Hunan freshwater diatom study describes two new Tryblionella species

A descriptive taxonomic study of diatoms collected from freshwater habitats in Hunan Province, China, documents the fine valve ultrastructure of three members of Tryblionella sensu lato and formally describes two species new to science: Tryblionella dongtingensis and Tryblionella hunanensis.

The study combined light microscopy with scanning electron microscopy. It also provides the first detailed documentation of the marginal ridge, proximal and distal mantles and fibulae of T. calida, a species for which earlier electron micrographs had provided limited detail.

The authors collected a surface-sediment sample from the littoral zone of East Dongting Lake on 5 March 2023, at 29°19′7″N, 112°47′32″E and 39 m above sea level. The sample contained T. calida, T. dongtingensis and T. hunanensis. A second sample was collected on 24 March 2024 from mud sediments on stone surfaces in a riffle of the Yongle River, a tributary of the Xiang River, at 26°55′08″N, 113°35′55″E and 119 m above sea level. It contained T. calida and T. hunanensis. Slides were deposited in the Herbarium of Jishou University, Hunan, China.

Tryblionella calida had linear-lanceolate valves with wedge-shaped, slightly protruding apices and an almost flat valve surface. Among 17 measured valves, lengths ranged from 38–61 μm and widths from 8.6–10.6 μm. The axial sternum was absent, continuous transapical ribs numbered 15–17 per 10 μm and rectangular to square fibulae numbered 9–10 per 10 μm. Scanning electron microscopy showed a high marginal ridge with long slit-like fenestrae at its base and a smooth, plain upper margin. The distal mantle was shallow and unornamented, while the deep proximal mantle often concealed the fibulae.

The newly described T. dongtingensis was based on the holotype specimen deposited under slide number DIA2026002, with the type locality in East Dongting Lake. Its 30 measured valves were linear-lanceolate, 33–65 μm long and 10.3–12.1 μm wide. A longitudinal fold along the valve midline divided the valve face into two non-coplanar halves. Transapical ribs numbered 9–14 per 10 μm and fibulae numbered 6–9 per 10 μm. Some transapical ribs crossed the valve face continuously, whereas others ended at the central fold. The species had a high marginal ridge with long slit-like fenestrae, but its upper margin was undulate rather than plain.

Tryblionella hunanensis was represented by the holotype on slide DIA2026003 from the Yongle River. Across 31 measured valves, it had panduriform outlines with cuneate to acute apices, lengths of 37–53 μm and widths of 8.9–13.2 μm. Its valve face was slightly undulate, with 9–12 transapical ribs per 10 μm and 7–10 rectangular to square fibulae per 10 μm. Electron micrographs revealed dense silica granules, four to seven longitudinal grooves in the raphe-canal wall on the valve side, and a high marginal ridge with a distinctly lobed upper margin. Unlike the ridges of T. calida and T. dongtingensis, its basal portion lacked long slit-like fenestrae and instead had a row of pores at the junction with the distal mantle.

Field measurements at the Dongting Lake locality gave conductivity of 494 ± 0.9 μS cm−1, pH of 8.4 ± 0.06 and water temperature of 27.8 ± 0.3 °C. The authors associated the environmental measurements and associated diatom assemblage with alkaline and mesotrophic to eutrophic conditions. At the Yongle River locality, conductivity was 111.7 ± 0.8 μS cm−1, pH was 7.6 ± 0.04 and water temperature was 22.3 ± 0.3 °C; the associated assemblage and measurements suggested alkaline and mesotrophic conditions. T. dongtingensis was reported only from its type locality, while T. hunanensis was reported from the Yongle River and East Dongting Lake.

Although the three species can coexist in East Dongting Lake and have overlapping valve dimensions, the study distinguished them through combinations of valve form, surface undulation, rib and fibula density, raphe-canal structure and marginal-ridge architecture. T. dongtingensis was characterized by its central longitudinal fold and undulate ridge margin, while T. hunanensis combined a panduriform valve, grooved raphe-canal wall, silica granules and lobed ridge margin. The paper placed all three in a morphological group characterized by a more or less undulate valve face, transapical ribs, uniseriate striae with rounded, rimmed inner areolar openings, bar-like ribs along the raphe-canal wall, a marginal ridge and squat or apically elongated fibulae.

The observations come from two collected samples and detailed microscopy of selected specimens rather than a survey of population abundance, seasonal variation or broad geographic distribution. The limited collection localities mean that the reported environmental associations and known distributions should not be treated as comprehensive. The authors also noted that the monophyly of a proposed group around T. calida remains unresolved: the study found that the fibulae of T. calida are non-perforated, contrary to the character state used in an earlier morphological consensus tree.

Subject of Research: Taxonomic and ultrastructural study of freshwater Tryblionella diatoms in Hunan, China

Article Title: Ultrastructure of three species of Tryblionella ( Bacillariophyceae ) found in freshwater habitats in Hunan Province, China, including descriptions of two new species

Article References: Ma, Y.-L., Yu, C.-P., Liu, B., Rioual, P., Ma, Y.-D., & Jiang, X.-Y. (2026). Ultrastructure of three species of Tryblionella (Bacillariophyceae) found in freshwater habitats in Hunan Province, China, including descriptions of two new species. PhytoKeys, 278, 259-277. https://doi.org/10.3897/phytokeys.278.204336

Image Credits: AI Generated

DOI: 10.3897/phytokeys.278.204336

Keywords: Tryblionella, diatom taxonomy, freshwater diatoms, Hunan Province, Dongting Lake, Yongle River, scanning electron microscopy, new species, valve ultrastructure, Ultrastructure, three, species