Journal of Oceanology and Limnology   2022, Vol. 40 issue(2): 712-728     PDF       
http://dx.doi.org/10.1007/s00343-021-0470-x
Institute of Oceanology, Chinese Academy of Sciences
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Article Information

YANG Lin, YU Pan, YOU Qingmin, LI Guisheng, WANG Quanxi
Morphological and phylogenetic analysis of a new Melosira species and revision of freshwater Melosira in China
Journal of Oceanology and Limnology, 40(2): 712-728
http://dx.doi.org/10.1007/s00343-021-0470-x

Article History

Received Dec. 11, 2020
accepted in principle Jan. 26, 2021
accepted for publication Apr. 8, 2021
Morphological and phylogenetic analysis of a new Melosira species and revision of freshwater Melosira in China
Lin YANG1,2, Pan YU2,3, Qingmin YOU2, Guisheng LI1, Quanxi WANG2     
1 College of Environmental and Geographical Sciences, Shanghai Normal University, Shanghai 200235, China;
2 College of Life Sciences, Shanghai Normal University, Shanghai 200235, China;
3 Institute of Hydrobiology, Chinese Academy of Sciences, Wuhan 430000, China
Abstract: Melosira is one of the most common diatom genera found in freshwater ecosystems. There are many freshwater species of Melosira, of which M. varians is the most common. In our investigation of periphytic diatoms in the Jinsha River, China, a new species, M. capsularum sp. nov, was characterized in combined morphological and molecular approaches. M. varians was also analyzed using molecular data. The new species is similar to M. varians, M. moniliformis, M. nummuloides, and M. lineata in morphology. The cells are capsular and join to filaments by mucilage pads that are secreted on the valve face and united into pairs by their cingula. The valve face is domed and covered with small granules, and the valve mantle edge has a milled appearance. The two strains of M. capsularum are in a single clade obviously away from other Melosira species, as determined in phylogenetic analysis based on nuclear small subunit (SSU) rDNA sequences and the chloroplast-encoded rbcL gene. Thus, the results of morphological comparisons and phylogenetic analysis based on molecular data provide strong evidence that M. capsularum is a new species, thereby increasing the total number of recognized freshwater diatom species in China. In addition, we have systematically reclassified the freshwater Melosira that have been recorded in China.
Keywords: Melosira    morphology    molecular phylogeny    new species    revision    
1 INTRODUCTION

During the identification of periphytic diatoms in the Jinsha River, China, we discovered a new species, Melosira capsularum sp. nov. When identifying this new species, we were confused in taxonomy concerning freshwater Melosira in China, and thus it seemed necessary to clarify the taxonomic status of the genus. Melosira was established by Agardh (1824), with M. nummuloides C. Agardh as the type species. M. nummuloides was described as having subspherical cells forming moniliform filamentous diatom communities. Since then, species that possess these characteristics have been placed in the genus Melosira. The genus comprised more than 800 species and varieties, including both freshwater and marine species, and other varieties (Guiry and Guiry, 2020). In China, a total of 48 taxa of freshwater Melosira have been reported (Qi, 1995).

In the 1970s, it was proposed by some studies (Florin, 1970; Ross and Sims, 1973; Gasse, 1975; Crawford, 1975a, 1979) that M. granulata and species conforming to its characteristics should be separated from Melosira as a separate genus, as they were not in accordance with the original description; the characteristics of the type species specified that the mantle should be highly ornamented and that the cells should often form colonies. Moreover, depending on the species, cells may be joined by linking spines. On which genera the separated species should be classified into, there is disparity among researchers. The genus Aulacoseira was established by Thwaites (1848), with A. crenulata (Ehrenberg) Thwaites as the type species. The genus was characterized as having cylindrical cells with siliceous cell walls and cells connected by small spines forming straight, curved, or even coiled filaments. However, since its establishment in 1848, Aulacoseira has largely been ignored by researchers and has long been in a state of disuse. It was not until 1979 that Simonsen resurrected the genus name and associated it with 59 common species, including M. distans (Ehrenberg) Kützing, M. granulata Ehrenberg, and M. italica (Ehrenberg) Kützing (Simonsen, 1979). Subsequently, some researchers successively revised species within Melosira that conformed to the characteristics of Aulacoseira (Hartley et al., 1986; Haworth, 1990; Krammer, 1991; Trifonova and Genkal, 2001; Houk, 2010; Ognjanova-Rumenova and Crawford, 2012). Thus, most species that formerly belonged to the genus Melosira have been reassigned to Aulacoseira. Additionally, Melosira includes an extensive range of taxa, some of them have been reclassified into other genera, including Paralia, Orthoseira, Stephanodiscus, and Ellerbeckia (Guiry and Guiry, 2020).

