Tetramelas phaeophysciae

Tetramelas phaeophysciae is an obligately lichenicolous lichen-forming fungus—meaning it lives on other lichens—developing its own body (thallus) mainly as a network of hyphae within the host thallus. The apothecia are small (about 0.10–0.55 mm across), typically scattered to abundant on the host thallus or directly on the host's apothecia. They are sessile, with discs that are initially urn-shaped and later flatten to convex. The margin is often distinct, although it can become excluded as the apothecia become strongly convex.^[2]

Microscopically, the hymenium is about 50–70 μm tall, with asci roughly 43–50 × 12–20 μm. Paraphyses (sterile filaments) have broadened tips (about 4–6 μm) with distinct apical caps. The ascospores are ellipsoid and consistently 1-septate, usually slightly curved, measuring about 16.5–22 × 6.5–9 μm. The outer spore wall (perispore) is finely wrinkled (rugulate) and only slightly thinner than the main wall. Pycnidia were not observed, and thin-layer chromatography detected no secondary metabolites (spot tests K, P, and C negative).^[2]

The species is known from arctic to subalpine parts of Greenland, Iceland, Norway, and Sweden. It grows within the thalli of Phaeophyscia and Physcia species that occur on rocks (and more rarely on soil), and it has most often been collected in damp settings such as riverbanks and lake shores.^[2] Its known range has since increased with records from Hungary,^[3] Mongolia,^[4] Svalbard,^[5] and the Kodar Mountains in the Transbaikal region of Siberia.^[6]

Among recorded hosts, Phaeophyscia sciastra is the most frequent, followed by Physcia caesia; other reported hosts include Phaeophyscia endococcina, Physcia dubia, and Physcia tenella. In the original account, Nordin and Tibell contrasted it with the closely related Tetramelas pulverulentus (the recombined Buellia pulverulenta), which is associated with Physconia hosts rather than Phaeophyscia—though both species can occur on Physcia.^[2]