Bactrospora flavopruinosa

The thallus of Bactrospora flavopruinosa is mostly immersed in the outer surface of the wood, and shows up only as a thin whitish patch with scattered yellow dots from its algal partner. In cross-section the thallus is very thin (about 20 μm) and homoiomerous (made largely of algal cells), with only a few short, colourless fungal hyphae reaching the surface. Some areas are exposed while others are still covered by a thin film of the wood, giving a mottled look under magnification. The photobiont is Trentepohlia-like, forming branched filaments of broadly spindle-shaped, rough-walled cells about 11–18 × 7–9 μm.^[2]

The apothecia are sessile, round to slightly top-shaped, 0.3–0.8 mm across and about 0.2 mm tall. Their discs are flat and coated in a bright lemon-yellow, powdery pruina; the margin is also pruinose, but looks glossy black where the coating has rubbed off, and is often toothed. Microscopically, the hymenium is clear and colourless, 100–150 μm tall, and iodine-positive; above it, the epihymenium contains bright yellow crystals that turn ochraceous in section after treatment with potassium hydroxide solution (K). The asci are cylindrical (60–75 × 6–8 μm) and each contains eight long, colourless ascospores with 9–15 septa; the spores are 45–55 × 1.5–2.5 μm, straight to slightly curved, and do not break apart. Pycnidia have not been observed. In K, the yellow pruina releases a yellow solution and briefly forms sulphur-yellow, needle-like crystals before dissolving again. Based on this reaction, the authors suggested the pruina may contain naphthopyran compounds such as simonyellin or protosimonyellin.^[2]

In a global identification key to Bactrospora, B. flavopruinosa keys out by its yellow-pruinose apothecia together with narrow ascospores.^[3] Several other Bactrospora species have similarly thread-like (filiform) ascospores of the patellarioides type and an iodine-positive (amyloid) excipulum, including B. patellarioides, B. brodoi, B. carneopallida, B. cascadensis, and B. mesospora. However, none of these combines such narrow ascospores with lemon-yellow, pruinose apothecia. On Bermuda, B. flavopruinosa occurs in the same habitats as B. myriadea and B. denticulata, but those species have black, non-pruinose apothecia and a non-amyloid excipulum. B. denticulata also has longer ascospores (over 70 μm) and strongly toothed (denticulate) apothecial margins, so B. flavopruinosa can be separated both in the field and under the microscope.^[2] Unlike some yellow pruina-producing members of the genus, the yellow pruina in B. flavopruinosa does not turn reddish-magenta in the K reaction.^[4]

Bactrospora flavopruinosa is a lignicolous lichen (growing on wood) known only from its type locality in the Walsingham Nature Reserve (the Walsingham "Jungle") in Hamilton Parish, Bermuda. The type specimen was collected about 3 m above sea level on a well-lit but sheltered, barkless trunk of the endemic Bermuda cedar (Juniperus bermudiana), on firm, sun-bleached wood near a protected bay in limestone terrain formed by partly collapsed cave roofs. The reserve preserves a small remnant of Bermuda's pre-colonial lowland woodland, dominated by native trees including Bermuda olivewood (Elaeodendron laneanum), Bermuda palmetto (Sabal bermudana), white stopper (Eugenia monticola), and hackberry (Celtis laevigata), which provide stable bark and wood surfaces compared with many introduced trees that shed bark quickly. The history of Bermuda cedar is especially relevant: a mid-20th-century scale-insect outbreak ("Cedar blight") killed about 99% of the population, but some dead, decorticated trunks and branches have persisted for decades because the resin-rich wood decays slowly, creating a long-lasting substratum for uncommon lichens. At Walsingham, B. flavopruinosa occurs alongside other Bactrospora species such as B. myriadea and the locally rarer B. denticulata, as well as wood- and bark-inhabiting lichens including Arthopyrenia minor, Caloplaca sarcopisioides, Chrysothrix xanthina, several Hafellia species, and Nadvornikia hawaiiensis; the authors suggested that protecting remaining dead cedar trunks, particularly in reserves like Walsingham, would help conserve these specialised lichen communities.^[2]