At present, the freshwater Melosira reported in China have not been systematically reclassified, and the taxonomic status of some species remains unclear. Therefore, while describing a new species of Melosira from the Jinsha River, we attempted to clarify the taxonomic position of freshwater Melosira genus of China.

2 MATERIAL AND METHOD 2.1 Sample collection, isolation, and cultures

The Jinsha River is the upper part of the Changjiang (Yangtze) River. Jinsha River runs through Sichuan and Yunnan provinces, with total length of 2 326 km and big drop of ~3 280 m, water area of 473 000 km2, and annual average flow of 4 750 m3/s (Gao et al., 2019). The Jinsha River accounts for ~26% of the Changjiang River basin area (Gao et al., 2019).

Periphytic samples were collected on May 2, 2020, from the Jinsha River (28°19′11″N, 103°55′2″E) from the surfaces of Cladophora sp. Samples were analyzed for total phosphorus (TP) using the ammonium molybdate spectrophotometric method (GB 11893-1989) and for total nitrogen (TN) using alkaline potassium persulfate digestion and the UV spectrophotometric method (HJ636-2012). Water temperature, conductivity, salinity, and pH were measured on-site with an YSIPro Plus multiparameter meter (YSI, Yellow Springs, OH, USA). Single diatom cells derived from clone cultures of subsamples were isolated using a Pasteur pipette and the capillary method under a Nikon Ts2 inverted microscope (Nikon, Tokyo, Japan). Cells were isolated and cultured in 24-well cell plates and each well contained 2-mL CSI medium. Nonaxenic unialgal cultures were maintained in CSI medium at 24 ℃ in a growth chamber under a 12-h: 12-h light/dark photoperiod. A list of all of the strains examined in this study with their GenBank accession numbers, geographic locations of the sample areas, and ecological parameters are presented in Table 1.

Table 1 List of strains examined in this study and their GenBank accession numbers
2.2 Light microscopy (LM) analysis

Field samples were treated with concentrated nitric acid using a microwave accelerated reaction system (MARS) (CEM Corporation, Charlotte, NC, USA) and a preprogrammed digestion scheme (temperature, 180 ℃; ramp, 15 min; hold, 15 min) (Luo et al., 2018). Cultured samples were treated with an alcohol gradient (30%→50%→70%→80%→100%). To remove the acid from the oxidized cultures, the samples were washed six times with distilled water, and the cleaned diatoms were then mounted in Naphrax® to obtain permanent slides for the LM analysis.

2.3 Scanning electron microscopy (SEM) analysis

Cleaned specimens were air-dried on glass cover slips and attached to copper stubs, coated with ~15-nm gold-palladium using a sputter coater (HITACHI E-1045), and examined using a Hitachi SU 8010 SEM (2 kV) at Shanghai Normal University, Shanghai, China. Both the cleaned material and slides were stored at the Laboratory of Algae and Environment of Shanghai Normal University. Diatom images were compiled with Photoshop CS4 (Adobe Photoshop CS4 Extended), and the terminology and identifications were based on previous publications.

2.4 Extraction of DNA and amplification

The total DNA of monoclonal culture strains was extracted using InstaGeneTM Matrix following the manufacturer's protocol. Fragments of SSU rDNA (~1 750 nt) and rbcL (~1 470 nt) were amplified by PCR using primers 66F and 1255R for the rbcL fragments and 11F and 1174R for the SSU rDNA fragments (Alverson et al., 2007).

Amplifications of the SSU rDNA fragments and partial rbcL gene fragments were carried out using premade ScreenMix (Evrogen, Moscow, Russia) for the PCR assays. The amplification conditions for the SSU rDNA fragments were as follows: initial denaturation of 3 min 30 s at 94 ℃, denaturation for 35 cycles at 94 ℃ for 50 s, annealing at 58 ℃ for 50 s, extension at 72 ℃ for 60 s, and a final extension at 72 ℃ for 10 min. The amplification conditions for the partial rbcL fragments were as follows: initial denaturation of 3 min 30 s at 94 ℃, denaturation for 35 cycles at 94 ℃ for 50 s, annealing at 53 ℃ for 50 s, extension at 72 ℃ for 60 s, and a final extension at 72 ℃ for 10 min. Each PCR mixture (50 μL) contained 17-μL ddH2O, 25-μL 2×EasyTaq PCR SuperMix (TransGen Biotech, Beijing, China), 2 μL of each primer (10 mmol/L) (BGI, Shanghai, China), and 4-μL DNA template.

