Lange-Bertalot (6.0%), Platessa hustedtii Lange-Bertalot (5.5%), and Achnanthidium minutissimum (Kitzing) Czarnecki (5.0%), and other taxa with lower abundance (less than 5%) such as Navicula capitatoradiata Ger- main, Achnanthidium rivulare Potapova & Ponader, Staurosira construens Ehren- berg, Punctastriata nyingchiensis Luo & Wang, Cocconeis placentula Ehrenberg,

PhytoKeys 259: 81-102 (2025), DOI: 10.3897/phytokeys.259.150757 86

Yun Li et al.: Four new Planothidium species

and Nitzschia palea Smith. Additional ecological information is presented in Table 1.

Remarks. Due to their similar valve outlines, Planothidium pseudoreichardtii sp. nov. and Planothidium liboensis sp. nov. were compared with several mor- phologically similar taxa, including Planothidium reichardtii Lange-Bertalot & Werum, Planothidium rostratum (@strup) Lange-Bertalot and Planothidium xinguense Morais, Wetzel & Bicudo (Table 2). However, P pseudoreichardtii sp. nov. can be easily distinguished from P liboensis sp. nov. because the former possesses a sinus, while the latter has a cavum on the center of the rapheless valve. Meanwhile, P. pseudoreichardtii can be differentiated from P reichardtii by several distinct features: (1) in LM, the central area of the raph- eless valve in P pseudoreichardtii has a broader hyaline area outlined by 3-5 striae, whereas in P reichardtii, the hyaline area is outlined by only 1-2 striae; (2) The striae on the rapheless valve of P pseudoreichardtii consist of 3-4 rows of circular areolae, with the outermost two rows being larger than the central ones, whereas P reichardtii has uniformly sized areolae in its striae; (3) the striae of P pseudoreichardtii terminates at the valve margin with a dis- tinctly enlarged areola, while P. reichardtii doesn’t have it. (4) irregular shallow slit-like depressions are present in the central and axial areas of the rapheless valve in P pseudoreicharatii, whereas no depressions were observed on the rapheless valve of P reichardtii.

Table 2. Comparison of morphological characteristics of Planothidium pseudoreichardtii sp. nov. & Planothidium liboen- sis sp. nov. and closely related taxa.

so tel gir .. Planothidium reichardtii Planothidium Rlpnounolum Planothidium xinguense pseudoreichardtii ; : rostratum (@strup) : Lange-Bertalot & Werum | liboensis sp. nov. Morais Sp. nov. Lange-Bertalot Reference This study Werum and Lange- This study Lange-Bertalot 1999; Morais et al. 2020 Bertalot 2004 Weizel et al. 2019

Valve outline Elliptical Elliptical to linear-elliptical Elliptical Elliptical Elliptical, asymmetrical Apices Rostrate Subrostrate to capitate Rostrate Short to long rostrate | Short to long rostrate Length 11.5-19.0 um 8-18 um 12-18 pm 6.5-15.0 um 12-14 um Width 55-7.5;m 4.0-6.5 um 5-7 um 4.0-6.5 um 6.7-8.2 um Central area (RV) Rectangular to Nd Rectangular to | Rectangular to slightly | Irregular, rectangular to

nearly circular nearly circular round slightly rounded Axial area (RV) Narrow linear linear Narrow linear Narrow, linear, Narrow linear

widening towards the central area Striae (RV) 14-18/10 um Nd 14-17/10 um 12-16/10 pm 12-14 um Areolae composition | 3-4 rows areolae Nd 4-5 rows areolae 3-4rows areolae 3-4 rows areolae of striae (RV) Axial area (SV) Narrow, linear to linear Narrow linear | Narrow, straight, linear | Narrow, linear, expanded lanceolate in the central area opposite to the cavum

Additional structure Sinus Sinus Cavum Cavum Cavum (SV) Striae (SV) 14-17/10 um Nd 15-16/10 um 12-14/10 pm 12-14 um Areolae composition | 3-4 rows areolae Nd 4-5 rows areolae 3-4 rows areolae 1-3 rows areolae of striae (SV)

Note: “Nd” indicates no data available in the reference.

PhytoKeys 259: 81-102 (2025), DOI: 10.3897/phytokeys.259.150757 97

Yun Li et al.: Four new Planothidium species

-

Figure 3. Planothidium liboensis sp. nov. LM; A-H. Rapheless valves; I-P. Raphe valves.

‘ . , = : 4 ~ = y = A ie = = 2 se 3 Rg ee —— wa ‘ ar F LX SO, ea er Pott Sri

Planothidium liboensis Q-M. You, P. Yu & J.P. Kociolek, sp. nov. Figs 3,4

Holotype. SHTU! Slide GZ201510051, holotype illustrated in Fig. 3F, N. Diatom samples are housed in the Lab of Algae and Environment, College of Life Sci- ences, Shanghai Normal University, China.

Isotype. COLO! Material 11117, Slides are housed in the Kociolek Collec- tion, University of Colorado, Museum of Natural History Diatom Herbarium, Boulder, U.S.A.

Type locality. CHINA. Xiaogikong Scenic Area, Libo County, Guizhou Province, 25°15'36'"N, 107°45'16'E, altitude: 780 m, collected by Wang Q.X. & Kociolek J.P.,on October 2, 2015.

