Synarthonia xanthonica

Synarthonia xanthonica is a species of crustose lichen in order Arthoniales,^[1] characterized by its UV-positive orange thallus and linear to ink spot-like ascomata (fruiting bodies). This lichen is corticolous, growing on the bark of trees in primary rainforests in Brazil.

Synarthonia xanthonica was described by André Aptroot in 2022 from specimens collected in the Cristalino nature reserve, Mato Grosso, Brazil. The species was identified as a novel member of the genus Synarthonia. The specific epithet, xanthonica, refers to the presence of xanthone compounds in the lichen, which contribute to the distinctive UV+ (orange) colouration of the thallus.^[2]

The thallus of Synarthonia xanthonica is crustose and continuous, lacking a cortex and featuring a dull, pale ochraceous-white colouration. It covers areas up to 3 cm in diameter and is under 0.1 mm thick, bordered by a dark brown prothallus line about 0.2 mm wide. The photobiont associated with this species is trentepohlioid (i.e., a member of the green algal genus Trentepohlia).^[2]

Ascomata (fruiting bodies) are solitary, superficial on the thallus, and present an irregularly linear to ink spot-like branched outline. They measure 0.3–0.8 mm in width and are under 0.05 mm high, with a dark grey, white pruinose disc. The margin of the ascomata is not differentiated. Ascospores are club-shaped (clavate), hyaline, and contain two septa; they measure 12–13.5 by 4.5–5.5 μm, and are not surrounded by a gelatinous sheath.^[2]

Chemical analysis revealed the presence of a xanthone, likely 1,8-dihydroxy-3,6-dimethoxyxanthone, contributing to the UV+ (orange) reaction of the thallus. Other chemical spot test reactions include C–, K–, KC–, and P–.^[2]

Synarthonia xanthonica is exclusively known to occur in its type locality within the primary rainforests of Mato Grosso, Brazil.^[2]