PCR products were purified using a SanPrep column DNA gel purification kit (Sangon, China). Then, PCR products, SSU rDNA fragments, and partial rbcL genes (decoded from two sides using forward and reverse PCR primers) were sent to the BGI Tech Corporation (Beijing, China) for sequencing on an ABI 3730XL sequencer. Sequences were submitted for a BLAST search of the National Center for Biotechnology Information (NCBI) database to find closely related sequences.

2.5 Tree construction

Newly determined sequences and GenBank sequences of 59 separate centric diatoms from different morphological groups were included in the alignments. One araphid diatom, Fragilaria crotonensis, was selected as outgroup taxon. The obtained sequences and their data were downloaded from GenBank (Supplementary Table S1).

Sequences were aligned using the Clustal W option in the BioEdit sequence analysis software (Thompson et al., 1997; Hall, 1999). Untrimmed bases from both ends were deleted to produce identical length alignments. Using the ModelTest v3.7 Software, the GTR model of nucleotide substitutions with gamma (G) distribution rates and equal proportions across invariable sites (Ⅰ) was the most appropriate evolutionary model for the SSU rDNA and rbcL alignments individually, and for the rbcL-SSU rDNA alignments in general (Posada, 2006) (Table 2). Finally, concatenated SSU rDNA + rbcL alignments of 59 taxa were constructed for which both SSU rDNA and rbcL sequences were available. Phylogenies were constructed based on this model using Bayesian inference (BI) and maximum likelihood (ML) analyses. PHYML software was used to generate ML trees and the bootstrap analysis was conducted using 1 000 replicates (Felsenstein, 1981; Guindon and Gascuel, 2003). Bayesian analyses were conducted using MrBayes v3.1.2 (Ronquist and Huelesenbeck, 2003). The Markov chain Monte Carlo (MCMC) algorithm running three hot Markov chains simultaneously and one cold Markov chain was used to estimate the posterior probabilities of phylogenetic trees. The Markov chains started from a random tree and ran for 2 000 000 generations with sampling every 1 000 generations for a total of 2 000 samples for each run. FigTree v1.4.2 and Adobe Illustrator CS6 were used to edit all resulting phylogenetic trees.

Table 2 Parameters of the nucleotide substitution model estimates using Modeltest v3.7
3 RESULT 3.1 Morphological investigations

Melosira capsularum L. Yang & Q. X. Wang sp. nov. (Figs. 16)

Fig.1 Live Melosira capsularum sp. nov. cells with LM observation under field-collected condition a–c. colony, chloroplast structure, and shape of the valve mantle; d–e. periphytic style and substrate of live cells. Melosira capsularum (arrows) epiphytic on the surfaces of Cladophora sp. Scale bars=20 μm.
Fig.2 Live Melosira capsularum sp. nov. cells with chloroplast structure and frustule shape under laboratory culture conditions a. colonies in 24-well cell plates; b–e. colony, valve mantle view. Arrows point to the girdle bands. Scale bars=10 μm.
Fig.3 LM micrographs of Melosira capsularum sp. nov. a–g, n–y. valve mantle view; h–m. valve face. Scale bars=10 μm.
Fig.4 SEM micrographs of Melosira capsularum sp. nov. a. external valve view; b. internal valve view, arrow points to the rimoportulae; c. valve mantle view; d. a ring of rib-like silica process arranged longitudinally on the valve mantle (arrow); e. numerous rimoportulae (arrow) and small granules on the valve face; f. the structure of rimoportulae in internal valve, arrow points to rimoportula with openings rounded or elongated in the internal valve view. Scale bars: 10 μm (a, c); 5 μm (b); 1 μm (d, e, f).
Fig.5 SEM micrographs of Melosira capsularum sp. nov. a–f. the structure of girdle bands. Black arrows point to valvocopula. The ligulae (white arrows) are triangular. Scale bars. 10 μm (a, c, d, e, f); 5 μm (b).
Fig.6 SEM micrographs of Melosira capsularum sp. nov. Colony derived from culture material strain, JSJ2A4. Scale bars: 10 μm (a, d); 5 μm (b, c).

