Malmidea densisidiata

The thallus of Malmidea densisidiata is crustose, mostly continuous and corticate (with an outer cortex). It forms a dull, pale greenish crust that can cover areas of up to 25 cm (10 in) across and is about 0.1  mm thick. It is surrounded by a white hyphal prothallus (a border of fungal threads), which also runs between thallus patches along cracks in the bark. The surface is densely covered with isidia. The isidia are pale yellowish green, short, cylindrical, and unbranched, about 0.2 × 0.1 mm. The medulla (inner layer) of both the thallus and isidia is pale yellow. The photobiont (algal partner) is trebouxioid. The ascomata are sessile (sitting directly on the surface), solitary, and constricted at the base. They are round to lobed, 0.4–1.5 mm wide and up to 0.5 mm high. The disc (central surface) is medium brown and flat to slightly convex. The whitish margin is barely raised above the disc, about 0.1 mm wide and bears small whitish papillae. In section, the excipulum is pale yellow and contains pockets of large hyaline crystals. The epihymenium is pale brown and the hypothecium is black. The hamathecium (tissue between the asci) is not inspersed (without scattered droplets). It turns blue with iodine (IKI+), mainly in the ascus tips. Asci are Porpidia-type. Ascospores are produced eight per ascus. The spores are hyaline, simple, ellipsoid, and measure 11–12.5 × 4.5–5.5 μm. They lack a surrounding gelatinous sheath. Pycnidia have not been observed. In standard spot tests, the thallus is negative in UV, C, KC, and P tests but reacts K+ (yellow). The yellow medulla also reacts K+, turning reddish. Thin-layer chromatography detected atranorin and several pigments, including chloroemodin, as the main lichen substances.^[2]

Malmidea densisidiata grows on tree bark in primary rainforest in the Reserva Cristalino region of Mato Grosso, Brazil, at about 250 and 350 m (820 and 1,150 ft) elevation. At the time of its original publication (2022), it had not been reported from outside Brazil.^[2] Within the type area it is locally common and represented by several additional collections.^[2] It has since been documented from Rondônia.^[3]