Buellia sulphurica

The lichen forms a thin crust comprising discrete, powder-yellow areoles—small, polygonal patches that sometimes lift slightly at the edges. No dark prothallus (border) is visible. The surface is matt and dusted with a fine whitish bloom, a result of minute crystals that dissolve in potassium hydroxide solution; coarser, inert mineral crystals up to 50 micrometres (μm) are also embedded in the thallus.^[2]

Reproductive structures are abundant but minute. Each black apothecium begins almost immersed in the thallus with a fleeting thalline rim, then soon becomes adnate to sessile, 0.15–0.25 mm across, and markedly convex. The proper margin is black and rapidly excluded, while stray flecks of yellow thallus tissue often cling to the disc as it emerges. Internally the apothecium has a very thin, pale receptacle (proper exciple) that merges into a clear hymenium; simple to sparsely branched paraphyses end in brown-capped tips. The eight-spored asci are club-shaped; their Bacidia-type amyloid reaction shows deep-blue sides with a pale core in iodine. Ascospores are one-septate, oblong and 10–13 × 6–7 μm, with a briefly thickened median wall in juvenile stages but no outer ornamentation. No pycnidia have been observed in B. sulphurica.^[2]

Chemically the cortex contains no lichen products detectable by the common spot tests (K, C, P all negative), whereas the medulla holds rhizocarpic acid, likewise unreactive to these tests but identifiable by thin-layer chromatography. The hymenium, unlike the thallus, gives a strong amyloid (blue) reaction in Lugol's iodine solution.^[2]

Buellia sulphurica is a saxicolous (rock-dwelling) crust restricted to basalt boulders and outcrops within a few metres of active sulphur fumaroles. The type material came from the north-north-west-facing inner wall of Volcán Alcedo at 1,055 m above sea level, in open scrub dotted with Tournefortia rufosericea and Adiantum concinnum. Additional specimens were later found on the outer slope of Volcán Chico on Sierra Negra, also on Isabela Island. The bright neon-yellow thalli stand out clearly against the dark lava, yet repeated searches on other Galápagos islands have failed to locate the species farther afield.^[2] It is one of 13 Buellia species that have been documented from the Galápagos islands.^[3]