Description. Light microscopy (LM) (Fig. 3A—P). Valves elliptical with slightly constricted ends and rostrate extensions. Valve dimensions (n = 75): Length 12-18 um, width 5-7 um. Rapheless Valve (SV) (Fig. 3A—H): Axial area narrow, straight, linear. Central area with a large, unilateral, horseshoe-shaped hyaline area containing a clearly visible cavum, on the opposite side, striae extend to the axial area. Striae radiate along the valve outline, 15-16 in 10 um (measured opposite the cavum). Raphe Valve (RV) (Fig. 3I-P): Axial area narrow, straight, and linear, widening slightly towards the central area. Central area irregular, rect- angular to slightly rounded, bordered by 2—3 shortened, asymmetrical striae on

indicates the different valves

any — =

of the same frustule. Q. SEM external view of an entire rapheless valve, scale bar as shown. Scale bars: 10 um (A-P).

PhytoKeys 259: 81-102 (2025), DOI: 10.3897/phytokeys.259.150757 39

Yun Li et al.: Four new Planothidium species

Figure 4. Planothidium liboensis sp. nov SEM; A. Internal view of an entire rapheless valve; B. External view of an entire raphe valve; C. Internal view of an entire raphe valve. Scale bar as shown.

each side. Raphe branches straight, with proximal raphe endings drop-like. Stri- ae radiate along the valve outline, 14-17 in 10 um.

Scanning electron microscopy (SEM) (Figs 3Q, 4A-C). Rapheless Valve (Figs 3Q, 4A): Striae composed of 4 to 5 rows of circular areolae, typically par- allel to each other. Near the central area, striae often narrow to 3 areolae adja- cent to the axial area. Striae extend onto the valve margin, and no areolae exist on the mantle. The axial area and central area exhibit irregular depressions. The cavum opening is broad, striae are distinctly wider than the virgae and

PhytoKeys 259: 81-102 (2025), DOI: 10.3897/phytokeys.259.150757 89

Yun Li et al.: Four new Planothidium species

sunken between them, areolae are covered by individual hymenes. Raphe Valve (Fig. 4B, C): Striae composed of 4 to 5 rows of circular areolae, with some striae consisting of 3 areolae near the axial area and valve margin. Striae extend onto the valve margin, and no areolae exist on the mantle. The central area typically displays 2 to 3 shortened areolae. Raphe branches are straight, with proximal raphe endings expanding into pores, surrounded by shallow, drop-like depres- sions. Distal raphe ends are curved in the same direction, briefly extending onto the mantle. Internally, proximal raphe endings are slightly deflected to the op- posite side, distal raphe endings terminate in a small helictoglossa. The central nodule is raised, striae are markedly wider than the virgae and sunken between them, and areolae are covered by individual hymenes.

Etymology. The species was named for the type locality, Libo County.

PhycoBank registration. http://phycobank.org/105528

Distribution and ecology. This species has currently only been found at its type locality, where it was collected from rocks and floating debris in ponds, as well as from rocks beneath a waterfall (water temperature 18.0-19.5 °C, elevation 629-780 m). The species was observed in samples GZ201510045 (1.0%), GZ201510051 (5.8%), and GZ201510066 (1.5%). In the type material (GZ201510051), P liboensis exhibited a relatively high abundance. The asso- ciated diatom flora included Sellaphora sp. (15.5%), Planothidium rostratum (12.5%), Fallacia sp. (5.0%), Achnanthidium minutissimum (5.0%), Nitzschia palea (5.0%), and other taxa with lower abundance (less than 5%) such as Pla- nothidium ellipticum (Cleve) Round & Bukhtiyarova, Sellaphora rotunda Wetzel, Ector, Van de Vijver, Compere & Mann, Achnanthidium spp., Punctastriata nying- chiensis, Staurosira construens, Cocconeis placentula, and Platessa hustedtii. Additional ecological information is presented in Table 1.

Remarks. In LM, P liboensis, P rostratum, and P. xinguense exhibit broadly sim- ilar valve outlines, and all display a cavum. However, P liboensis can be distin- guished from other taxa by the following features: (1) P. liboensis has a denser arrangement of striae, typically consisting of 4-5 rows of similarly-sized round areolae, and the striae are distinctly wider than the virgae. In contrast, the striae of P rostratum in the rapheless valve consist of 3-4 rows of areolae, where the two middle rows are smaller than the outer rows. The striae of P xinguense in raph- eless valve are composed of 1-3 rows of areolae; (2) The central area of raph- eless valve in P liboensis exhibits distinctive irregularly shaped shallow depres- sions, whereas P rostratum features shallow, slit-like depressions. P xinguense also exhibits irregularly shaped shallow depressions in the central area, but their shape differs from those in P liboensis. Additionally, P xinguense has slit-like de- pressions in the axial area; (3) compared to P rostratum, the cavum of P liboensis has a more open aperture; (4) compared to P. xinguense, P. liboensis exhibits weaker striae radiation and a narrower central area on the rapheless valve.

Planothidium angustirostratum Q-M. You, P. Yu & J.P. Kociolek, sp. nov. Figs 5, 6

Holotype. SHTU! Slide GZ201510051, holotype illustrated in Fig. 5F, N. Diatom samples are housed in the Lab of Algae and Environment, College of Life Sci- ences, Shanghai Normal University, China.

PhytoKeys 259: 81-102 (2025), DOI: 10.3897/phytokeys.259.150757 90

Yun Li et al.: Four new Planothidium species

Sf wie

“aw = —

Figure 5. Planothidium angustirostratum sp. nov. LM; A-H. Rapheless valves; I-P. Raphe valves. “=” indicates the different valves of the same frustule. Q. SEM external view of an entire rapheless valve, scale bar as shown. Scale bars: 10 um (A-P).

Isotype. COLO! Material 11117, Slides are housed in the Kociolek Collection, Uni- versity of Colorado, Museum of Natural History Diatom Herbarium, Boulder, U.S.A.

