Chrysosphaerella baikalensis Popovskaya 1971

Chrysosphaerella baikalensis Popovskaya (1971: 9) emend. Bessudova, Firsova & Likhoshway Description: There are both individual cells with two flagella, slightly elongated, ranging from obovate to nearly spherical, measuring 9–15 × 7–12 μm, and lacking mucous membranes, as well as cells grouped in colonies of 3–6, forming spherical or oblong clusters, 18–31 μm in diameter. Each cell contains two chromatophores, typically elliptical and laterally compressed. The two flagella are of unequal length: the longer reaching up to 36 μm, and the shorter measuring 7 μm (Popovskaya 1971). The cells are covered with numerous plate-like and spine-like siliceous scales. Four types of plate-like scales are present: small elliptical (1.4–2.5 × 0.9–1.3 μm) (Figs. 2 and 5), large elliptical (3.6–4 × 2–3.2 μm) (Figs. 2, 5 and 7), large rounded (3.2–4.7 × 2.4–3.9 μm) (Figs. 2, 8 and 9), and large subcircular (2.8–3.9 × 3–4.2 μm) (Fig. 9) scales. The plate-like scales have a raised, unpatterned rim along the edge, measuring 0.3–0.6 μm in width. On the inner surface of the rim, a series of short radial ribs, 0.1–0.32 μm in diameter, converge towards the center, enclosing a central unpatterned area of elliptical or round shape (Figs. 7 and 8). The spine scales, measuring 21–37(47) μm in length, taper slightly towards a forked tip and consist of a shaft and a bobbin-like structure with two bases. The basal base plate is 3.5–3.9 μm in diameter, while the second base plate measures 4.1–4.7 μm, spaced 0.8–1 μm apart (Figs. 2 and 5). A large circular hole, 0.7–1.1 μm in diameter, is present at the base of the spine (Fig. 3). The shaft’s thickness from the base to the tip has a ratio of approximately 1.5:1. Neotype (here designated): Portion of a single gathering of cells on SEM stubs No. 18851 deposited at the herbarium of the Limnological Institute, Siberian Branch of the Russian Academy of Sciences, Irkutsk (LIN). Material from Lake Baikal, in the Irkutsk region, Russia, on June 22, 2023. Figure 2 illustrates the neotype. Type locality: Lake Baikal, Latitude/Longitude: 53°13′ N / 107°45′ E. Ecology: The species grow in Lake Baikal during the under-ice period from March to May (Popovskaya 1971) at a temperature of 0.04–0.6 ℃ (Bessudova et al. 2023; Bashenkhaeva et al. 2024 in press) and during the open water period (June) at a temperature of 3.6–4.5 °С; individual plate and spines scales are found up to 11.5 °С (Bessudova et al. 2024). Stomatocyst 507 Firsova & Bessudova nov. (Figs. 10–18) Biological affinity: When examining samples on these cysts, plate-scales of Chrysosphaerella baikalensis were often found, so we attribute the new morphotype to this species. Negative number:18851_Ch bay_stomat_014 Location: Lake Baikal SEM description: The stomatocyst is spherical, with a diameter ranging from 10.8 to 13.5 μm, and may be slightly oblate, measuring 12.5–13.5 × 10.8–12.2 μm. It lacks a collar. The pore is regular, measuring 0.2–0.3 μm in diameter, located in a depression that is 1.4–1.7 μm in diameter and 0.2–0.4 μm deep. This pore is encircled by an asymmetric, planar pseudoannulus that is 0.1–0.4 μm wide. We identified two morphotypes of stomatocysts in a single sample of this species, as follows. Forma A: The surface of this stomatocyst is densely covered with short, needle-like outgrowths. We hypothesize that this form represents immature stomatocysts. Forma B: The surface of this stomatocyst is either smooth or psilate. Ecology: Both morphotypes were found during the open water period in June at a temperature of 3.9–4.5 °С.