Big-disk pebble lichen

Porpidia macrocarpa is a crustose lichen with a variable thallus, typically appearing immersed (embedded in the substrate) to thin and continuous, but it can sometimes be thicker and become cracked (rimose) or divided into small sections (areolate). The surface of the thallus is rough and web-like (subarachnoid) with a colour range from pale grey to greenish-grey. It frequently shows patches or continuous areas of orange to rust-red due to oxidation. The medulla (internal tissue) does not react to iodine (I–). The prothallus (initial growth stage) is indistinct at the margins of immersed thalli but becomes wavy, black, and somewhat distinct at the margins of more superficial thalli. The apothecia (fruiting bodies) are up to 3 mm in diameter, usually sessile (sitting directly on the thallus), abundant, and can be scattered or crowded together. They are constricted at the base with a thick, swollen true exciple (rim) that is persistent, black, shiny, and raised, ranging from entire to somewhat wavy (flexuose). The hyphae (filamentous fungal cells) measure 3–9 μm in diameter. The disc of the apothecia can be slightly concave to flat or convex, black or brown-black, with a matt or shiny finish, and sometimes covered with a grey powdery coating (pruina). Larger apothecia often appear to divide through the formation of secondary margins within the disc. …

In Nepal, Porpidia macrocarpa has been reported from 2,950 to 4,000 m elevation in a compilation of published records.

Lichenicolous (lichen-dwelling) fungi that have been recorded growing on Porpidia macrocarpa include Cecidonia xenophana, Endococcus propinquus, E. rugulosus, Muellerella pygmaea, and Sclerococcum australe.

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