Type locality. CHINA. Xiaogikong Scenic Area, Libo County, Guizhou Province, 25°15'36'"N, 107°45'16'E, altitude: 780 m, collected by Wang Q.X. & Kociolek J.P., on October 2, 2015.

Description. Light microscopy (LM) (Fig. 5A-P). Valves elliptic-lanceolate with narrowly rostrate to subcapitate apices. Valve dimensions (n = 40): Valve length 17-21 um, width 6.5-7.0 um. Rapheless Valve (Fig. 54—H): Axial area narrow, linear, slightly broadened in the middle. A horseshoe-shaped hyaline area present on one side of the central area, containing a cavum. Striae weakly radiate along the valve outline, 13-14 in 10 um (measured opposite the hyaline area). Raphe Valve (Fig. 5I-P): Axial area narrow, straight, linear. Central area subcircular to rectangular, bordered by 3-4 slightly shortened striae on each side. Raphe branches straight, with proximal raphe endings drop-like. Striae ra- diate along the valve outline, 13-14 in 10 um.

Scanning electron microscopy (SEM) (Figs 5Q, 6A-C). Rapheless Valve (Figs 5Q, 6A): Striae composed of 3-4 rows of circular areolae, narrowing near the central area adjacent to the axial area. Striae extend onto the valve mantle. Irregular circular depressions present between the axial area, central area, even

PhytoKeys 259: 81-102 (2025), DOI: 10.3897/phytokeys.259.150757 91

Yun Li et al.: Four new Planothidium species

s / Px aN ee es: atte Figure 6. Planothidium angustirostratum sp. nov. SEM; A. Internal view of an entire rapheless valve; B. External view of an entire raphe valve; C. Internal view of an entire raphe valve. Scale bar as shown.

between striae. Internally, areolae covered individually, striae wider than virgae and sunken between them. Cavum opening slightly constricted. Raphe Valve (Fig. 6B, C): Striae composed of 3-4 rows of areolae, striae near the central area regularly shortened and narrowing towards the axial area. Striae extend onto the valve mantle. Raphe branches straight, with proximal raphe endings expanding into pores surrounded by drop-like depressions. Distal raphe ends are curved in the same direction, slightly extending onto the valve mantle. Inter- nally, proximal raphe endings slightly deflected to the opposite side, and distal raphe endings terminating in a faint helictoglossa. Internally, areolae covered individually, striae wider than virgae and sunken between them.

Etymology. The species was named for its narrowly rostrate valve apices.

PhycoBank registration. http://phycobank.org/105529

PhytoKeys 259: 81-102 (2025), DOI: 10.3897/phytokeys.259.150757 99

Yun Li et al.: Four new Planothidium species

Distribution and ecology. This species has currently only been found at its type locality, where it was collected from floating debris in a pond (water temperature 18.0 °C, elevation 780 m). The species was observed only insample (GZ201510051) at a low relative abundance (0.75%). In the type material, P angustirostratum was rare. The associated diatom flora included Sellaphora sp. (15.5%), Planothidium ros- tratum (12.5%), Planothidium liboensis (5.75%), Fallacia sp. (5.0%), Achnanthidium minutissimum (5.0%), Nitzschia palea (5.0%), and other taxa with lower abundance (less than 5%) such as Planothidium ellipticum, Sellaphora rotunda, Achnanthidium spp., Punctastriata nyingchiensis, Staurosira construens, Cocconeis placentula, and Platessa hustedtii. Additional ecological information is presented in Table 1.

Remarks. Due to the broadly similar valve outline in LM and the presence of a cavum, Planothidium angustirostratum sp. nov. can be compared with Planothidium brasiliense Wetzel & Blanco, Planothidium potapovae Wetzel & Ector and Planothid- ium rostratoholarcticum Lange-Bertalot & Bak (Table 3). In addition to differences in striae density and valve length and width, Planothidium angustirostratum sp. nov. also can be distinguished from these similar species by the following features: (1) with narrower rostrate to subcapitate apices and a narrower cavum aperture; (2) a smaller central area, ranging from rectangular to nearly circular; (3) its striae denser on the raphe valve, consist of 3-4 rows of circular areolae; (4) circular shal- low depressions are present in the central area, axial area, and between the striae, unlike the slit-like depressions observed in P potapovae and P rostratoholarcticum.

Table 3. Comparison of morphological characteristics of Planothidium angustirostratum sp. nov. and closely related taxa.

Planothidium angustirostratum sp. nov.

This study

- Planothidium Planothidium brasiliense Planothidium potapovae ree e : rostratoholarcticum Lange- Weizel & Blanco Weitzel & Ector Bertalot & Bak

Wetzel et al. 2019 Weitzel et al. 2019 Bak and Lange-Bertalot

2014; Wetzel et al. 2019

Elliptic-lanceolate Lanceolate, elliptic- Lanceolate to broadly Elliptical lanceolate elliptic-lanceolate Apices Narrowly rostrate to Rostrate Strongly rostrate Subrostrate subcapitate

Length 17-21 um 20-28 um 11.5-14.5 um 6.4-13.5 um Width 6.5-7.0 um 7.0-8.0 um 5.0-6.0 um 3.8-6.4 um Central area (RV) Subcircular to rectangular Rectangular Wide rectangular Irregular Axial area (RV) Narrow linear Narrow linear Wide rectangular Narrow, linear Striae near to central | 3-4 shortened striae on 2-3 shortened striae on 1-3 shortened striaeon | Usually 1 shortened striae area (RV) each side each side each side on each side Raphe Straight Straight Straight Straight Striae (RV) 13-14/10 um Nd Nd 14-17/10 pm Areolae composition 3-4 rows areolae Nd 4 rows areolae 2-3 rows areolae of striae (RV)

Axial area (SV) Narrow linear

Additional structure Cavum

(SV)

Striae (SV) 13-14/10 um Areolae composition 3-4 rows areolae of striae (SV)

Narrow, linear, expanded in the central area

Cavum

15-17/10 ym Nd

Note: “Nd” indicates no data available in the reference.

