Fragilaria saxoplanctonica Lange-Bertalot & Ulrich, 2014

Lee, Ha-Eun, Kim, Taehee & Ki, Sang Deuk Lee and Jang-Seu, 2023, Description of an unrecorded diatom Fragilaria saxoplanctonica Lange-Bertalot & Ulrich (Bacillariophyceae) from Paldang Reservoir in Korea, Journal of Species Research 12 (4), pp. 313-320 : 315-319

Fragilaria saxoplanctonica Lange-Bertalot & Ulrich

2014 ( Fig. 2 )

Reference: Lange-Bertalot & Ulrich 2014, p. 30, fig. 13: 1 - 9, 14: 1 - 8.

Description: The frustules of the diatom shaped needle and narrowly rectangular. Each cell existed solely, and no colony formation was observed. The external valve showed no linking or marginal spines, and the ends of the valves were rounded. The valve length was 67.2 - 70.2 μm, width was 1.4 - 2.9 μm, and the ratio of width-to-length was 1: 23.2 - 50.1. Striae were alternate or opposite, composed of 2 - 4 areolae near the apices and 5 - 7 areolae in the middle except the central area with ghost striae. The number of striae in 10 μm was 24 - 26. In addition, one rimoportula per valve existed.

Taxonomic remarks: According to the original description, the similar species of F. saxoplanctonica was F. tenera var. nanana , which was once called F. nanana Lange-Bertalot. SEM analysis, however, revealed differences such as low density of striae (18.5 - 20 at 10 μm), pyramidal spines, and the capitate ends (Table 1). Two species, F. aquaplus Lange-Bergalot & Ulrich and F. gracilis showed more similar characteristics such as narrow, linear, and needle shaped valve view, no spines, and opposite striations (Table 1). Despite of relative similarities, F. aquaplus has shorter valve length (22 - 45 μm), and F. gracilis has wider valve width (3.6 μm) and lower striae density (20 in 10 μm). In addition, both species have subcapitate ends. All characteristics of the cells in this study were consistent with F. saxoplanctonica isolated from a lake in Saxony, Germany ( Lange-Bertalot and Ulrich, 2014).

Ecology: This species was collected from freshwater. Temperature at sampling was 14.8℃, pH was 9.3.

Site of collection: Paldang Reservoir , Gyeonggi-do, Korea ( GPS code: 37°29′16″N 127°17′14″E) .

Date of collection: April 13, 2022.

Gene sequences: partial 18S rRNA (GenBank No. OQ359101), and partial rbc L (GenBank No. OQ361707) gene sequences.

Molecular affiliation by 18S rRNA and rbc L: 18S rDNA (1,645 bp) and rbc L (978 bp) sequences of F. saxoplanctonica were determined by using the Korean isolate. There were no recorded sequences of F. saxoplanctonica in GenBank database. That is, the 18S rDNA and rbc L sequences of F. saxoplanctonica were sequenced for the first time. The 18S rDNA sequence of our species was nearly identical (99.82% similarity) with F. tenera var. nanana (AM497739), and rbc L sequence was 98.06% similarity to F. heatherae (MK672907). Due to the lack of 18S rDNA from F. heatherae and rbc L from F. tenera var. nanana , the relationship between the three species could not be compared.

Our 18S rDNA and rbc L ML trees both include three classes ( Coscinodiscophyceae, Mediophyceae, and Bacillariophyceae), and within Bacillariophyceae, there are three orders: Thalassiophysales, Licmophorales , and Fragilaria les ( Figs. 3 & 4 ). Both trees show that Fragilaria members cluster together, and the genus is closely related to genus Ulnaria , forming a separate clade from Fragilaria . Within the Fragilaria clade, F. thornata Tan & Kim forms a sister relationships with F. saxoplanctonica in the 18S rDNA ML tree ( Fig. 3 ), and with F. rumpens and F. radians in the rbc L ML tree ( Fig. 4 ). Although both trees share many common species, it showed considerable differences within the Fragilaria clade at the species level. These results show that the sequences from our isolated diatom clearly belong to the genus Fragilaria .

In conclusion, morphological and molecular characteristics of freshwater diatoms isolated from Paldang Reservoir were analyzed and identified as F. saxoplanctonica . This species has the same morphological key characteristics compared to the known F. saxoplanctonica , and its 18S rDNA and rbc L sequences were clustered within the clade of Fragilaria . These results confirmed that our species was F. saxoplanctonica , and it was the first record in Korea.

Figures

Fig. 2. SEM micrographs of the Korean Fragilaria saxoplanctonica isolated from Paldang Reservoir. A, B: valve in external view displaying the outline and striae. C: external view of central area with ghost striae. D: the striae patterns. E, F: external view of valve ends. G: girdle view. The arrow shows the external view of rimoportula. Scale bar=10 μm in A and B.

Fig. 3. A phylogenetic tree of diatoms using the maximum likelihood(ML) method based on 18S rRNA sequences. The genus Fragilaria is highlighted in brown color, and our species is specified in bold. Coscinodiscophyceae was used as an outgroup.

Fig. 4. A phylogenetic tree of Fragilaria using the maximum likelihood (ML) method based on chloroplast rbcL sequences. The genus Fragilaria is highlighted in green color, and our species is specified in bold. Strains marked with asterisk (*) are taken from Kahlert et al. (2019). Coscinodiscophyceae members were used as an outgroup.