Mallomonas prominentia Ignatenko, E.S. Gusev, & Yatsenko-Stepanova, 2025

Ignatenko, Marina, Gusev, Evgeniy & Yatsenko-Stepanova, Tatyana, 2025, Two new species of the genus Mallomonas (Chrysophyceae) from ephemeral pools in the steppe zone of the Southern Urals, Russia, Phytotaxa 705 (1), pp. 86-96 : 90-91

Mallomonas prominentia Ignatenko, E.S. Gusev, & Yatsenko-Stepanova sp. nov. ( Figs. 16–25 )

Cells are 13.5–14.3 × 6.5-7.0 μm, armoured with apical domed collar scales, domeless body scales and small rear scales with short spines. The body scales are rhombic, slightly asymmetrical, 3.0–3.7 × 2.0–2.5 μm, with a shield, posterior submarginal rib, anterior and posterior flanges, and a posterior rim. The shield is raised above the anterior flange and posterior submarginal rib, especially in the proximal part, where it forms a raised protrusion with distinct, thickened margins in the shape of the letter “V”. The proximal part of the shield is separated from the posterior submarginal rib by a deep groove. The shield is patterned with a reticulated meshwork, creating a dense system of irregular, polygonal apertures and delicate papillae on the surface. In the central part of the shield, the papillae are scattered irregularly, and along the anterior flange and partly along the posterior submarginal rib they form distinct rows in such a way that a diamond-shaped figure is formed. The anterior flange is wide, with weakly developed 10–15 struts on each side, radiating from the shield. There is a papilla at the distal end of each strut near the margin of the anterior flange. The posterior submarginal rib is wide, smooth, with internal struts, visible only in TEM images. There is one pit with rimmed base plate pore in the angle of the posterior submarginal rib. The posterior rim is wide, smooth, encircles half of the scale, with a depression in the angle. The posterior flange is narrow. The collar scales are 3.5–3.7 × 2.0–2.2 μm, asymmetrical, elongate and have a broadly rounded, proximal end and narrower distal end, with a well-developed dome. The shield of the collar scales has a meshwork pattern similar to the body scales. There is a thickened rib and groove along the posterior submarginal rib. There are several pits with rimmed base plate pores in the angle of the posterior submarginal rib. The dome is oval in outline, with reticulation, papillae and a sharply pointed peak at the apex. The rear scales are similar in structure to the body scales, with a groove along the posterior submarginal rib, but smaller in size, 2.6–2.9 × 1.6–2.0 μm, more asymmetrical, bearing small spines, and with the less raised shield. Bristles are 6.9–8.6 μm in length, smooth, slightly curved and pointed. Cysts were not observed.

Holotype (here designated):— Portion of a single gathering of cells on SEM stub 54_I_2 deposited at the Herbarium of the Steppe Institute of the Ural Branch of the Russian Academy of Sciences , Orenburg ( ORIS). Figure 25 is a representative scale from the type specimen.

Type locality:— RUSSIA: Orenburg region, small unnamed ephemeral pond (“Svetlinskii” Biological Reserve). Latitude/Longitude: 51°02′19″N, 60°47′09″E, 03 May 2023. Leg. M.E. Ignatenko.

Etymology:— The epithet “prominentia ” (protrusion) refers to the characteristic elevation of the proximal part of the shield.

Distribution: — This species was found in the type locality only.

Ecology: —The species has been observed in small shallow ephemeral pond at water temperatures of 14–20 ⁰C and pH of 6.3–8.6.

Figures

FIGURES 16–19. Mallomonas prominentia sp. nov. (SEM). Figures 16, 17. Whole cells. Figure 18. Collar and body scales. Figure 19. Body and rear scales. Scale bars: figs. 16, 17—5 μm; figs. 18, 19—2 μm.

FIGURES 20–25. Mallomonas prominentia sp. nov. Body scales. Figures 20–22. TEM. Figures 23–25. SEM. Scale bars: figs. 23–25—2 μm; figs. 20–22—1 μm.