PhytoKeys 259: 81-102 (2025), DOI: 10.3897/phytokeys.259.150757

Linear—lanceolate,widening in the central area*

Cavum

12-13/10 ym

3-4 rows areolae

Narrow, straight, linear

Cavum

13-17/10 pm

2-3 rows areolae

93

Yun Li et al.: Four new Planothidium species

Planothidium maolanensis Q-M. You, P. Yu & J.P. Kociolek, sp. nov. Figs 7,8

Holotype. SHTU! Slide GZ201510099, holotype illustrated in Fig. 7D, L. Diatom samples are housed in the Lab of Algae and Environment, College of Life Sci- ences, Shanghai Normal University, China.

Isotype. COLO! Material 11165, Slides are housed in the Kociolek Collection, University of Colorado, Museum of Natural History Diatom Herbarium, Boulder, U.S.A.

Type locality. CHINA. Maolan Nature Reserve, Libo County, Guizhou Province, 25°17'32'"N, 108°04'16'E, altitude: 650 m, collected by Kociolek J.P. & Wang Q.X., on October 4, 2015.

Description. Light microscopy (LM) (Fig. 7A-P). Valves elliptical, with round- ed ends not extended. Valve dimensions (n = 50): Length 9.5-18.0 um, width 5.5-7.5 um. Rapheless Valve (Fig. 7A—-H): Axial area linear-lanceolate, with central area slightly broadened. One side of the central area has two striae with

Wii

iii >»

TMT Au TT\\\ oe -

7 ahi

~ = ~~ — _— -~ - _ — -_ = = =

TTP

rere

Figure 7. Planothidium maolanensis sp. nov. LM; A-H. Rapheless valves; I-P. Raphe valves. “=” indicates the different valves of the same frustule. Q. SEM external view of an entire rapheless valve, scale bar as shown. Scale bars: 10 um (A-P).

PhytoKeys 259: 81-102 (2025), DOI: 10.3897/phytokeys.259.150757 94

Yun Li et al.: Four new Planothidium species

Figure 8. Planothidium maolanensis sp. nov SEM; A. Internal view of an entire rapheless valve. The white arrow points to a distinctly wider virgae; B. External view of an entire raphe valve. White arrows indicate the areolae on the mantle. C. Internal view of an entire raphe valve. Scale bar as shown.

widened spacing, opposite side striae slightly shortened. Striae weakly radiate along the valve outline, narrowing towards axial area, 10-12 in 10 um (mea- sured opposite hyaline area). Raphe Valve (Fig. 7I-P): Axial area linear-lance- olate, slightly widened in the central area. Central area subcircular, bordered by 2-3 slightly shortened striae on each side. One side of the central area has two striae with widened spacing. Striae radiate weakly, 10-12 in 10 um. Raphe branches straight, occasionally slightly curved.

PhytoKeys 259: 81-102 (2025), DOI: 10.3897/phytokeys.259.150757 95

Yun Li et al.: Four new Planothidium species

Scanning electron microscopy (SEM) (Figs 7Q, 8A-C). Rapheless Valve (Figs 7Q, 8A): Striae composed of 4-5 rows of circular areolae, markedly narrow near the axial area, reducing to 2-3 rows of areolae, striae almost extend onto the valve margin, and some areolae exist on the mantle. Irregular depressions pres- ent on the axial area, central area and even on the virgae. Internally, areolae covered. Virgae widen gradually from the ends towards the center, with striae sunken between virgae, a distinctly wider virgae present on one side of the cen- tral area (Fig. 8A, White arrow). Raphe Valve (Fig. 8B, C): Striae composed of 5-6 rows of small circular areolae, markedly narrowing near the central area, reducing to 2-3 rows of areolae. striae don't extend onto the valve margin, and some areolae exist on the mantle (Fig. 8B, White arrows). Raphe branches straight, occasionally slightly curved. Externally, proximal raphe endings expand into pores, distal raphe ends are curved in the same direction, and extend onto the mantle. Internally, proximal raphe endings slightly deflected to the opposite side, and distal raphe endings terminate in a small helictoglossa. Internally, are- olae covered individually by membranes, striae sunken between virgae.

Etymology. The species was named for the Maolan Nature Reserve, where the type specimen was collected.

PhycoBank registration. http://phycobank.org/105530

Distribution and ecology. This species has currently only been found at its type locality, where it was collected from rocks in a subsurface stream and in rapidly flowing river water (water temperature 18.0 °C, elevation 650-811 m). The species was observed in samples GZ201510099 (3.75%), GZ201510100 (0.25%), and GZ2015108 (0.75%). In the type material (GZ201510099), P maolanensis was rare. The associated diatom flora included Cocconeis pla- centula (23.0%), Achnanthidium delmontii (7.5%), Navicula Iundii (7.5%), Navicula antonii Lange-Bertalot, Navicula sp. (6.0%), Achnanthidium minutissimum (5.5%), and other taxa with lower abundance (less than 5%) such as Platessa hustedtii, Achnanthidium spp., Encyonema hophense Krammer, and Achnanthes prominula Levkov & Tofilovska. Additional ecological information is presented in Table 1.

