Cosmarium lineatum F.M. Costa, G.J.P.Ramos & C.W.N.Moura sp. nov. (Figs. 2, 6–14)

Diagnosis: —Cells slightly longer than broad, median constriction deep, sinus closed, linear, one prominent papillae at the extremity of sinus; semicells subreniform, basal angles rounded; cell wall covered by granules, a central longitudinal row with four warts, one isthmial wart in each semicell; lateral view rectangular, apical view elliptic. Cell dimensions: length 35.7–36.7 μm, breadth 27.8–31.4 μm, breadth of isthmus 8–11.7 μm.

Holotype:— Material numbered (HUEFS 263035!) [pro parte], deposited at the Herbarium of State University of Feira de Santana, population partially illustrated here (Figs. 2, 6–14).

Paratypes:— Material numbered (HUEFS 263039) [pro parte], deposited at the Herbarium of State University of Feira de Santana.

Type locality:— BRAZIL. Bahia: Camaçari, Camaçari-Mirim River, Lat. 12°42’23.1’’ S, Long. 38°15’27.2’’ W, 29 November 2019, F. M. Costa & J. T.X. Neto s/n (HUEFS 263035).

Habitat:— Periphytic on Eleocharis acutangula; water conductivity 0.05 mS. cm-1 (± 0.01); dissolved oxygen 7.5 mg.L- 1 (± 1); pH 7.8 (± 0.6); total dissolved solids 0.02 ppt (± 0.01); water temperature 27.3 ºC (± 2.5).

Etymology:— The specific epithet refers to the surface ornamentation composed of warts linearly arranged on the face of the semicells.

Differential diagnosis:— This taxon differs from other Cosmarium species in having subreniform semicells with a central longitudinal row composed by four robust warts, and one prominent papillae at the extremity of sinus.

The newly described species resembles Cosmarium boeckii Wille (1880:28) due to the presence of an oblong wart near the isthmus and three rounded warts forming a row. However, the latter species differs in having much smaller granules on lateral to the central row, forming a concentric circle. Also, it differs because of the semicircular cell outline and smaller cell dimensions (length 29 μm, breadth 27 μm, isthmus 8 μm). Another species similar is Cosmarium isthmochondrum Nordstedt (1873:12), but this one differs in the ornamentation arrangement (two granules on the semicell face and two granules next to the apical margin).

An interesting feature observed in the population was the presence of a thick mucilaginous layer (Fig. 6). Mucilage production usually represents an adaptation mechanism that aids in algal flotation and protects against predation (Book 1981; Coesel 1994). Recently, it was also observed that the mucilage envelope can protect desmids against fungal parasite action (Van Den Wyngaert et al. 2022).