Mallomonas rimosa Gusev, Tran, Shkurina & Martynenko, 2023

Gusev, Evgeniy, Tran, Hoan, Shkurina, Nataliya & Martynenko, Nikita, 2023, Mallomonas rimosa sp. nov. (Synurales, Chrysophyceae) - a rare tropical species from Vietnam, Phytotaxa 630 (4), pp. 295-300 : 297-298

Mallomonas rimosa Gusev, Tran, Shkurina & Martynenko sp. nov. ( Figs. 2–13 ).

Cells are armoured with apical domed collar scales, domeless body scales, and small rear scales with spines. The body scales are rhomboidal and often slightly asymmetrical, 2.4–3.0 × 1.6–2.2 μm. The shield is covered with a thick siliceous layer and patterned with small, circular, and evenly spaced depressions (pits). There is a larger depression of irregular shape in the angle of the posterior submarginal rib lacking secondary layer (‘window’). The ‘window’ usually contains one rimmed pore on the base plate. On some scales, the area without a secondary siliceous layer is expanded along the arms of the posterior submarginal rib. In this case, there may be up to 3 pores in this area. The posterior submarginal rib is smooth and fused with the arms of the anterior submarginal rib. The anterior submarginal rib is well developed, raised above the shield, wide, and smooth. The anterior flange is narrow, with one row of papillae on each side. The posterior border is narrow and smooth. The posterior flange is narrow and smooth, sometimes with additional ribs parallel to the posterior submarginal rib. The rear scales are similar in structure to the body scales, but more asymmetric, bearing stout spines at the anterior end, 2.1–2.3 × 1.4–1.5 μm. The collar scales are larger than body scales, asymmetrical, elongated, and have a broadly rounded proximal end and narrower distal end, 3.3–3.5 × 1.9–2.2 μm. Submarginal ribs are well developed. There is a single row of papillae in the flange of the proximal end. Ornamentation on the shield is the same as found on the body scales. There are 2–6 base plate pores in the angle of the posterior submarginal rib. The dome on the collar scales is ornamented with papillae and ribs and has a small sharply pointed peak at the apex. Bristles and cysts were not observed.

Holotype:— Portion of a single gathering of cells on SEM stub number TTH55 deposited at the Herbarium of the Papanin Institute for Biology of Inland Waters RAS, Borok ( IBIW). Material from unnamed reservoir in Thua Thien Hue Province, Vietnam. Sample collected 28 October 2022 by E. Gusev. Figure 2 is a representative scale from the type specimen.

Type locality:— Vietnam: unnamed swamp water body in Thua Thien Hue Province, Phong Điền District, Latitude/ Longitude: N16°39.777’ E107°21.905’.

Etymology:—The epithet “rimosa ” (“drilled”, “full of holes”) refers to the presence of a numerous circular pits on the shield.

Distribution: —To date, M. rimosa has been observed in two localities in Phong Điền District, Thua Thien Hue Province, Vietnam. In addition to the type locality, it has been found in Bau Sen Lake (Latitude/Longitude: N16°34.173’ E107°26.502’).

Observations: — Mallomonas rimosa was found in acidic conditions (pH 5.0–5.3), the specific conductance in the localities varied from 17 to 37 μS cm-1, temperature from 26 to 33ºC, total phosphorus concentration in the type locality was 68 μL-1, Kjeldahl nitrogen concentration 406 μL-1.

Figures

FIGURES 2–13. Mallomonas rimosa sp. nov. Figure 2–3. Body scales, SEM. Figures 4–10. Body scales, TEM. Figure 11. Rear scale, TEM. Figure 12–13. Collar scales, TEM. Figure 2 is a representative scale from the type specimen. Scale bars: Figures 4–5, 7, 10–13: 2 μm; Figures 6, 8–9: 1 μm; Figures 2–3: 0.5 μm.