Remarks. Due to the broadly similar valve outline in LM and the absence of cavum, Planothidium maolanensis sp. nov. can be compared with P hauckianum (Grunow) Bukhtiyarova, P kaetherobertianum Van de Vijber & Bosak, and P. iber- ense Rovira & Witkowski (Table 4). P maolanensis can be easily distinguished from P. kaetherobertianum and P. iberense by valve outline and striae density, while it shows greater overall similarity to P hauckianum. However, P maolan- ensis can still be differentiated by the following features: (1) In PR maolanensis, only two striae on one side of the central area show slightly increased spacing, while the remaining striae are evenly arranged. In contrast, P hauckianum has more striae with increased spacing on both sides of the central area. (2) The striae of PR maolanensis consist of 4-5 rows of circular areolae on rapheless valve and 5-6 rows on raphe valve. In P hauckianum, the striae consist of 3-4 rows of circular areolae on the rapheless valve and 2-4 rows on the raphe valve. (3) Although the striae in P maolanensis and P hauckianum both gener- ally narrow toward the axial area, the narrowing in P maolanensis is not gradual or consistent. Sometimes, the middle of striae contracts, making it narrower than the ends near the axial area. (4) P maolanensis has irregularly shaped shallow depressions in axial area and between the striae, while P hauckianum exhibits slit-like depressions.

PhytoKeys 259: 81-102 (2025), DOI: 10.3897/phytokeys.259.150757 96

Yun Li et al.: Four new Planothidium species

Table 4. Comparison of morphological characteristics of Planothidium maolanensis sp. nov. and closely related taxa.

Reference

Valve outline Apices

Length

Width

Central area (RV) Axial area (RV) Raphe

Striae (RV)

Areolae composition of striae (RV)

Axial area (SV)

Additional structure (SV)

Striae (SV)

Areolae composition of striae (SV)

Planothidium maolanensis | Planothidium hauckianum

sp. nov. (Grunow) Bukhtiyarova This study Kulas et al. 2020 Elliptical Linear-lanceolate or elliptic- lanceolate Rounded ends not Very weakly protracted,

extended slightly rostrate to broadly rounded in smaller valves 9.5-18.0 um 7-30 um §.5=7-5.0m 4.5-7.0 um Subcircular Asymmetrical

Narrow linear Narrow, linear Straight to slightly curved 10-12/10 um

5-6 rows areolae

Straight to weakly curved 8-10/10 um

2-4 rows areolae Narrow linear Narrow linear Absent Absent

10-12/10 ym

4-5 rows areolae

7-11/10 um

3-4 rows areolae

Planothidium

kaetherobertianum Van de

Vijber & Bosak

Van De Vijver and Bosak

2019

lanceolate to elliptic- lanceolate in smaller valves with cuneately rounded

Only very weakly protracted

apices

7-13 pm 4.0-4.5 um

Rounded to rectangular

Very narrow, linear Straight 16-17/10 um

2-3 rows areolae

Linear to narrowly lanceolate

Absent

15-16/10 um

2 rows areolae

Planothidium iberense Rovira & Witkowski

Rovira et al. 2011

Ellipticallanceolate

Moderately produced, obtusely rounded

17-26 um 6.5-9.5 um Circular Narrow, linear Straight 12-15/10 pm

4 rows areolae

Narrow, linear

Absent

14-16/10 pm

3-4 rows areolae

Discussion

The four new species described in this study exhibit multiseriate striae on both the raphe valve and the rapheless valve. On one side of the central area of the rapheless valve, Planothidium pseudoreichardtii sp. nov. features an uncovered depression known as a “sinus”, Planothidium liboensis sp. nov. and Planothid- ium angustirostratum sp. nov. possess a cavum, and Planothidium maolanensis sp. nov. lacks both sinus and cavum. The four new species were compared with several related and morphologically similar taxa (Tables 2, 3, 4).

As acomplement to morphometry, Wetzel et al. (2019) proposed several dis- tinguishing features in SEM, such as the pattern of valve surface smoothness on the rapheless valve, the number of areolae per stria, and whether the striae on the rapheless valve are interrupted towards the valve mantle. During the obser- vation of these new Planothidium species, we found that the morphology of the striae and the number of areolae rows per stria on the rapheless valve are rela- tively stable within the same species. For instance, the rapheless valve striae of P. pseudoreichardtii sp. nov. are composed of two outer rows of larger areolae and 1-2 smaller rows in the center, with an enlarged areola present at the end of each stria. The presence or absence of striae on the rapheless valve mantle is also stable within the same species. Striae are present on the valve mantle of P angustirostratum sp. nov. and P maolanensis sp. nov., whereas no striae are observed on the valve mantle of P liboensis sp. nov. and P maolanensis sp.

PhytoKeys 259: 81-102 (2025), DOI: 10.3897/phytokeys.259.150757 97

Yun Li et al.: Four new Planothidium species

nov. Furthermore, we observed that the feature of valve surface smoothness on the rapheless valve is consistent within each species. Based on the shape of the depressions, these features can be roughly classified into three types: P. an- gustirostratum sp. nov. exhibits circular shallow depressions on the valve sur- face; P. liboensis sp. nov. and P maolanensis sp. nov. display irregularly shaped depressions; while P pseudoreichardtii sp. nov. and P. rostratum, also observed in the Xiaoqgikong Scenic Area, exhibit slit-like shallow depressions.