Description: Cells are capsular and joined by mucilage pads secreted on the valve face consisting of paired groups united by the cingula. Diameter: 13.0–16.0 μm; mantle height: 10.0–14.5 μm. Based on the SEM observation, the valve face is convex hemispherical with small granules. Areolae are arranged randomly or in rows radiating from the center of the valves. The valve mantle is deep; the edge of the mantle in both valves has the same milled edge appearance (arrow in Fig. 4d). In addition, numerous rimoportulae are scattered over the valve face and mantle and occur in a ring near the mantle edge (arrow in Fig. 4e). From the internal valve view, rimoportulae are with openings rounded or elongated pores, 0.2 μm in diameter, and there is a narrow groove around the rimoportulae (arrow in Fig. 4b & 4f). There are two to eight open cingular bands, and the pores in the bands are in distinct rows nearly parallel to the pervalar axis or slightly curved. All cingular bands but the valvocopula (black arrow in Fig. 5) are ligulate; the ligulae are triangular (white arrow in Fig. 5). Cingular bands are covered on the valve mantle. Each band is composed of one row of elongated pores that is close to the valvocopula, an unornamented area, and transverse 1–6 rows of circular or elongated pores. It is precisely due to the existence of the cingula that cells are distinctly united into pairs or triplets.

Holotype: Slide No. JSJ200501-1, Lab of Algae and Environment, College of Life Sciences, Shanghai Normal University, Shanghai (SHNU), China.

Etymology: The specific epithet refers to the shape of the frustule.

3.2 Molecular analysis

Pairwise comparisons of putatively related taxa showed that M. capsularum strains exhibited 100% similarity. However, they had an evolutionary distance of 0.06–0.08 with other Melosira diatoms and > 0.09 p-distances with Aulocoseira based on the partial ribulose-1, 5-bisphosphate carboxylase large subunit. The p-distances between Aulocoseira species ranged from 0.00–0.06 (Table 3). The pairwise uncorrected p-distances based on the partial SSU rDNA gene showed that two M. capsularum strains had a p-distance of 0.00, indicating that they exhibited 100% similarity. However, these strains had p-distances that ranged from 0.09–0.16 with other Melosira species and from 0.14–0.17 with Aulocoseira species. The p-distances between Aulocoseira species ranged from 0.00–0.05 (Table 4).

Table 3 Pairwise uncorrected p-distances of 21 strains based on the partial ribulose-1, 5-bisphosphate carboxylase large subunit
Table 4 Pairwise uncorrected p-distances of 21 strains based on the partial SSU rDNA gene

The concatenated alignments of the rbcL and SSU rDNA matrix included 59 centric diatom strains and one outgroup taxon, and consisted of 1 307 characters, of which 487 (37.4%) were variable and 403 (30.9%) were parsimony-informative. There were 815 (62.6%) conserved sites. The average percentages of T, C, A, and G were 32.0%, 15.4%, 30.0%, and 22.6%, respectively. Two molecular markers, SSU rDNA and rbcL, were included in the ML and BI analyses for phylogenetic inference (Fig. 7). Both analyses produced similar topologies. All of the centric species were subdivided into three main lineages with different levels of statistical support, namely, Melosirales, Biddulphiales, and Thalassiosirales. Melosira formed a lineage next to Aulocoseira (1.00/98.5). Another lineage consisted of Pleurosira, Terpsinoe, Hydrosera, Urosolenia, and Orthoseira (0.51/-). Within the remaining centric diatom group, there were two main clades: one included the genera Cyclotella and Thalassiosira, and the other included Skeletonema, which is a sister genus toStephanodiscus, Cyclostephanos, Lindavia, and Discostella.

Fig.7 Bayesian tree of Melosira and Aulocoseira species constructed from a concatenated alignment of 59 partial rbcL and SSU rDNA sequences of 1 307 characters Bootstrap support values obtained from the RAxML analyses are shown below the horizontal lines; values < 50 are not shown. Bayesian posterior probabilities are shown above the horizontal lines; values < 50 are not shown. All sequences have associated strain numbers (if available) and GenBank numbers (Supplementary Table S1). Fragilaria crotonensis was used as the outgroup. Sequences of our culture strains that were new species in this paper are highlighted in red. * means sequences of our other culture strains.

Within the Melosira species, M. nummuloides formed a single branch in the clade with the other Melosira species included in the analysis (1.00/100.0). The other Melosira species subdivided into three lineages. The first lineage consisted of M. dubia and M. moniliformis (1.00/98.9). Two strains of M. capsularum formed the second linage next to M. varians with low statistical support (0.82/59.6), but this single branch had high statistical support (1.00/100.0). The third lineage was comprised of three strains of M. varians (1.00/100.0).

