Stigonema dinghuense Song et Li, 2015

Song, Gao-Fei, Xiang, Xian-Fen, Wang, Zhong-Jie & Li, Ren-Hui, 2015, Polyphasic characterization of Stigonema dinghuense, sp. nov. (Cyanophyceae, Nostocophycidae, Stigonemaceae), from Dinghu Mountain, south China, Phytotaxa 213 (3), pp. 212-224 : 214-218

Stigonema dinghuense Song et Li , sp. nov. ( Fig. 1 )

Thallus macroscopic, caespitose, cushion like, dull-brown to blackish-green. Sheaths thin, firm, colorless, yellow or yellowbrown. Mature filaments polyseriate, heteropolar, 41.3–77.1 μm (=57.8μm) wide, richly branched at the base. Branches Ttype and narrower than the main filament, 14.4–50.2 μm (=34.8 μm) wide, with long, broadly developed lateral branches or with short mammilliform hormogonia. Hormogonia short, uniseriate, arising on both sides, 13.2–27.9 μm (=19.6 μm) wide and 35.2–59.2 μm (=45.8 μm) long. Cells spherical, 8.2–18.3 μm (=12.6cμm) wide and 8.8–18.5 μm (=13.1μm) long, in older parts regularly barrel-shaped. Heterocytes mostly lateral, nearly spherical, 6.2–13.8 μm (=9.2 μm) wide and 6.2–12.9 μm (=8.5μm) long. Akinetes not observed.

Type: — CHINA. Guangdong Province: Dinghu Mountain, 23°9’34.19” N, 112°33’23.62” E, 415 m a.s.l., G. F. Song, 22 January 2013 (holotype CHCB! DH- 2013012030-1).

Etymology: —The name of the species was chosen for the sampling location.

Habitat: —Wet rocky walls in the temperate region.

Notes: —Several species morphologically similar to S. dinghuense have been described (see a comparison in Table 2). This species was found only in unpolluted soil habitats and under mosses on wet rocky walls from the temperate region. These populations are similar to S. mamillosum according to the descriptions in the literature, but there are doubts about their identity outside North Europe and North America. Despite many records of S. mamillosum outside of this area of distribution have been reported, there are few morphological descriptions, indicating that the S. mamillosum requires a revision comparing the tropical, subtropical, and temperate populations with those from subfrigid and frigid zones.

Pigment examination: —Observations under fluorescence microscope indicated that S. dinghuense simultaneously contained chlorophyll a, phycocyanin, and phycoerythrin ( Fig. 2 ). The fluorescence intensity of branches was significantly higher than that of the main filaments. Chlorophyll and phycocyanin are widely present in cyanobacteria, and phycoerythrin is a taxonomically useful marker for certain cyanobacterial groups/species.

Phylogenetic analysis: —The direct PCR without routine DNA extraction resulted in obtaining two 16S rRNA gene, two rbc LX and one nif H sequences. The two 16S rRNA gene sequences from the population of S. dinghuense showed 99.2% similarity, and these two sequences shared 96.5% and 96.7%, 96.2% and 96.2%, 97.6% and 98.3% similarities with Stigonema ocellatum SAG 48.8, S. ocellatum SAMA 35, and S. turfaceum CBFS A-031, respectively. The S. dinghuense and other Stigonema sequences obtained in GenBank shared relatively high similarities, within the range of 95.9%─98.3% ( Table 3). The similarities in nif H and rbc LX sequences among heterocytous cyanobacteria including S. dinghuense examined in this study, are shown in Tables S2 and S 3.

Based on 16S rRNA gene sequences from 81 cyanobacterial taxa, including both S. dinghuense sequences in this study, the phylogenetic trees were constructed using the NJ, ML, and Bayesian methods. The ML tree is shown in Fig. 3 , with support values from the three methods at each node. Phylogenetic analysis indicated that S. dinghuense was in a very deep branch within the heterocytous cyanobacterial group, together with S. turfaceum . The heterocytous cyanobacteria were separated into five clusters (A, B, C, D and E), and S. dinghuense was located within the Stigonemataceae (B1) ( Fig. 3 ). The phylogenetic trees based on nif H and rbc LX sequences are also shown in Figs. 4 and 5 respectively.

Figures

FIGURE 1. Main characteristics of Stigonema dinghuense. A. Habit of thallus, B. Mature filaments. C. Lateral branches. D. Hormogonia. Scale bars: 200 μm (A), 50 μm (B, C), and 20 μm (D).

FIGURE 2. Microphotographs by fluorescence microscope of a free living filament. A. No filter. B. Using the channel for chlorophyll a. C. Using the channel for phycocyanobilin. D. Using the channel for phycoerythrobilin. Scale bar = 50 μm.

FIGURE 3. Maximum likelihood (ML) phylogenetic tree of cyanobacteria based on 16S rRNA sequences (928 bp) representing the current GenBank data on heterocytous cyanobacteria. Bootstrap values greater than 50% with distance/ parsimony methods are indicated at the nodes in this form: NJ/ML/Bayers. Sequences from GenBank are indicated by accession numbers. The main clades are indicated by numbers. Sequences from this study are denoted with solid circles.

FIGURE 4. Maximum likelihood (ML) phylogenetic tree of cyanobacteria based on nifH nucleic sequences (205 bp) representing the current GenBank data on heterocytous cyanobacteria. Bootstrap values greater than 50% with distance/ parsimony methods are indicated at the nodes in this form: NJ/ML/Bayers. Sequences from GenBank are indicated by accession numbers. Sequences from this study are denoted with solid circles.

FIGURE 5. Maximum likelihood (ML) phylogenetic tree of cyanobacteria based on rbcLX nucleic sequences (769 bp) representing the current GenBank data on heterocytous cyanobacteria. Bootstrap values greater than 50% with distance/ parsimony methods are indicated at the nodes in this form: NJ/ML/Bayers. Sequences from GenBank are indicated by accession numbers. Sequences from this study are denoted with solid circles.