Currently, numerous species of the genus Planothidium have been discov- ered in various aquatic environments and habitats worldwide (Blanco et al. 2013; N’Guessan et al. 2014; Wetzel and Ector 2014a, 2014b; Kopalova et al. 2016; Riaux-Gobin et al. 2018; Stancheva 2019; Cantonati et al. 2021; Lai et al. 2021). Additionally, some studies have investigated the ecological distribution and physiological characteristics of Planothidium species (Sbihi et al. 2014; Stancheva et al. 2020; Juchem et al. 2023). Despite significant advancements over the years, the taxonomy of Planothidium is relatively well-developed, but several questions remain open for further exploration. Future research may re- quire additional populations or increased taxa sampling to examine the stabili- ty of certain morphological features within the genus. Furthermore, integrating more ecological data and molecular techniques could provide deeper insights into the variability of these features under different environmental conditions and their taxonomic applicability.

Acknowledgements

We would like to thank Dr. Lang Li for assistance in the preparation of literature. We also thank the editor and reviewers for their valuable comments.

Additional information

Conflict of interest

The authors have declared that no competing interests exist.

Ethical statement

No ethical statement was reported.

Funding

This research was supported by the National Natural Science Foundation of China (Nos. 32170205 & 32100165), Natural Science Foundation of Shanghai (No. 23ZR1446800).

Author contributions

Data curation: HWZ. Formal analysis: YL. Funding acquisition: QMY. Investigation: HWZ, QXW, PY, JPK, LXL. Methodology: QMY. Project administration: QXW, WTP. Resources: WTP, QXW. Validation: PY, QMY, YL, LXL. Writing — original draft: YL. Writing - review and editing: JPK, QMY.

Author ORCIDs

Yun Li © https://orcid.org/0009-0005-1251-7941 Lin-Xin Lu © https://orcid.org/0009-0004-5919-2083 Hui-Wen Zhou ® https://orcid.org/0009-0004-3926-4887

PhytoKeys 259: 81-102 (2025), DOI: 10.3897/phytokeys.259.150757 98

Yun Li et al.: Four new Planothidium species

Pan Yu © https://orcid.org/0000-0001-7937-2381

John Patrick Kociolek © https://orcid.org/0000-0001-9824-7164 Wan-Ting Pang © https://orcid.org/0000-0002-4231-4046 Quan-Xi Wang ® https://orcid.org/0000-0002-0317-6613 Qing-Min You ® https://orcid.org/0000-0002-5538-8503

Data availability

All of the data that support the findings of this study are available in the main text.

References

Alvarez-Blanco |, Blanco S (2013) Planothidium galaicum sp. nov. (Bacillariophyta, Achnanthidiaceae), a new diatom species from Galician coast, Spain. Phytotaxa 151: 44. https://doi.org/10.11646/phytotaxa.151.1.4

Bak M, Lange-Bertalot H (2014) Four small-celled Planothidium Species from Central Europe proposed as new to science. Oceanological and Hydrobiological Studies 43: 346-359. https://doi.org/10.2478/s13545-014-0152-9

Blanco S, Alvarez-Blanco |, Cejudo-Figueiras C, Espejo JMR, Barrera CB, Bécares E, Del Olmo FD, Artigas RC (2013) The diatom flora in temporary ponds of Dofiana Nation- al Park (southwest Spain): Five new taxa. Nordic Journal of Botany 31: 489-499. https://doi.org/10.1111/j.1756-1051.2013.01691.x

Borrego-Ramos M, Blanco §, Olenici A (2018) Diatoms from the valporquero cave (Leon, NW Spain), with the description of Germainiella legionensis sp. nov. Journal of Caves and Karst Studies 80: 181-189. https://doi.org/10.4311/2017MB0128

Cantonati M, Bilous O, Angeli N, Van Wensen L, Lange-Bertalot H (2021) Three new dia- tom species from spring habitats in the Northern Apennines (Emilia-Romagna, Italy). Diversity 13: 549. https://doi.org/10.3390/d13110549

Guiry MD, Guiry GM (2024) AlgaeBase. World-wide electronic publication, National Univer- sity of Ireland, Galway. https://www.algaebase.org [Searched on 05 December 2024]

Jahn R, Abarca N, Gemeinholzer B, Mora D, Skibbe O, Kulikovskiy M, Gusev E, Kusber W-H, Zimmermann J (2017) Planothidium lanceolatum and Planothidium frequentissi- mum reinvestigated with molecular methods and morphology: Four new species and the taxonomic importance of the sinus and cavum. Diatom Research 32: 75-107. https://doi.org/10.1080/0269249X.2017.1312548

Juchem DP, Schimani K, Holzinger A, Permann C, Abarca N, Skibbe O, Zimmermann J, Graeve M, Karsten U (2023) Lipid degradation and photosynthetic traits after pro- longed darkness in four Antarctic benthic diatoms, including the newly described species Planothidium wetzelii sp. nov. Frontiers in Microbiology 14: 1241826. https:// doi.org/10.3389/fmicb.2023.1241826

Kociolek JP. You QM, Stepanek J, Lowe RL, Wang QX (2016a) A new Eunotia (Bacillario- phyta: Eunotiales) species from Karst formations of southern China. Phytotaxa 265: 285. https://doi.org/10.11646/phytotaxa.265.3.10

Kociolek JP. You QM, Stepanek JG, Lowe RL, Wang QX (2016b) New freshwater dia- tom genus, Edtheriotia gen. nov. of the Stephanodiscaceae (Bacillariophyta) from south-central China. Phycological Research 64: 274-280. https://doi.org/10.1111/ pre.12145

Kociolek JP You QM, Lou F, Yu P Lowe RL, Wang QX (2019) First report and new freshwa- ter species of Germainiella (Bacillariophyta) from the Maolan Nature Reserve, Guizhou Province, China. Phytotaxa 393: 35. https://doi.org/10.11646/phytotaxa.393.1.3