3.3 The revision of the freshwater Melosira in China

At present, of the 48 taxa of the genus Melosira that have been reported in China, 17 remain within the genus Melosira, 18 taxa have been transferred into Aulacoseira, and 5 taxa have been transferred into other genera (Ellerbeckia, Stephanodiscus, Orthoseira, and Paralia). In addition, there are seven varieties and forms for which their classification status needs to be reassessed. Therefore, according to the detailed morphological description and hand drawings, we have proposed seven new combinations and have rearranged the freshwater Melosira in China. The details required for seven new combinations are therefore given below. The arrangement of 48 freshwater Melosira taxa reported in China is shown in Table 5.

Table 5 Taxonomic status revision of the freshwater Melosira species in China

1 Aulacoseira lirata var. seriata (Müller) L. Yang & Q. X. Wang, comb. nov.

Basionym: Melosira lirata var. seriata Müller 1898. Bacillariales aus den Hochseen des Riesengebirges. Forschungsberichte aus der Biologischen Station zu Plön 6: 8, pl. 3: Fig. 34.

Synonym: Melosira distans var. lirata f. seriata (Müller) Hustedt 1927a. Die Kieselalgen Deutschlands, Österreichs und der Schweiz unter Berücksichtigung der übrigen Länder Europas sowie der angrenzenden Meeresgebiete. Bd. Ⅶ: Teil 1: 264.

Aulacoseira distans f. seriata (Müller) Davydova in Glezer et al. 1992. The diatoms of the USSR fossil and recent. Vol. Ⅱ. fasc. 2: Stephanodiscaceae, Ectodictyonaceae, Paraliaceae, Radialiplicataceae, Pseudopodosiraceae, Trochosiraceae, Melosiraceae, Aulacosiraceae. pl. 80.

2 Aulacoseira granulata var. curvata (Grunow) L. Yang & Q. X. Wang, comb. nov.

Basionym: Melosira granulata var. curvata Grunow in Van Heurck 1882. Synopsis des Diatomees de Belgique: 87: Fig. 18.

Synonym: Melosira granulata f. curvata (Grunow) Hustedt 1927b. Fossile Bacillariaceen aus dem Loa-Becken in der Atacama-Wüste, Chile. Archiv für Hydrobiologie 18(2): 250.

Aulacoseira ambigua var. curvata (Grunow) Simonsen 1979. The diatom system: ideas on phylogeny. Bacillaria 2: 56.

3 Aulacoseira italica f. curvata (Pantocsek) L. Yang & Q. X. Wang, comb. nov.

Basionym: Melosira crenulata f. curvata Pantocsek 1902. Kieselalgen oder Bacillarien des Balaton. Resultate der Wissenschaftlichen Erforschung des Balatonsees, herausgegeben von der Balatonsee-Commission der Ung. Geographischen Gesellschaft. Bd 2(2): 103; pl. 15, Fig. 327.

Synonym: Melosira italica f. curvata (Pantocsek) Hustedt 1927a. Die Kieselalgen Deutschlands, Österreichs und der Schweiz unter Berücksichtigung der übrigen Länder Europas sowie der angrenzenden Meeresgebiete. Bd. Ⅶ: Teil 1: Liefrung 1. In: Rabenhorst's Kryptogamen Flora von Deutschland, Österreich und der Schweiz: 260.

4 Aulacoseira italica var. hankensis (Skvortzov) L. Yang & Q. X. Wang, comb. nov.

Basionym: Melosira italica var. hankensis Skvortzov 1929. Freshwater diatoms from Amoy, South China. The China Journal 11(1): 42, pl. 1: Fig. 8.

5 Orthoseira roeseana f. spinosa (Skvortzov) L. Yang & Q. X. Wang, comb. nov.

Basionym: Melosira roeseana var. epidendron f. spinosa Skvortzov 1938. Subaërial diatoms from Pin-Chiang-Sheng Province. Philippine Journal of Science. Section C 65(3): 265, pl. 3: Fig. 2.

6 Orthoseira roeseana var. xizangensis (Chen) L. Yang & Q. X. Wang, comb. nov.

Basionym: Melosira roeseana var. xizangensis Chen in Chen & Zhu 1985. Studies on the freshwater Centricae of China. Acta Hydrobiologica Sinica 9(1): 81, Figs. 1, 2.

