Mallomonas arenosa Ignatenko, Nguyen Ngoc Lam, Phan Trong Huan & E. S. Gusev 2024, sp. nov.

Mallomonas arenosa Ignatenko, Nguyen Ngoc Lam, Phan Trong Huan & E.S. Gusev sp. nov. (Figs 2 –15) Cell dimensions unknown. Body scales rhombic, 2.1–2.4 × 1.4–1.6 μm, with a small shallow dome, a shield, a V-rib, an anterior submarginal rib, and flanges. The shield is covered with transverse ribs, which can be connected by longitudinal ribs, forming a reticulum of meshes of various sizes. There are pits inside the meshes, which usually do not penetrate the base plate. These pits are located along transverse ribs or mesh boundaries. There is a single pore on the shield at the angle of the V-rib. The V-rib is slightly hooded and continuous with the anterior submarginal rib. The anterior submarginal rib is well developed and raised above the shield and anterior flange. There is a single row of small papillae on one or both arms of the anterior submarginal rib. Weakly or well-developed struts extend from the anterior submarginal rib toward the anterior flange. There are 1–2 pits between the struts. The posterior rim is narrow and smooth. The posterior flange is narrow and smooth. Caudal scales are smaller than the body scales, 1.8 × 1.3 μm, and slightly asymmetrical. Ornamentation on the shield is the same as that found on the body scales. Holotype: — Portion of a single gathering of cells on SEM stub number 56_I_1 deposited at the Herbarium of the Steppe Institute of the Ural Branch of the Russian Academy of Sciences, Orenburg (ORIS). Material from unnamed acidic water body in Thua Thien-Hue Province, Viet Nam. Sample collected 28 October 2022 by E.S. Gusev. Figure 8 is a representative scale from the type specimen. Type locality:— Viet Nam: unnamed swamp water body in Thua Thien-Hue Province, Phong Dien District, Latitude/Longitude: N16°39.777′ E107°21.905′. Etymology: —The epithet “arenosa ” (“sandy”) reflects the fact that this species is found in waterbodies, located among the sands. Distribution: —To date, Mallomonas arenosa has been observed in two closely located water bodies in Thua Thien-Hue Province, Phong Dien District, Viet Nam. In addition to the type locality, it has been found in another unnamed water body of a similar type (Latitude/Longitude: N16°38.576′ E107°22.515′). Ecology: — Mallomonas arenosa was found in acidic conditions (pH 4.8–5.0), the specific conductance in the localities was 18 µS cm –1, temperature 26 ºC.