PhytoKeys 259: 81-102 (2025), DOI: 10.3897/phytokeys.259.150757 99

Yun Li et al.: Four new Planothidium species

Kopalova K, Zidarova R, Van De Vijver B (2016) Four new monoraphid diatom species (Bacillariophyta, Achnanthaceae) from the Maritime Antarctic Region. European Journal of Taxonomy 217: 1-19. https://doi.org/10.5852/ejt.2016.217

Kulas A, Gligora Udovié M, Ector L, Van De Vijver B (2020) Analysis of the type material of Achnanthes hauckiana Grunow (Achnanthaceae, Bacillariophyceae). Botany Letters 167: 439-452. https://doi.org/10.1080/23818107.2020.1808527

Kulikovskiy MS, Andreeva SA, Gusev ES, Kuznetsova IV, Annenkova NV (2016) Molecular phylogeny of monoraphid diatoms and raphe significance in evolution and taxonomy. The Biological Bulletin 43: 398-407. https://doi.org/10.1134/S1062359016050046

Lai GG, Ector L, Luglié A, Sechi N, Wetzel CE (2018) Sellaphora gologonica sp. nov. (Bacil- lariophyta, Sellaphoraceae), a new diatom species from a Mediterranean karst spring (Sardinia, Italy). Phytotaxa 356: 145. https://doi.org/10.11646/phytotaxa.356.2.4

Lai GG, Ector L, Padedda BM, Wetzel CE (2021) Planothidium marganaiensis sp. nov. (Bacillariophyta), a new cavum-bearing species from a karst spring in south-western Sardinia (Italy). Phytotaxa 489: 140-154. https://doi.org/10.11646/phytotaxa.489.2.3

Lange-Bertalot H (1999) Neue Kombinationen von Taxa aus Achnanthes Bory (sensu lato). In: Lange-Bertalot H (Ed.) Iconographia Diatomologica, Vol. 6: Phytogeogra- phy-Diversity- Taxonomy. Koeltz Scientific Books, K6nigstein, Germany, 276-289.

Li YH, Nagumo T, Xu KD (2022) Morphology and molecular phylogeny of Amphora baot- uensis sp. nov., anew freshwater benthic diatom from a karst spring in China. Diatom Research 37: 145-153. https://doi.org/10.1080/0269249X.2022.2063390

Li L, Huang YY, Nong QZ, Lai JX, Li YH (2024) Planothidium pseudolinkei sp. nov. (Bacil- lariophyta), a new marine monoraphid diatom species from the coast of Guangxi, China. PhytoKeys 246: 237-249. https://doi.org/10.3897/phytokeys.246.128068

Liu Y, Fan YW, Wang QX (2015) Study on the Achnanthoid Diatoms from the Great Xing’an Moutains. Acta Hydrobiologica Sinica 39: 554-563.

Liu Y, Kociolek JP, Wang QX, Fan YW (2016) Newly recorded of freshwater monoraphid diatom from Hainan Island, China. Acta Hydrobiologica Sinica 40: 1266-1277.

Lowe R, Kociolek J, You QM, Wang QX, Stepanek J (2017) Diversity of the diatom genus Humidophila in karst areas of Guizhou, China. Phytotaxa 305: 269-284.

Morais KS, Costa LF, Bicudo CEDM, Ector L, Wetzel CE (2020) A new Planothidium spe- cies (Achnanthidiaceae, Bacillariophyceae) from Xingu Ria, Amazon River basin, Brazil. Phytotaxa 477: 194-204. https://doi.org/10.11646/phytotaxa.477.2.4

Morales EA (2006) Small Planothidium Round Et Bukhtiyarova (Bacillariophyceae) taxa related to P. daui (foged) Lange-Bertalot from the United States. Diatom Research 21: 325-342. https://doi.org/10.1080/0269249X.2006.9705673

N’Guessan KR, Wetzel CE, Ector L, Coste M, Cocquyt C, Van De Vijver B, Yao SS, Ouattara A, Kouamelan EP, Tison-Rosebery J (2014) Planothidium comperei sp. nov. (Bacillar- iophyta), a new diatom species from Ivory Coast. Plant Ecology and Evolution 147: 455-462. https://doi.org/10.5091/plecevo.2014.981

Novis PM, Braidwood J, Kilroy C (2012) Small diatoms (Bacillariophyta) in cultures from the Styx River, New Zealand, including descriptions of three new species. Phytotaxa 64: 11. https://doi.org/10.11646/phytotaxa.64.1.3

Riaux-Gobin C, Witkowski A, Igersheim A, Lobban CS, Al-Handal AY, Compére P (2018) Planothidium juandenovense sp. nov. (Bacillariophyta) from Juan de Nova (Scattered Islands, Mozambique Channel) and other tropical environments: A new addition to the Planothidium delicatulum complex. Fottea 18: 106-119. https://doi.org/10.5507/ fot.2017.019

PhytoKeys 259: 81-102 (2025), DOI: 10.3897/phytokeys.259.150757 100

Yun Li et al.: Four new Planothidium species

Round FE, Bukhtiyarova L (1996) Four new genera based on Achnanthes (Achnanthidium) together with a re-definition of Achnanthidium. Diatom Research 11: 345-361. https:// doi.org/10.1080/0269249X.1996.9705389

Rovira L, Witkowski A, Trobajo R, Ruppel M, Ibafiez C (2011) Planothidium iberense sp. nov., a new brackish diatom of the Ebro Estuary, northeast Spain. Diatom Research 26: 99-107. https://doi.org/10.1080/0269249X.2011.587645