7 Aulacoseira youngii var. tenuissima (Skvortzov) L. Yang & Q. X. Wang, comb. nov.

Basionym: Melosira youngii var. tenuissima Skvortzov 1937. Neogene diatoms from Eastern Shantung. Bulletin of the Geological Society of China 17(2): 195, pl. 1: Figs. 1-3.

4 DISCUSSION 4.1 Habitat analysis of the new species-Melosira capsularum L. Yang & Q. X. Wang

Most Melosira species are planktonic and mass reproduction of certain species is through the formation of diatomite deposits. In this study, we combined morphological and molecular methods to analyze a putatively new Melosira species that was collected from the surfaces of Cladophora sp. Because these diatoms grow in a given location, they accurately reflect the water quality and changes in plankton populations in rivers, lakes, and oceans with higher flow rates. Therefore, the diatoms of the genus are good indicators for water environmental assessments. M. capsularum was also observed in specimens collected in May and November, 2019. Therefore, we deduced that this species has existed in this location (i.e., it was not affected by water temperature).

4.2 Morphological comparison between M. capsularum and other similar species

Based on the morphological characteristics and molecular results, M. capsularum is similar to M. varians, M. nummuloides, M. moniliformis, M. hummii, M. hustedti, M. lahuensis, M. muscigena and M. lineata. All of these species exhibit the following characteristics: the position and arrangement of rimoportulae; the shape of girdle bands, and the arrangement of cell connections. However, they have obvious morphological differences, as summarized in Table 6.

Table 6 Morphological characteristics of seven Melosira species
4.3 Phylogenetic analysis of M. capsularum and other centric diatoms

In our phylogenetic analysis, most of the sequences for freshwater centric genera were obtained from GenBank. Nuclear SSU rDNA sequences and chloroplast rbcL sequences were applied in a previous phylogenetic analysis of centric diatoms of Thalassiosirales (Alverson et al., 2007), where Cyclotella species formed a clade with Thalassiosira species, and Discostella, Cyclostephanos, and Stephanodiscus species clustered together on a large branch. This clustering result is consistent with our findings (Fig. 7). Our data support results by Gargas et al. (2018) that Orthoseira is not closely related to other melosiroid diatoms, but it is closer to multipolar diatoms. Orthoseira is more closely related to Terpsinoe and Hydrosera than to Melosira or Aulacoseira. Phylogenetic trees based on SSU rDNA and rbcL sequences showed that M. capsularum was distinct from other Melosira species. Differences in both morphological measurements and molecular analyses supported M. caspularum as a new species. In our phylogenetic tree, Melosira formed a lineage next to Aulacoseira species, indicating that these genera have a close relationship. This result was consistent with our morphological observations. In the phylogenetic tree, M. dubia formed a branch with M. moniliformis and had higher statistical support (1.00/98.9). However, in AlgaeBase, M. dubia is regarded as a synonym of Podosira dubia (Kützing) Grunow. Therefore, the phylogenetic position of M. dubia/P. dubia requires further investigation, including morphological observations and molecular analyses of available strains.

4.4 Current status in phylogeny of the genus Melosira

In the genus Aulocoseira, 45 species have been phylogenetically analyzed using morphological and gene sequence data (SSU rDNA and rbcL) (Edgar and Theriot, 2004). Compared to Aulocoseira, there is a lack of comparative systematic phylogenetic studies on Melosira and the sequences of Melosira species are lacking as well. Thus, further work on additional Melosira taxa, including both morphological and molecular analyses, is required in order to form a more complete picture of the evolutionary history of this genus.

5 CONCLUSION

A new species Melosira capsularum, which was collected from the surfaces of Cladophora sp. during an investigation of periphytic diatoms in the Jinsha River, China, was identified in this study. The results of both the morphological comparisons and phylogenetic analysis support the finding that this algal taxon is a new species. In addition, after a series of revisions of the 48 taxa of freshwater Melosira that have been reported in China, 18 taxa (13 species and 5 varieties) were retained the genus Melosira genus; 22 taxa (13 species, 7 varieties, and 2 forms) have been transferred into the genus Aulacoseira; 1 species and 1 variety were transferred into Ellerbrckia; 4 taxa (1 species, 3 varieties, and 1 form) were transferred into Orthoseira; 1 species was transferred into Paralia and 1 species was transferred into Stephonodiscus.

6 DATA AVAILABILITY STATEMENT

All of the data obtained and/or analyzed in this study are available from the corresponding author upon request.

Electronic supplementary material

Supplementary material (Supplementary Table S1) is available in the online version of this article at https://doi.org/10.1007/s00343-021-0470-x.

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