Sbihi K, Cherifi O, Bertrand M, El Gharmali A (2014) Biosorption of metals (Cd, Cu and Zn) by the freshwater diatom Planothidium lanceolatum: A laboratory study. Diatom Research 29: 55-63. https://doi.org/10.1080/0269249X.2013.872193

S6mek H, Hamilton PB, Solak CN, Beauger A, Yilmaz E, Sevindik TO (2025) A new diatom (Bacillariophyta) species—Fallacia cinariana sp. nov.—from Kaklik Cave in the West- ern Anatolian Karst Region, Republic of Turkiye. Phytotaxa 694: 173-183. https://doi. org/10.11646/phytotaxa.694.2.5

Stancheva R (2019) Planothidium sheathii,a new monoraphid diatom species from rivers in California, USA. Phytotaxa 393: 131. https://doi.org/10.11646/phytotaxa.393.2.4

Stancheva R, Kristan NV, Kristan Ill WB, Sheath RG (2020) Diatom genus Planothidium (Bacillariophyta) from streams and rivers in California, USA: Diversity, distribution and autecology. Phytotaxa 470: 1-30. https://doi.org/10.11646/phytotaxa.470.1.1

Van De Vijver B, Bosak S (2019) Planothidium kaetherobertianum, a new marine diatom (Bacillariophyta) species from the Adriatic Sea. Phytotaxa 425: 105-112. https://doi. org/10.11646/phytotaxa.425.2.5

Van De Vijver B, Wetzel C, Kopalova K, Zidarova R, Ector L (2013) Analysis of the type material of Achnanthidium lanceolatum Brébisson ex Kiitzing (Bacillariophyta) with the description of two new Planothidium species from the Antarctic Region. Fottea 13: 105-117. https://doi.org/10.5507/fot.2013.010

Van De Vijver B, Wetzel CE, Ector L (2018) Analysis of the type material of Planothid- ium delicatulum (Bacillariophyta) with the description of two new Planothidium spe- cies from the sub-Antarctic Region. Fottea 18: 200-211. https://doi.org/10.5507/ fot.2018.006

Wang YL, Yu P, Cao Y, Wang QX, You QM (2019) New records of species of Achnanthi- diaceae (Bacillariophyta) in Ganzi, Sichuan, China. Zhiwu Kexue Xuebao 37: 10-17. https://doi.org/10.11913/PSJ.2095-0837.2019.10010

Werum M, Lange-Bertalot H (2004) Diatoms in Springs from Central Europe and elsewhere under the influence of hydrologeology and anthropogenic impacts. In: Lange-Bertalot H (Ed.) Iconographia Diatomologica 13. A.R.G. Gantner Verlag K.G, Ruggell, 3-417.

Wetzel CE, Ector L (2014a) Planothidium lagerheimii comb. nov. (Bacillariophyta, Achnanthales) a forgotten diatom from South America. Phytotaxa 188: 261. https:// doi.org/10.11646/phytotaxa.188.5.3

Wetzel CE, Ector L (2014b) Taxonomy, distribution and autecology of Planothidium ba- gualensis sp. nov. (Bacillariophyta) a common monoraphid species from southern Brazilian rivers. Phytotaxa 156: 201. https://doi.org/10.11646/phytotaxa.156.4.2

Wetzel CE, Van De Vijver B, Blanco S, Ector L (2019) On some common and new cavum-bearing Planothidium (Bacillariophyta) species from freshwater. Fottea 19: 50-89. https://doi.org/10.5507/fot.2018.016

You QM, Kociolek JP, Cai MJ, Lowe RL, Liu Y, Wang QX (2017) Morphology and ultra- structure of Sellaphora constrictum sp. nov. (Bacillariophyta), a new diatom from southern China. Phytotaxa 327: 261. https://doi.org/10.11646/phytotaxa.327.3.5

PhytoKeys 259: 81-102 (2025), DOI: 10.3897/phytokeys.259.150757 101

Yun Li et al.: Four new Planothidium species

You QM, Yu P, Kociolek JP Wang YL, Luo F, Lowe R, Wang QX (2019a) A new species of Achnanthes (Bacillariophyceae) from a freshwater habitat in a karst landform from south-central China. Phycological Research 67: 303-310. https://doi.org/10.1111/ pre.12381

You QM, Cao Y, Yu P, Kociolek JP, Zang LX, Wu B, Lowe R, Wang QX (2019b) Three new subaerial Achnanthidium (Bacillariophyta) species from a karst landform in the Guizhou Province, China. Fottea 19: 138-150. https://doi.org/10.5507/fot.2019.005

You QM, Zhao K, Wang YL, Yu P. Kociolek JP. Pang WT, Wang QX (2021) Four new spe- cies of monoraphid diatoms from Western Sichuan Plateau in China. Phytotaxa 479: 257-274. https://doi.org/10.11646/phytotaxa.479.3.3

Yu P, You QM, Kociolek JP Lowe R, Wang QX (2017) Nupela major sp. nov., a new diatom species from Maolan Nature Reserve, central-south of China. Phytotaxa 311: 245. https://doi.org/10.11646/phytotaxa.311.3.4

Zhou HW, Yu P, Guo LY, Kociolek JP Wang QX, You QM (2024) Cocconeis crisscrossis sp. nov., anew monoraphid diatom (Bacillariophyta) from southern China. PhytoKeys 242: 39-50. https://doi.org/10.3897/phytokeys.242.123316

PhytoKeys 259: 81-102 (2025), DOI: 10.3897/phytokeys.259.150757 102